8x4 overlapping boards

Give this man the can of Sardines. (sorry, TIN . . . Bloody yanks that can't speak da queen's english!) I don't think the GOOZ card has much value though.

Shadetree has done an excellent job of verbally describing what I can visualize but would struggle to explain to someone else.

By the way, I assume Del's shed is on English soil. I thought you blokes were metricized? How is it you have a 20' x 20' roof with 4' x 8' sheets for covering?

The roof was built in the 1960's.
I love the way we still by 8x4 boards but they are quoted as 2400x1200 mm .

Del

Oh, Del. Don't start down that road. Aaaaaargh!

Engineered timber boards are sized in accordance with the American Plywood Association's standards, which are in imperial units. Cut timber is sized according to pan-European standards, which are in metric units - hence the 0.3m sizing criterion used by timber distributors locally. 0.3m? It's a "metric foot". Which is closer to 11⁵/₈in............

It's enough to make a skilled carpenter weep.

Then make the mistake of trying to buy16D (penny) nails by the pound, (lb<- that one).

"Sorry sir, you can only get 4 inch by the kilo"???!!!

........But the best one was, "Oh yeah, everything's metric now, plywood only comes 1200mm X 2400mm".....said with a straight face!

Ha..excellent, I was hoping there may be a cunning way to avoid that 2' cut sheet...but it's hardly a difficult job, blimey a quick swipe with a claw should do it .

(Sardines and Gooz card duly awarded, I had to open the tin to check the sardines were ok_{....<munch munch>} yup they were fine)

Del

Del,

'I was hoping there may be a cunning way to avoid that 2' cut sheet'

Your hope has arrived (:

Start at top left lay 4x8 sheet of foam down with length horizontal but hang 1/2 length off roof edge. Abut next 4x8 sheet and so on when top is done start next row with full 4x8 sheet on roof and continue, start bottom edge half off again until roof is covered with foam. Minimize walking on foam by starting lay wood at bottom left edge with length vertical abut next sheet horizontal continue chimney block pattern never stepping on foam and it all fits well. Cut off over hanging pieces and you're done.

Hi, Shadetree!

Wow! The overlap, the order of laying, the method, and the ease of following those directions! Nice. ...GA.

Mark

Except....start at the bottom (not the top/ridge) for nailing. Handling sheets much easier. Which means you need to measure the total layout and mark as necessary to ensure squareness and coverage, and mandatory gaps between sheets, before driving a nail.

An image possesses a value equal to 10³ pieces of verbage.

Assuming you have a flat roof.

No, you should stagger the joints as well.

The insulation boards are PUfoam with an aluminium skin either side...what's the best way to cut 'em? Will a knife do it? Or is something like a wallboard saw better.

Oh boy this armchair work is sooo tiring, must get more toast & tea... and maybe a nap.

Del

Del,

The way I cut those boards is to lay a 2x4 down on the board and use it as a cuting guide. Once you have cut through one side with your knife, pick the board up (having first removed the 2x4) and break it. that should leave you one foil to cut. with it partially folded you can easily score it and there you have it.

There is no way that I can see that you can avoid having 1/2 sheet of each board left over. Perhaps you can use it on your dog house. You do have a dog, don't you? My cat does. He has some humans too.

Cheers, I shall order my boards on Monday...I have the rubber already... and I can get the ply locally...I'l take pics and may be do a blog.

I'll have to wait for a decent weather window....probably September if the current weather is anything to go by .

Del

If you cover one with the other you can make your choice. You have to cut only ONE sheet. There are for sure even better arrangements but it is for you a start. One cobination covers 12 gaps and the other 13 .

Enjoy your work

Nickname

Hi, nick name!

Your plan covers the area but there are places that one side or another matches the edges below. Edge matches = Cold spot + Noise admission.

Del asked for overlap.

Overlap = Zero to pinhole cold spots (If insulation taped before second layer = zero cold spots & zero noise admission). Add the tar paper & shingles without taping = no noise admission.

Mark

Everyonr knows the answer to that is stagger the joints. Daaaaa. You stagger them because it make for a tighter and stronger fit .

Sort of...

If sheets were aligned vertically--and not diagonally or randomly--there would be double nailing, one in each abutting sheet, along the entire length of certain rafters underlying sheet joints. These would be overly nailed into, and somewhat splintered (and weakened) rafters.

Del, I personally wouldn't worry about minimizing cutting, but overlap the layers as much as you can

To simplify, start horizontally, and then adhere the second layer vertically. Ensure that the first layer for structural purposes is either horizontal or vertical for maximum strength

Changing direction will be fine although I'm not sure why you would. Just remember that there is virtually no strength in the insulation so make sure to run the ply perpendicular to the joist system and don't skimp on the nails.

I see you have the stagger part down, but here's something that will help too. Since you aren't looking for strength n the insulation start with the rip (1/2 sheet) on the bottom. You can stand on the ceiling joists while you do this, after you get 1 1/2 rows of insulation on you can put a row of plywood.
Then you can stand on the part you've done to work your way to the top. To insure you get it straight measure down from the top to get a line for the top of the bottom row. I do this kind of work for a living and this is how we've always done it from Ohio to Colorado to Georgia.. It really is alot easier to work uphill than down.

This is the best answer yet. Oh by the way in construction You always give the width first be it windows, boards , plywood ect. So it should be stated 4x8 sheets.

oilcan13

"...in construction You always give the width first be it windows, boards , plywood etc."

True.

In the US, the most common structural lumber is the 2-by-4, nominally 2 inches by four, though actually considerably less than that. When I saw a personalized license plate some years back, reading 2 X 4 X 8, I figured out its true meaning in a few minutes. For those of you with an actual system of measurements (we have a hodgepodge, nothing systematic about it!) I will explain that the third digit is the length, and in feet, not inches. Challenge: in a word, what does the plate say?

Dimensional lumber ie 2 x 4's etc are cut to size then dressed (planed and dried) to give the 1 1/2 x 3 1/2 finished dimension. As for sheet good plywood, sheetrock, insulaton etc it would be identified as 1/2" or 3/4" or 1" x 4 x 8 or 12.and with plywood the 1/2" would be 15/32". If you save 1/32 per piece times 10,000 pieces per lot your gain in a year is nothing to sneeze at.

2X4X8=STUD...Daaaaa

How long is an 8' stud (trick question)

96" OFCOURSE Why would you ask a question like that?/

Hi Michael Sadleson!

Probably because the other two dimensions have changed over time, the length question was a "trick question". Due to "saw width" at the sawmill (an excuse given by the suppliers so they could actually charge more for less), what used to be a truly dimensioned 2" X 4" piece of lumber now measures 1 1/2" X 3 1/2". At least the reduction doesn't interfere with general construction, and the building codes have been altered to adjust to this reality, along with the changed dimesions of 2 X 3, 2 X 6, 2 X 8, 2 X 10, and 2 X 12 lumber and related code requirements.

Fortunately, the key length of 96" was not able to be changed because it would interfere with normal stud usage too severely, so you still get what you're paying for in terms of length.

Unlike doors, whose dimensions have now changed to the point where if you don't buy a door in a frame, you often have to go to suppliers smart enough to carry a full-sized door, or, frequently, special order one to fit an existing door opening.

Mark

"Fortunately, the key length of 96" was not able to be changed because it would interfere with normal stud usage too severely, so you still get what you're paying for in terms of length."

As DGC1 has already pointed out, the suppliers actually HAVE changed the length - but in at least some lumberyards and home centers (here in the US, at least) it depends upon whether you order them as "studs" or as dimensional lumber without mentioning the purpose. They may offer "eight-foot studs" that are 92-5/8" long, and "eight-foot 2-by-4s" that are 96" long in the next bay of the same building. Depending upon my purpose, I will usually SAY that I want eight-footers, and am willing to make sure that everyone in the store knows what an idiot the clerk is if I find my vehicle being loaded with shorter ones. But actual lumberyards tend to have fairly knowledgeable people in those positions, and MOST of the home centers do too. Still find the losers there every so often, and do my best to make them uncomfortable about trying to fake their way through.

Hi, DGC1!

As Ron correctly pointed out, I was fooled by the trick question. My excuse is that in my work the 2 X 4 material was mainly repair and reno materials, and the purchase of extra materials was usually all to the good and the costs of materials always covered by the clients. So I rarely if ever purchased the 92 5/8" stud size of 2 X 4, even for studs, unless the stud material was on sale.

Ya got me!

Mark

How about angle deflection of a 6/ 12 pitch roof on 4000sq.' house because of hurricanes ?

As I said "trick question" it's 92 5/8" for 8' and 104 5/8" for 9' add 4 1/2" for a single bottom plate and 2 top plates gives 1 1/8" over finish height for 1/2" floor and 5/8" sheetrock ceiling. It took awhile to get a bite on that one.

Hi, DGIC!

I was referreing to the length of a standard 2 X 4 X 8. The length of a 2 X 4 X 8 at the Home Depot stores around here is 96" off the shelf. Not more, not less.

Did your "trick question" imply something else?

Mark

Sorry for the sizes, but here are a couple suggestions.

Or if you wanted to stagger your boards:

Here's a new slant on the problem:

I know it means more work up front, but I think you might get better results in the long run.

Hi, 3doug!

Neat! Bit of tricky cutting, though, having to measure 4' down from the second edge and then cutting from an exact corner to the mark. Probably a little more open to corner breakage due to the pressue on the straight edge. But a really good finish with no laying complications. GA

Mark

Del,

As you know, and I have found out after visiting this discussion place for the past year. Opinions are like #@@%&!$##. Every one has one. There are tons of ways to lay this out. But in a previous life, I constructed houses and churches.

Rule of thumb. Every time you can, stagger the joints. This is as true with sheeting panels as well as boards, or planks.

If you were to take a 4 X 8 sheet (plywood, drywall, or insulation) they are designed to be supported with bracing across the shorter side, (5 braces spaced 24" OC [on center]). The end of the sheets or boards are the weakest because you are fastening at less than half of the support board (the 2 joints share one board)

Because wood materials are flexible, any load on the ends of the sheeting will cause movement and can eventually loosen the fasteners or panels. Where as there is greater strength and resistance to movement towards the center of the panel. Staggering joints allows the weaker joints to be supported by the strength of the next board. (suggestion, use panel clips)

With this in mind, a 20' x 20' shed roof would require 13 - 4' x 8' panels. Assuming roof trusses or joists running to a peak, run the panels horizontal (or perpendicular, as mentioned before). First row, 2 full panels and 1 half panel, starting with a full sheet. Second row start with the other half panel and then 2 full sheets. 5 rows - 10 full sheets and 3 sheets cut in half, with one half sheet left over. Which ever way the roofing boards run, run the panels perpendicular to them.

Seeing you would install the insulation board under the plywood, preferabily you would want NO seams shared between the two layers, unless they were tongue and grooved and NO share joints. Which would mean start your insulation boards with a 2' X 8' row. (cut the insulation, the strength is in the plywood, and it is much easier)

RXR does a good job explaining standard practice. You can't argue with experience.

But like the saying goes:

I'm not trying to show off (well, maybe a little), it's just that I need the practice with ACAD, and I realize that not everyone has the software to produce an illustration.

Because cats are a curious sort (another saying), so, Del, I wouldn't blame you if you did experiement with another pattern. Just remember what that old adage says about curious felines, and take an accounting of how many lives you have to spare before you act.

Del what is the measurements of the joists under your roof, 16", 18", 21", 24"?

Use tongue and groove or clips to keep the sheets flush with each other. The long edges of the sheets of plywood or OSB (oriented strand board) should have the studs/joists perpendicular under them and the ends centered on a stud/joist. No I'm not a carpenter but I'm an expert at making sawdust in my shop.

If you want real 2X4s get them ruff cut from the mill and wear gloves. They are a lot stronger.

Brad

Rafters in the US are spaced standard at 16",19.2", or 24" on center, however on old bldgs that have plank decking it was common to guess at the spacing making it somewhere between here and there. ply clips are required on 24" ctrs and sometimes used on 19.2" but rarely on 16" and then only as a preference by the DIY'r These are code here. 3/4' ply comes tongue and groove of you have load concerns and wide rafter spacing.

Yes if the rafter spacing is wide it would provide more structural strength. This can be important if the roof is flat and or if there is a snow load that needs to be considered. A lot of older roofs were done in 1 x ?, some t & g or just plank. The only other way to get the same structural integrity is with multiple layers of say 3/8" or double 1/2". Sorry I am not familiar with the weather there, we have a 40 psf live load figure here and you could also use Iso for the insulaton in thicknsses of I believe up to 4". It is alot more rigid than the foil backed stuff available at Home Depot.

But my favorite remodel was a hand built (no power tools) house by an unknown Japanese carpenter. It was an engineering masterpiece in visual art. It looked perfect yet no corner was square and no surface level. The only way to repair anything was by eye.

I make no assumption to Del's rafter spacing other than it is most likely in inches.

If Del is wanting it insulated he might consider SIPs

Most structural insulated panels (SIPs) are load-bearing laminates of foam and wood sheathing materials used for building the exterior walls, roof, floor and even foundations in homes and commercial buildings. Manufactured in factories, the panels are shipped by truck to a building site and assembled by framing crews. When properly assembled, the resulting home or commercial building provides an extremely strong, well-insulated and comfortable building shell. Some 60 different companies manufacture panels throughout North America.

A structure built with SIPs looks the same from the outside as any other. Yet its strength and efficiency characteristics are exceptional. Tests performed by national laboratories prove the product's energy efficiency. And SIP structures that have survived hurricanes, earthquakes, tornadoes and even indoor fires attest to SIPs extraordinary strength. These and other benefits are generating plenty of news coverage and lots of sales.

From here http://www.sipweb.com/about/default.asp

Just an idea

Brad

The joist are nominally 24" between centres, but I shall just screw through to the existing sound/solid plywood deck.

Now I know some will throw their hands up in horror at this.
The existing roof deck is sound but the felt is due for replacement, so I intend to take off all the raised edges and fascias, clean it down and roof over the top.
I know that 'best practice' says I should lift off the old deck and start from scratch...

Well I did that on the other section of my flat roof...and it was a huge waste of time and meant I got in trouble when we had a cloud burst of tropical proportions before I had the EPDM stuck down.

Doing it this way it will be quicker, easier, and the roof will be weatherproof at all times.

The only reason I lifted the boards on the last roof is...wait for it...you'll love it.
All the boards were good solid ply, except the last one (over the garage) was chip board (not even decent OSB!!!)
They's obviously run out of ply and just used any old thing!!It had gone all soft of course, so I ended up ripping up 15 good boards because of one bad.)

I shall take pics of the progress and bore you all rigid with a blog when it's done, assuming it ever stops raining here...

Del

Hi, Del the cat!

Don't ignore a new, well tarred flashing built right into the parapet around that newly laid flat roof, and not just laid up against it.

Mark

Here's a pic of one I did earlier .

You can see I've taken the EPDM up the wall, right into the horizontal joint and pointed it in. I had to do that roof before I could do the fun bit with the solar panels.

Del

Whoa you had one them rare type chip boards that can't be sized by a single replacement board...I'm loving it (: ain't remodeling fun!

Hello Del,

How thick is the new rigid insulation? How thick is the existing plywood deck? If you are relying on screws, it would be much better to hit the joists at 24" centers, but that may not be easy to do. If the old deck is only 3/8" thick, you won't get much of a bite with a wood screw.

So far, everyone seems to be of the opinion that you should stagger the joints between the insulation and new plywood deck. I would think that staggering the joints of the existing deck and the new deck is more important, but if you can stagger all three, so much the better.

If the sheets of insulation are really 2400x1200, they won't be modular with your joist spacing...or do they mean 2438x1219? I looked in the supplier's data but could not find exact dimensions for the sheets.

I assume that plywood comes in 96"x48" size in your locale.

Insulation 40mm.
Existing deck ~3/4" thick, so plenty of bite .

No chance of hitting the joists as there is roofing felt disguising the position and if I try to mark it up from the ends of the joists the parallax and the fact that the joists aren't true would make it impossible.

The boards arrive tomorrow ... Dunno when the Sun will arrive

Del

I'm impressed! Your response arrived when I still had 3.4 minutes of edit time left. I thought it would be held back until all the time had elapsed just in case I wanted to change the message.

Anyway, with 3/4" thickness of existing deck, I agree you will get lots of bite. And furthermore, the existing deck is so rigid that the staggering of joints in the upper two layers is not of much concern. I would opt for the labor saving solution, i.e. minimizing the cuts.

Cheers.

Hi, Del the cat!

"No chance of hitting the joists as there is roofing felt disguising the position"

Don't you have a stud finder?

Mark

..Consider the chance of putting a screw through 1/2" of ply and 40mm of insulation board and hoping to hit a stud, even if you know where the stud is...

Maybe if I had X-ray vision and a helper...?

Hi, Del the cat!

It's obvious from the picture that you think you know who/where one particular stud is.

In other news...With a good electronic stud finder, you should also be able to locate the other kind of "stud"...the roof joists (or ceiling joists beneath your flat roof) kind. Mark their locations out using the stud finder and use a chalk line or straight edge to show the joists. Use lots of deck screws when fastening the new base down, to prevent shifting due to temp and pressure changes, and clear nail polish, which you may borrow from your super friend, to coat the heads against rust.

Returning to the initial story...Have your friend do the rust-proofing, and be certain that she is working down wind and in a direction away from you. That way, you won't need the X-ray glasses.

Mark

M.S. scribed "... You stagger them because it make for a tighter and stronger fit ." I stagger when I'm tight, and my sweety throws a pretty strong fit.

You are a sick, sick man my friend lol

Sorry I'm a bit late for this party. This design has no overlapping joints (unlike the others except the sloping one). Ok you need one extra insulating sheet and you need to cut out those four 6' X 2' corners, but at least cutting of the wood is kept to the absolute minimum (one four foot cut).

Hi, Randall!

A+

Mark

This thread reminds me of an old puzzle. You have a chess board with two diagonally opposite corners missing, and, 31 dominoes. One domino exactly covers two squares of the chess board. Can you cover all the 62 squares with the dominoes without cutting any?

Here's a template you can print out to try it.

I did it with 27. Do I get points for efficiency?

"I did it with 27. Do I get points for efficiency?"

I'd guess no; your notched chessboard is eight columns wide but only seven tall, for (56-2)= 54 squares; with each domino covering two squares, that takes 27. But the puzzle is and eight-by-eight, and (64-2)= 62 squares, and would require 31 dominoes IF it can be done.

I realized that as soon as I posted it, and I couldn't get it edited in time. Too bad that CR4 doesn't allow you to delete your own posts.

Edit: I don't think it can be done. The grid has 6 rows of 8 and 2 rows of 7. That odd number just won't divide into an even one, because it is prime.

The grid has 6 rows of 8 and 2 rows of 7. That odd number just won't divide into an even one, because it is prime.

Not sure I understand this, especially as your original grid had 5 rows of 8 and 2 of 7, and, you managed to cover it easily enough. Anyway I've re-posted the problem as a mini puzzle in the general section.

Solution posted in thread:-

http://cr4.globalspec.com/thread/19702

The roof dimension makes it fairly simple.

Due to roof size the pattern will be that which minimizes cuts--2.5 boards lengths will be an exact fit in one direction (20/8 = 2.5), 4 whole board widths in the other direction. This means you can cover the roof with only 1/2 board left over on both layers. The only only thing you need be concerned with is that boards must always be laid with longest dimension (when there is a longest dimension) across the rafters. Also you will need to calculate for the gaps (for clips) between sheathing boards. This will result in a small amount of rake and eave overlap which can be left as is or trimmed. Here's a pic of layer one, and another of layer two which is just the flip of layer one.

This pattern aligns ALL FOUR horizontals through both layers, weakening the roof and adding leakage-prone direct paths through. That's why post #1 splits the foam (easier than splitting plywood); #35 improves procedure to minimize walking on foam & eases work effort by building solid platform under you as you go upward.

Point taken...but not so much for reasons of "weakening" (since the foam adds no significant structure) or "leakage shorts" (so long as single ply does its job--albeit that aligned horizontal abutting could increase likelihood of film failure) but, rather for purpose of better nailing distribution; always having "virgin" wood as each nail (in above, sheathing layer) is driven. No matter, though, in terms of determining minimum materials purchase (sheet count) which I took as the reason behind Del's question. Given the roof dimension, the calculation will be the same no matter which direction a "layer" is laid: each layer will require 13 sheets with (roughly) one-half sheet total sacrifice.

Realistically, though, there will be (it would seem) a bit more sacrifice of the insulation than the sheathing (plywood or OSB), given that the insulation would probably be cut and fit/affixed between the joists/rafters—the joists/rafters providing insulation between foam boards—here I've not taken into account shrinkage of foam which could cause (probably insignificant) thermal shorts over the long term. In this case the foam insulation direction is dictated by joist/rafter direction...and has no material bearing on sheathing direction...which will need in all cases to traverse across joists/rafters for best structural (and dimensional) integrity. So, in the end, my drawings still provide the "solution" to materials purchase and "offsetting," so long as one pic is rotated 90 degrees.

Thanks for the feedback.

UG

"...given that the insulation would probably be cut and fit/affixed between the joists/rafters..."

But Del has told us that there is an existing deck (of 3/4 inch plywood) that will not be removed, into which he will be screwing the fasteners; he will not access the space between rafters/joists. The foam will be laid upon the deck over a layer of roofing felt paper. See previous posts # 27, 43, 63, & 64 for a quick recap.

Thanks, Ron, for apprising me of the overlooked realities. Given the robustness of Del's existing, panelized sub roof (and as I have attempted to expound below), I am inclined, like #64 (I believe it was), to think that least materials and labor will get the job done adequately.

There was one thing, two actually, in Del's comment that piqued my curiosity: his mention of the felt's needing replacement—as if damaged; and his election to paper underneath (rather than atop) the foam sheets.

I would first ask, in the former instance, how the paper came to be damaged, if not solely by the ravages of time and heat; or by exposure to the elements due to failure of the overlying...whatever. This question ties in to the (answer to) the next question: whether the felt should underlie the foam insulation.

Seems to me that doing this (laying impermeable foam boards over permeable felt) will result in entrapment of condensing moisture (from the house interior) underneath the foam—moisture which, unless allowed to escape, could cause damage to underlying elements—moisture which, in the past, might have led to premature degradation of the felt (say, by rot)! (As I understand, felt is typically applied below the single ply (same as with shingles...where the felt is actually the primary roofing material).

I would think it worthy of consideration...whether some form of ventilation might not be desirable underneath the insulation...that is, if it can be done. I forget the name of it...but it could be a porous blanket something like the black pre-filters surrounding the HEPA cores in air purifiers...anything which would allow air circulation (and vapor escape) underneath the insulation boards. Where this is not practical or do-able (and, yes, this takes us back to exposing joist bays) then the only recourse (the better part of wisdom) might be to forgo the shortcut, temporarily remove the existing roof sheathing, and place the insulation within the joist cavities. Of course I could be missing something and look forward to whatever illumination you and others might provide.

UG

"There was one thing, two actually, in Del's comment that piqued my curiosity: his mention of the felt's needing replacement—as if damaged; and his election to paper underneath (rather than atop) the foam sheets.

I would first ask, in the former instance, how the paper came to be damaged, if not solely by the ravages of time and heat; or by exposure to the elements due to failure of the overlying...whatever. This question ties in to the (answer to) the next question: whether the felt should underlie the foam insulation.

Seems to me that doing this (laying impermeable foam boards over permeable felt) will result in entrapment of condensing moisture (from the house interior) underneath the foam..."

Good and proper questions, I think, UG. And between posts #43 & #63, I'm uncertain of exact conditions. My thought was that the felt was punctured by the fasteners for whatever roofing had been applied and since removed, but that involves assumptions that I cannot justify. I believe Del himself will need to weigh in. The references to the earlier fiasco involving a rainstorm that hit while the house was "naked", and in various forms to current rainy conditions suggests that it is being left in place as a safety measure. If it ought to be removed for ventilation reasons, maybe that can be done piecemeal immediately ahead of the laying down of foam; a tarp (tarpaulin) can be kept at hand if rain threatens, to cover the exposed area temporarily.

I had a 20 x 40 foot tarp on hand when my home's roof was replaced some years ago - and a damned good thing, too! In my case, there was no possibility of leaving the felt: in three applications of shingles, not a square inch of felt was applied. The last layer was of self-sealing shingles - and they had left the release paper in place, so that the adhesive was never exposed to do its job. And those weren't the WORST goofs they'd made . . .

I know it needs doing because the felt is starting to crack in some places (over joints) and I have had to patch it before.
I know the deck is sound because I've removed a section of fascia and seen the boards & joists.

Couple this to the fact that I've done the adjoining flat roof which was built at the same time.

Just to reiterate.

It's a 'flat' roof...with a very slight fall for drainage.

The construction is 3/4" ply over the joists, with 2 layer roofing felt stuck down with bitumen. Over that was about 1/22 of fine ballast (pea shingle, maybe this word 'shingle' caused confusion in a previous post, being misinterpreted a 'shingles')

I will try and incorporate some ventilation, enough to allow air movement but not the entry of Wasps!

My insulation boards didn't arrive yesterday....coming Monday now..

Del

PS...Thanks for all the help/interest.... if ever I need to move Stone henge in I know where to come for advice

Hi, Guest!

Horizontal layout overlaps nicely. Vertical doesn't appear to offer overlapping. Am I missing something?

Mark

Hi, Mark.

I hope the revised reply (to Ron just above) satisfies your question. Let me know if not.

UG

Hi, Guest!

Even rotating the second pic 90^o will leave an aligned edge here and there with the first pic in your original posting.

The coverage is economical, yet leaves occasional 4' lengths of aligned edge.

Mark

Mark,

...leaves occasional 4' lengths of aligned [overlying] edge [gap].

...what precisely constitutes offsetting, some might say: no full-length edge corresponds to an underlying full-length edge(s). But, if this exercise is strictly theoretical (and we presume the insulation to be structural elements; and the cap sheet to be water permeable), then, yes, the most obvious "solution" would be going with a different, diagonal alignment (say, staggering sheets at 1/3 of length and breadth); or even a random alignment; or, my proposal would work provided that wherever there might be overlying abutment gaps (other than square gaps), a foot or so of length was added, or subtracted, from underlying &or overlying sheets. Either of these would, in all likelihood, entail additional waste of material, not to mention significant additional labor and difficulty. Moreover, additional material and labor would be required for the installation of bridging to provide for "odd-edge" nailing—most especially if the roof is to bear the weight of shoes, ponding water, or rooftop fixtures.

As to any trade economy (in the sense of benefit), in the real world, of going to such lengths, I would call it a false economy in that the added structural rigidity, if any, or any degree of supposed "advantage" as regards hermeticity are of such little significance as to hardly justify the additional cost.

If, on the other hand, Del's inquiry was only meant as bait for willing fish (perhaps so he could show off his roof once again?), then my hat goes off to you and others who caught on much quicker than myself.

UG

Hi, Guest!

Check out responses #1, #31, and #53. Fully offset. Full, 1/2, and 1/4 ply sheets. No trick cutting. No aligned edges. Minor scrap.

Mark

Check #1 again, you get cracks over cracks the way he's described it, or, no stagger in the individual layers if you want to avoid cracks over cracks.

Hi, Randall!

I checked. In #1, laying begins with an upper corner. The first insulation piece laid is a 2 X 8, and the first row is 2 X 8's etc. The second row is 4 X 8's. The first ply row over the foam starts with a 4 X 4. the rest pretty well follows automatically. No alignments, only pinholes (except around the perimeter!).

Mark

Next row from left to right starts with 4'x4' followed by 2 4'x8' pieces.

This is his first full width row of insulation, when you lay your first 4X4 ply you get a 2' crack over crack. With the scheme in #1 you have to put up with this or forego staggering in individual layers.

I'm not saying I'd go with my insane scheme of post #53 for practical reasons, but, it does meet the theoretical challenge.

Hi, Randall!

I think we have a differing view of the information given in blog #1. Here's how I see it, with no lineups, only pinholes.

Mark

You may be right that that's what he means, but, it's not what he says:-

Your interpretation is probably a better way of doing it especially as Del's fixing to an existing strong roof, but, you don't get the stagger between the rows of each individual layer.

Well, by golly, Randall!

I'm absolutely certain that he meant the little diag I posted, and just got that second line of insulation accidentally reversed. Because he was doing it in writing rather than from a dwg, and because re-reversing the second line fits right in with his intention.

I knew instantly from his first insulation row and the first 4 X 4 ply piece what he was up to, and I think the other 3 GA contributors saw it too. Hey, Shadetree! Repair that blog, and start that second row of insulation with the 4 X 8 you intended to start it with!

Comments, Shadetree?

Mark

Another perspective: