Info On Size and Spacing For Perlins For Roof and New Building

Welcome to CR4. Your Do-It-Yourself attitude shows that you are one of us.

I don't know if anyone with the proper skill set will add to this thread. I do have one comment to make. Whenever you do anything safety related you need to be very careful about using comments from internet postings where the people don't even have real names. I don't think that any "CR4 old timer" would give you bad information related to safety. Postings from CR4 Guests usually but not always have good intentions.

You might want to add a little more information to your posting in case someone with knowledge wants to help. Are you in the US? If so, adding your state would provide some idea of the snow loading you might have. Do you have building codes and building inspectors? I was surprised last year to learn that there are lots of areas in the US that have no building codes and the only inspector is the tax man.

Last cheap comment, when in doubt do it safer. You or a member of your family will be the ones that the roof falls on if something goes wrong.

Bruce

Thanks for your response. I am located in Weld County in Colorado about 40 miles east of the front range of the Rockies. We do get heavy snows occasionally.

I will be using 29 ga. corrugated metal roofing. I presume 22" or 20" spacing would be advisable. What do you suggest?

Thanks again.

wrancher

2x4's on 24" centers is lighter than any of the load tables go in Architectural Graphic Standards, so this doesn't look enough. A fairly quick consultation with a structural engineer would give you good information, taking into account local codes and conditions. Loads may include snow and high wind loads, depending on your locale.

Thanks for your response. I am located in Weld County of Colorado just east of the front range of the Rocky Mountains. We do get heavy snow occasionally as well as wind.

Does the Structural Engineer Standards manual show a spacing of 22" or 20" for 2' x 4's and what spacing would you recommend for my area.

Thanks again,

wrancher

In about 1998, in Yakima, WA, USA, there was a snow accumulation of a foot or so, followed by rainfall that that soaked into the snow. This created a snow load of ~60psf on roofs that had been designed for 40psf. Many collapsed.

The lightest roof load given in my tables is 20psf. The required design in your region may be 30psf or 40psf, which is as high as my table goes, but even then might not be enough.

The allowable spans for lumber depend on:
1. The extreme fiber tensile strength f of the wood, which varies from 900 to 2050psi for different species.
2. The modulus of elasticity E, which varies from 1,000,000 to 1,760,000.
3. The depth (primarily) and thickness of the purlins.
4. The spacing between purlins.

Item (1) affects the breaking strength of the wood from bending.
Item (2) affects the deflection (sag), and tends to be the more stringent limitation.

In my table, the only entry allowing 2x4's to span 8ft is for the highest strength woods on 12" centers for a 20psf live load.

Going to 2x6's gains a lot. My table doesn't include 24" centers, but even the weaker wood 2x6's on 16" centers can span 9' on a 30psf load. So this is at least getting in the ball park.

Tentatlvely, a select structural grade of Douglas fir or Western hemlock 2x6's on 24" centers might do it for 30psf; but I would verify it with a structural engineer. For 40psf, that's probably still be a bit shy.

This is a lay, not professional, estimate. I don't have any corrugated sheet metal info, so no comment on that part.

You got good advice on the building. Additionally, USDA some 60-80 years ago had an extensive program, and published it. I believe, you can gain a lot from it.

As far as tornado shelter concerns, here is my commentary. Tornadoes are frequently accompanied by heavy downpours. Basement arrangement gives you wet feet or drown you. You can build one with its own steel reinforced foundation into the earth. Then proceed with one or two parallel course of cement blocks, plus heavy duty rebar vertically and horizontally every single course. Finished, fill it up with good concrete. Heavy duty steel door with locking bars openable inside or out - think of the work of the rescuers.The roof has to be tied in mesh to the rebar allaround. BUT, it cannot be airtight. Air pressure differential between normal air and vacuum is 1 ton per square meter (or roughly square yard), and will explode your shelter, if allowed to work on it. Smaller is sturdier. For guidance: I wisited Homestead in Florida a year after that hurricane with nearly tornado speed winds. Normal construction was mostly destroyed. Prestressed prefab concrete parking garage and office building did not suffer more, than some blown out windows, not even that very much.

If you are in CO. Personally, I would use 2X10 16"OC for the roof. End of story.

If it's your money, and you have plenty of it, go right ahead! I suspect this is as much overkill as the OP's first choice is underkill. But I know of a guy who would probably use 2x12's!

I have built many structures like this. You told what the space between the rafters are. But what about the span of the roof? I.E. the supports for the rafters and "lower chord of the refters." Will they rest on beams or walls? What will the sheathing be?

From what I understand though, I would make my spacing of rafters closer and space the purlin more depending on the sheathing... And where snow is a concern, I would increase the pitch also...

A pair of 2x4's sitting edge-ways over an 8 foot span with plywood over would be adequate for access use.

Is this like what your building?

Thanks for your reply. The span of the roof is 24 feet or total span including the pitch is 25 1/2 feet. I have steel I beams spaced 8' apart and the rafters are already delivered. They will rest on the I beams. I am already committed to the 4-12 pitch.

I will be using 29 gauge steel ribbed panel roofing. Panels are 38" with 36" coverage. The panels will be attached directly to the perlins.

I have a similar, but slightly smaller, shop building that was built in 1986, which has 2x4 perlins with 24" on center spacing, but the span is 5 or 6 feet less, so I would like to go with a reduced spacing or change to 2x6's.

My rafters on each end are two ply: Bottom chords are 2x10 and top chords are 2x8's. Other inner rafters are one ply with same size chords. I hope that is enough info.

Thanks again

wrancher