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Author Topic: Roof truss spacing for small shed  (Read 391 times)

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Offline lepton

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Roof truss spacing for small shed
« on: September 28, 2020, 10:28:09 PM »

I’m trying to build a simple shed for my pool equipment and childrens’ bikes. I’d like to use post-frame construction. The floor plan will be no bigger than 6’ x 18’, squeaking me under the 108sf limit that I can do without a permit.

I’m thinking of putting down 8 posts, 4 on each side, so 6’ spacing. Ideally, I’d have trusses only on the posts. I’m looking at Don P’s secret truss calculator to size things up (Untitled). Let’s assume I’m using the cheapest dimensional lumber I can get, so probably SS SPF.

The live load in my area is just under 35 psf, and let’s put 10psf dead load. The spacing is 72 inches between rafters if they only sit on posts, and the horizontal span is also 72 inches. For now, assume the pitch is 4-12.

The calculator says that 2x8s (1.5x7.25) would do. But can I trust the calculator in this situation? That truss is going to be solid wood! From the edge to the center of the roof is 3’, so at 4-12 pitch that’s just a 1’ rise, while the rafter takes up just under 8.5” — so the triangular gap between the two top chords is only a hair over 3.5”. I bet the secret calculator fails in this case. In some sense here the whole chord is “end” and there’s very little “middle.” Don what do you think?

Apparently a 3x5.5 also satisfies the calculator, but the same concerns persist, and two 2x6s are not truly the same as a 3x5.5 either.

Should I place beams atop the posts and increase the number of rafters? Is there any other reasonable solution here?

Online Don P

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Re: Roof truss spacing for small shed
« Reply #1 on: September 29, 2020, 03:21:45 PM »
Hi lepton, judging from your knowledge of obscure links, welcome from the shadows  ;D
My first question is what is spanning the 6' between rafters?
For rafter span in the calc plug in 36", or really ~ the plate thickness less than 36".
Use #1/2 SPF, SS is the highest grade, #1/2 is typical off the shelf stuff.
I'm getting a pass with 2x4 material using 34.5 as the span.

 

 
A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart

Offline lepton

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Re: Roof truss spacing for small shed
« Reply #2 on: September 29, 2020, 10:09:24 PM »
Aha! Thanks, I guess I didn't read carefully enough to understand the meaning of rafter span. Very nice. Your picture is also very helpful in visualizing what the end result could look like! :laugh:

Between the rafters I thought I'd attach purlins. I was going to hang them on edge between the rafters. Not sure yet how many or what their spacing would be. Your simple beam calculator should help me there. Then I'll put maybe 1/2" plywood sheathing and shingle it.

I didn't mention the nails; that confused me too. If I select 16d common nails, it says I need more than 11 nails at the end of the rafter? That seems like a lot.

Online Don P

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Re: Roof truss spacing for small shed
« Reply #3 on: September 29, 2020, 10:51:55 PM »
That calc came out of a class at VA Tech taught by Dr Woeste. I just wrote his equations into a calc. His rafter to ceiling joist nail count is higher than I would use because he is resisting the axial load coming down the rafter, I can see that in a typical truss type of cut at the heel. I generally use a birdsmouth catching the axial load into the plate with a ceiling joist nailed nailed alongside. Then IMO the tension in the ceiling joist is the force to be resisted and you can use the nail count in the box labelled ceiling joist lap, yeah, it is more than anyone ever imagines. And usually in construction it isn't the beams that fail, its the connections. I can about guarantee you've never driven a 16 common (.162"), what we all call a 16 penny nail is a sinker(.148"), in a gun I shoot a 16 box nail(.131"). His counts are correct, those are the connection strengths a factory truss would be shooting for. Don't destroy the wood but do connect well. Of equal importance is connecting the roof to the posts to keep the wind from taking it off.
A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart

Offline lepton

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Re: Roof truss spacing for small shed
« Reply #4 on: October 02, 2020, 09:49:17 PM »
Great; this seems to be coming along. I thought the birds-mouth was standard anyway, and it helps to align the trusses on the posts.

I used the uniformly loaded simple beam calculator to estimate the number of purlins. Remember the trusses are basically 6' apart. Since I plan to put the purlins upright, I'm using 1.5" as the width and 3.5" for the depth. For beams & strings, the max fiberstress is just 575 PSI for No. 2 SPF, the weakest (safest) value. With these inputs, the max load is about 195lbs.

If we consider half the roof between two trusses, that's an area of 3' x 6'. I'm using (35 + 10)psf loading, so that's 810lbs. Four purlins is almost enough; five passes the test. Is this a reasonable way to determine the number of purlins? It's a fairly close spacing over the 3'; basically 8".

Next to size the posts, I looked at the column capacity calculator. I think the middle side posts each carry 810lbs, and the corner posts slightly less. The column length is 96", and I set the depth d=3.5" and the width d2=1.5". So the narrow side faces the outside of the structure. I set E=1(million PSI) as per the chart attached to the beam calculator, but for the maximum allowable compression parallel to the grain Fc I had to do a bit of work. I dug up the AWC 2018 chart somewhere online, and it seems like this value should be 1150 PSI for No 1/2 SPF. And that passes!

So my posts can just be 2x4's set on edge to the walls. I think that's reasonable for sheathed walls, but I was planning to have a fairly open long side with barn-style doors for easy access. I'm concerned that a good kick could crack one of those posts and take out the structure. Perhaps I'll nail a couple together or use 4x4 posts --- something that can take a bit of day-to-day beating.

Online Don P

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Re: Roof truss spacing for small shed
« Reply #5 on: October 02, 2020, 10:07:04 PM »
For the purlins, it sounds like you are using the design values for heavy timber, since they are dimensional lumber 2-4" thick use this calc;
https://forestryforum.com/members/donp/ddsimplebeam.html

For columns you have the correct Fc value but there is one other rule in there  on slenderness and height and your gut is right. The length to depth ratio should not exceed 50, so a 1.5" thick dimension plays out at 75" tall maximum unbraced height before buckling becomes a problem. Your 4x4 post sounds better, like you said in the real world they get bumped too.
A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart


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