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Rafter design & load limits

01/30/2008 1:52 AM

Have tried to obtain pro info on my rafter needs but it seems many $$$$$ are reqd to get this info--retired--don't have many $$$$$. Thought someone with structural background would be so gracious to advise .

Have a 16'x36' room with std side wall framing. Want & need atttic space for goodies

have designed rafter/joice sets as follows (wish i could figure how to get dwg here)

ceiling joice-16'- 2x6, rafters-12'-2x6 (8/12 pitch) w/ apx 22" overhang, ridge board is spliced 2x8x38' , rafter are in steel hangers & have ties across the ridge, rafters & joice are tied at the side walls with steel hangers, screwed together & lagged to the the side wall headers' , rafter/joice ties are set in 4' from each side wall giving me an 8' usable attic width. Flooring is 5/8 4x8 osb set perpendicular to the bldg. length. Roofing is 5/8 osb w/ staggered joints & std shingles. Rafter ties in every section set at approx 3' spacing w/ joices tied (screwed) by the flooring.

We have aprox 1000 lb of goodies to put up there, which equates to aprox 3.5 lb/sq'. Although the rafter pitch is substancial, the added weight of the ceiling drywall concerns me when mama starts loading her goodies up there.

Would appreciate an expert/educated opinion!

Al

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#1

Re: rafter design & load limits

01/30/2008 4:12 AM

There are issues of indemnity and liability to consider in making a strategic judgement before proceeding with the work.

Here is a recent example. An extension to a timber-framed dwelling needed detailed calculations to take into account the design and construction of a Mansard roof, which is unusual in this part of the world. These calculations were sub-contracted to a specialist building design company, which produced the calculations necessary to satisfy the Building Regulations team at the local authority. Sure, a fee was paid for the calculation and the report document based upon them. The effect is that the calculations have been carried out by someone who is up-to-speed with current regulations and calculation algorithms, they have been checked by a second person at the firm, a copy is lodged with the Client, a copy is lodged with the local authority, and as a result, the Client is protected. In the unlikely event of a structural issue arising, the Client's insurer has the fullest of access to the information to determine liability and therefore the greatest opportunity of recovering its costs in putting right the defect. Now, would one achieve that level of protection by the comments of an anonymous individual called "Guest", not necessarily in the same country even, who just happened to want to create mischief in a global Engineering forum one day, one wonders?

On balance, it's probably worth making the investment in a professional local assessment!

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#2

Re: rafter design & load limits

01/30/2008 5:17 AM

Hello donzi

Let's hope your room is not built over one of Florida's notorious invisible "sinkholes", as there have been spectacular instances of these opening up below the unsuspecting..

As far as calculations go, and mentioned by PWSlack above, while it may be OK to do your own work, and it may all be quite satisfactory, there are insurance issues, not to mention the legal aspects if something goes wrong during construction, after completion, or in event of storm damage or similar.

While I could knock together something for you quite easily, I'm far away, and shipping costs to Florida are exorbitant and unreliable.

Best to get a local builder to do the job - put it to tender, or ask around your friends for a suitable local constructor, who is familiar with local rules.

Kind Regards....

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#3

Re: rafter design & load limits

01/30/2008 10:02 AM

Probably the cheapest thing to do is go down to your local Building Department (if there is one), if there isn't a local Building Dept. then maybe there is one in a larger city nearby, and inquire about what you plan to do. There is a lot to consider before you add ANY weight to the ceiling joists such as weight of roofing materials (if comp shingles are used how many layers), wind speeds, and snow loads.

Most larger building departments have literature that will help you. I would search query "building department", "forms", (and the local jurisdiction).

In the USA there were commercials for a product that was supposed to be an instant way to hang anything (it would use pressure to pinch two points of the rafter), I couldn't believe it when I saw it, because I knew that they were (eventually) going to be the cause of roof failures. If you live in an established neighborhood and drove around you would probably be able to spot a house or two that have done what you are proposing (without any engineering review) by the way the roof of the houses seem to sag at points.

What ever you decide to do "proceed with caution". You may be able to find a structural engineer that will sign off (or require some fixes) of what you plan to do at a rate that is cheaper than you would have thought.

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#4

Re: rafter design & load limits

01/30/2008 11:28 AM

Check with your local, friendly, roof truss designer/fabricator/supplier. Tell them what you want, and they can design it for you. Of course, they will want you to buy their trusses, which they will stand behind, and it saves you $$$ in on-site labor.

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#5

Re: Rafter design & load limits

01/30/2008 7:51 PM

You live in a hurricane prone area and you guys are having trouble with homeowner's insurance. Don't give the insurance company a solid reason to reject your claims. Go down to the local building inspector's office and ask which pre-made joist companies he is happy with. They should be able to deliver these to you, properly designed and approved, for not much more than you can do yourself. Even if you do decide to do-it-yourself, you will have seen what the pre-made guys came up with. BOCA puts you guys at 90-100 mph wind loading and I'd worry more about that than Mom's knick-knacks.

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#6

Re: Rafter design & load limits

01/30/2008 11:38 PM

You can use Google or Yahoo Search engines and type in Joist and Rafter spans and hit search. there are many more available then the two below.

You will get the Calculator for maxium span at http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

And the sizes calculator at http://www.planhelp.com/public/98.cfm this will tell you the sizes and dimension and type of wood needed.

From your comments, i think this will fill you in for the information you are asking for.

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#7
In reply to #6

Re: Rafter design & load limits

01/31/2008 3:24 AM

Just what i was lookin for==thank you "scissorlady3"==unfortunately a previous reply, using rather eloquent english and choice trade yibberich, to elevate his self esteem, put me in my place and scared off most subsequent replys.

Some others did offer other avenues to answer my question, but have you ever walked into your local handyman outlet and found anyone who knew anything about anything ? Im sure i'm hear bout this!

I have a new found respect for the opposite sex ! ( I hope i'm not making an errorinous assumption about your gender).

thanks again--it woked out fine.

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#12
In reply to #7

Re: Rafter design & load limits

02/01/2008 12:11 PM

HI,

I understand your frustration with trade language. Don't be scared off! What you are looking for is a engineer who is structural and certified, call around, you will be surprised at the pricing. One who has his calculations on a computer system will be much cheaper nearly 1/4 of the long hand approach. I have worked with a few in Berkeley, CA who did his work this way(he was licensed in several states, I don't recall which ones). I was a Project mgr of monumental skylights when I needed his services to get approvals from state and federal agencies. I believe online you will also find the load specifications for the connectors(go to webs for the mfgr of the connectors) that Florida will require due to hurricane imposed loads. One option that might be less expensive is the use of steel to handle long runs as structural members, the back draw to this is attachment of wood members for fire blocking and roofing materials. These steel members are much smaller than the wood dimensions. (sometimes this is an advantage), check with building dept for codes, some states will require fastening to a structural (rebar reinforced) pier and depth is usually 24" but can more or less depending on code. Most lumber yards just sell lumber and won't take liability for any structural loads. If the home has been built in the last 5 years then the calculations for the building will be at the building dept, which you can obtain. You will need that as you are going up another story(higher than existing roof ridge) in the building and they want to know if the building can withstand additional loads, if older you might have a huge expense as the above structural piers would have to be added at grade level. I hope this helps a little in your search.

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#10
In reply to #6

Re: Rafter design & load limits

01/31/2008 3:13 PM

I can only ask the fine lady if she knows where I can find this sort of help using steel as a material to work with? Many thanks before hand.

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#8

Re: Rafter design & load limits

01/31/2008 8:09 AM

hello Donzi.

You're in a situation most home-owners are... looking for a bit more storage... and looking at the attic.

I have to say you're heading in the wrong direction with your lumber sizing.

Your profile says you're in Northeast Florida, as it happens, I'm a Florida architect, so I can give you some insight into our unique wind loads.

Let's start with the roof: You've tentatively selected 2x6 with an 8/12 pitch. Wind loads in Florida do not favor steep roofs. You'll get more over-turning moment instead of uplift, which means your hurricane ties between roof joists and top plates of wall will have to be stiffer than normal. (when you select your hurrican ties, be sure to get the ones that extend down to make an attachement to the stud as well as the top plate... we've seen a number of top plates rotate up). You didn't indicate what spacing you intended to use for roof joists, so let's make the assumption that you will go with a standard 24 inch center to center of rafter... This looks like it should be a 2x12 if we're using the lesser modulus of elasticity for common lumber of 1.2 million psi for the span you've indicated. You'll want each rafter set to have a 2x12 "collar tie" to avoid spreading.

Ceiling joists: I know you've indicated that your anticipated load is to be 3 1/2 lb/sq ft... but you're going to be up there walking around aren't you? That sounds like a point load... a live load, if you will, (pun intended). Again, you didn't indicate what spacing, so we have to make another assumption - if we place in intermediate between the roof rafters with a rim board connection at the end (between rafters) you can get your attic floor joist spacing to 12 inches on center, which would get your joists to 2x10.

Your selection of OSB is good... the caveat there is to be sure to consult with your local building department regarding the recommended nailing spacing to resist uplift... we spend more time trying to keep things from blowing away around here rather than from falling down.

We won't even go into how you're going to ferry things up into the attic... you'll be visiting your local building department frequently would be my guess anyway.

keep us posted as to your progress... and God Bless you.

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#11
In reply to #8

Re: Rafter design & load limits

02/01/2008 11:05 AM

It's good to talk to someone who's willing to share their expertise without sniping at another ones' ignorance--A retired engineer of 40 yrs myself. I know your numbers don't lie but I have spent 8 mos' detailing every phase of this project from slab to rafter--In fact I have another "twin" bldg of lesser length which withstood the winds we experienced and you probably remember well--In fact we experienced a tornado spawned by "fancis" ,twisted a young live oak, 15' from the side of the bldg, around itself, went over the bldg & twisted several large oaks around themselves before tearing up the neighbors property without so much as a lifted shingle. Since wind loads were a major concern for me I have enough "steel" in this bldg to make two cars--If the roof goes it will have to take the mono slab .

If you would be so good to take a minute to review this rather busy dwg I threw together ( lost all original dwgs in original computer--man says he can't find appropiiate equip to restore) , I would certainly appreciate it.

PS-original bldg has over 2000# of junk --when mama goes up there-(170#) nothing moves. (I didn't tell you that).

thanks--DONZI---sorry/tried to import dwg--i give up--thanks anyway!

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#18
In reply to #8

Re: Rafter design & load limits

02/04/2008 8:11 PM

HELLO ARCHITECT3451

TRIED TO FORWARD DWG THRU CR4 AND UNFORTUNATELY THEY DON'T FORWARD DWGS/ATTACHMENTS VIA THEIR NETWORK--SUGGESTED I SECURE YOUR EMAIL ADD -GIVE YOU MINE-THAT WAY I COULD FWD DWG VIA EMAIL W/ATTACH FOR YOUR REVIEW , IF YOU EVEN HAVE THE TIME OR INTREST---

MINE IS farrell753033@bellsouth.net

thanks again for comments! DONZI

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#9

Re: Rafter design & load limits

01/31/2008 11:43 AM

Dear Donzi,

No matter how detailed your explanations are, there is always room for misunderstanding / misinterpretation.

It would be great, if you draw a Cross-Section for your Joist & Rafters typical arrangement with dimensions and sizes of the structural members, typical details (support on the head, connections and the ridge joint) and the plan of the attic room with overlayed rafters and spacing dimensions center-to-center.

Than, you could scan your drawings and attach to the thread (or e-mail). By the way, these drawings might help you to get organized for ordering materials and further construction.

If your structural system is relatively regular, we could limit ourselves with the design of one typical Rafter & Joist Frame.

Than, the tributary area width should be determined as the maximal spacing whereas Area Load magnitudes for the joist and for the rafters shall be the result of the worst possible load combination:

Loads Combination = Factor1 * Dead Load + Factor2 * Live Load + Factor3*Climatic Loads

where:

1. Dead Load is the self-weight of structures and weight of all flooring, roofing, ceiling, drywall and other non-removable materials, fixtures and attachments,

2. Live Load is 'Your Goodies', all your family and guests gathered in the attic and more goodies you might eventually put in the attic,

3. Climatic Load is Wind and Show on on your roof,

4. Factors represent the degree of uncertainty and/or probability.

5. All these values, except Dead Load (calculate it yourself), should be taken from the last editions of the National Building Code (for Canada) or the Universal Building Code (for the US)

By the way, the Building Code should also tell you if have seismic in your region (look in the appendixes). If you do, you should consider another Load Combination and find out what governs for lateral: Wind or the Static Equivalent Force from Seismic.

Normally, a Professional Structural Engineer performs these kind of calculations and takes legal responsibility covered by his/her professional insurance. If you don't want to pay for engineering services, you have plenty of time and you think you'd be able to learn these specifics of structural engineering, I wish you good luck.

Cheers, Len

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#14
In reply to #9

Re: Rafter design & load limits

02/02/2008 1:24 AM

DEAR BOGDANOVL

FIRST OF ALL, BEING A RETIRED ENGINEER OF 40 YRS I NEVER DO ANYTHING WITHOUT EXTENSIVE RESEARCH & DOCUMENTATION-I DRIVE OTHERS CRAZY.

THAT ASIDE, ALL MY DRAWINGS (WHICH DOCUMENT EVERY PHASE/TOPIC OF CONSTRUCTION), WERE LOST IN A FAILING COMPUTER (NO BACKUP) AND IM STILL WAITING FOR RECOVERY FROM AN EXPERT. I DID A QUICK DUPLICATE OF THE PERTINENT STRUCTURAL MEMBERS AND ATTEMPTED FOWARDING TO A ARITECTURAL RESPONDER IN MY AREA--DON'T KNOW IF I WAS SUCCESSFUL IN THE TRANSMISSION OF THIS DOCUMENT.

I GET MY MATERIAL LISTS FROM THESE DOCUMENTS AND SUBMIT SAME WITH ITEM REQUIREMENTS ( GRADE, ETC) TO AT LEAST 3 BIDDERS--DONT ALWAYS ACCEPT LOW BIT BUT MORE ON THEIR RESPONSE AND THE WAY THEY RUN THEIR BUSINESS.

YOU'RE COMMENTS ARE WELL TAKEN AND SINCERLY APPRECIATED--YOU SOUND LIKE THE KIND OF GENT I COULD HAVE AN INTRESTING CONVERSATION WITH.

PS--YOU MAY BE ABLE TO SECURE THAT DWG FROM CR4 (IF IT WENT THROUGH)----IF NOT, AND MY FIRST ATTEMPT WAS SUCCESSFUL, I WOULD FORWARD THE SAME DOCUMENT TO YOU--IF YOU WOULD BE INTERESTED.

THANKS AGAIN--DONZI

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#13

Re: Rafter design & load limits

02/01/2008 1:38 PM

Hi Donzi

What you describe sounds like a "Queen Post Truss". Call your local Truss Fabricator and tell them what your needs are and how your going to use the space. They will be able to design, build, deliver and raise the truss cheaper than you could do it yourself. Do not try to make trusses yourself. Not all dimensional lumber has the same load capacities, such as yellow pine, Douglas fir, so called white wood ( mixture of different types of pine, etc). Trusses made in a professional truss shop are engineered using the proper types of wood and are fasten together using plates pressed together under pressure (stronger than you may be able to make). They have all the proper calculations needed to do the job right. Still have your local building inspector approve the addition. Just make sure the existing building will take the additional load. Follow local building codes when installing or have a reputable contractor do it for you. Working on roofs can be very dangerous and is not a typical DIY type of job. Putting this part of the job in the hands of a professional is the safest and least costly way of doing it. It may seem like a small job, but one slip can be disastrous.

Good luck and be safe. BigEd, Professional Woodworker

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#15
In reply to #13

Re: Rafter design & load limits

02/02/2008 2:11 AM

BIGEDM

APPRECIATE THE INFO--AT 70+ YOU WONT FIND ME WALKING THE REFTERS--I'LL SEND MAMA UP. YEA-FAT CHANCE.

YOU'RE RIGHT- DOES RESEMBLE A "QUEEN POST TRUSS" AND I'M HAVING TROUBLE ACCEPTING THE EXTREME LIMITATIONS, PLACED ON MY SIZING, SINCE THE JOIST (16'-2X6) WITH A TRUSS SUPPORT SILIMAR TO A "QUEEN POST TRUSS", COMBINED WITH THE EXTREME RAFTER PITCH (8/12) , REDUCES THE USABLE ATTIC AREA TO 8' WHICH IS FURTHER STIFFENED BY 5/8-4X8 OSB LAID/SCREWED PERPENDICULAL TO THE LENGTH OF THE BUILDING, WITH JOIST TIES ACROSS THE ENTIRE LENGTH.

THE ONLY "DEAD" LOAD WOULD BE THE OSB FLOORING AND THE DRYWALL SHEATING BELOW WHICH AMOUNTS TO APPROX 2.5#/sq' .

THERE'S APPROX 3400# OF DEAD LOAD ON THE ROOF/RAFTER STRUCTURE, AND WIND LOAD--(NO SNOW LOAD) WHICH COMPUTES TO 4.5#/SQ'. + WIND LOAD .WITH THE EXTREME PITCH VERY LITTLE STRESS IS PLACED ON THE RAFTER MEMBERS --

ANYWAY, YOUR POINTS ARE WELL RECEIVED & I THANK YOU FOR YOUR COMMENTS.

DONZI

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#16
In reply to #15

Re: Rafter design & load limits

02/02/2008 5:03 PM

Hi Donzi

In my younger days I built houses, and this may be similar to what you are trying to do. We called this type of house a story and a half. On the second floor you had rafters that would go up similar to an A frame. Collar ties were placed near the top of the rafters to make up the ceiling. Short stud walls or as we called them Knee walls were placed near the bottom ends of the rafters to make up the side walls. This was all done with conventional framing methods using 2x6's for rafters and 2x4's for knee walls. As long as you use a sturdy floor joist 2x10 or 2x12 placed on the appropriate centers (look up the the load capacities for the type of lumber you'll be using) there should be no problem. After 35 years these houses are still standing, amazing. Since the rafters birds mouth cut will be sitting on the floor joist the additional height of the floor will not take away your ceiling height. As always check the local codes before doing the job.

If you need even more room, after you build your second floor deck, you could use a mansard styled roof, if it fits into your designs. This would give you a lot more floor and ceiling space for storage. Just another option.

Good luck, BigEd

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#17
In reply to #16

Re: Rafter design & load limits

02/04/2008 5:26 PM

hi BigEd

Seen many of these in my days -70+-in fact lived in some up north--popular in the bungalo style homes.

that's exactly what i did--selected a steep pitch,( to reduce the horizontal dead load on the rafter elements), used collar ties and steel rafter ties (mainly because i don't trust those steel rafter saddles by themselves) and installed rafter/joist ties you refer to as "knee walls"--these ties were primarly to assist in the support of the floor/ceiling joists which had to span 16'--the ties are placed 4' in on both side walls leaving a usable storage area of 8' the length of the bldg--because the rafter/joist elements are mechanically tied together at their ends and then , together, tied mechanically to the outer walls the only load on this section is the dead weight of the 4x8 sheets of 5/8"osb (screwed to the floor joists),( actual storage area) and the drywall below--amounts to 3#/sq', The rafter elements "bird mouth" (are resting entirely on the outer wall structure and not on the joists-- as i explained to another--have a "twin" bldg to this plan --been there 12yrs--survived 3 hurricanes a tornado which twisted trees around themselves within 15' of both sides of the structure without so much as a turned up shingle while the surrounding structures (neighbors) were hit pretty hard. Also , being retired engineer myself, visited some of the sites nearby to see how structures (carports,out-bldg,roof overs, trailors and other homes) all with the stamps of approval for our areas hurricane codes fared--not too good--i wonder! maybe just lucky--for once.

I guess if i'm overruled on the span distance 16' and the selection of joist sizing i can always "stiffen" these joists by laminating ply to their sides and watchin what ma puts up there.

Again-thanks for the info ---DONZI

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Architect3451 (1); BigEdm (2); Bill (1); BogdanovL (1); donzi (6); MCofSacramentoCounty (1); PWSlack (1); sc6chuck9 (1); scissorlady3 (2); Sparkstation (1); TVP45 (1)

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