HELP! Birdsmouths on treated wood for run rafters?


7 Years
Apr 14, 2012
Palmer, Alaska
We're constructing a run that is 24' long by 10' wide. We're building it out of treated wood, and would like to cut the stuff as LITTLE as possible. Do we have to cut birdsmouths out of the 2x4x12s that will be spanning the 10' side? Can we get away with rafter/hurricane brackets? The run height will be 8' on one side and 5' on the other, there will be a rafter ever 2' and perpendicular 2x3s running across them also at 2' spacing. It will be covered in clear Suntuf pannels (so the only real weight will be snow weight).

I think the hurricane ties will be sufficient. Cutting birdsmouths out of 2x4's will reduce the strength of the rafters (assuming you will have an overhang for eves) and have minimal structural benefit. I think for a 10' span, you might consider using 2x6's rather than 2x4's though. I'm not familiar with snow conditions around Palmer AK, but here in Michigan, if you have snow on the roof and then get a good rainfall, I doubt that 2x4 rafters would take the weight. I have been accused of overbuilding, so hopefully others will chime in.

Thanks for your reply! We don't often get rain in the winter, but we do get a decent amount of snow. I do have a roof shovel and put this roof at a pretty good pitch, the metal roof on our house does a pretty good job of having the snow just slide right off, I was hoping that would sort of be the deal with this one too.

I was hoping with the 2' spacing of the 2x4s we would distribute weight well enough, with our structure that was 13 2x4x12s (one foot overhang in each direction) and just the lumber for this project (not to mention the roofing I decided on) has put me so completely overbudget I don't want to think about it. The man at Lowes (we talked for a long while, and he was pretty knowledgeable about structures it seemed) thought 2x4s would be okay, but sometimes when people hear "chicken structure" they just think "doesn't matter" even though he was familiar with the type and brand of roof I was putting on it even though they don't currently carry it.
I understand budget constraints, but just for fun I ran the numbers through a rafter calculator.

I used these parameters:
Spacing: 24"
Wood Species: spruce/pine/fir
Deflection Limit: L/180 (the lower the number, the more deflection. Higher numbers, L/360, L/480, are used when less deflection is required, like when ceilings are plastered to prevent cracking etc)
Lumber Grade: No. 1 (best case, but I doubt your treated 2x4's will all be No.1)
Snow Load: 20 lbs/sq ft. (just a guess.....check locally)

Max span for these parameters is 7' 8".
Going to 2x4's on 16" centers increases span to 9' 5".
Using 2x6's on 24" centers increases span to 11' 3".

Not trying to spend your money for you....just trying to take the guess work out of it.

Wow, thank you so much for running those numbers. The information (and source for reference) are VERY helpful!! I think we will have another trip into town tomorrow, better to do things right than to try to get done with what we have, this is costing enough money as is no use cutting corners here.

It doesn't matter that we're running purlins perpendicularly across the the rafters also at 2' intervals, does it? I thought that would add to the stability... I must be overestimating their importance.
DH just took the 2x4x12s for the rafters back, we will be building with 2x6s. Thank you SO much Paul for your help, and I'm definitely bookmarking that site!
Correct....purlin spacing has minimal effect on rafter requirements. It's more to do with how rigid your roofing material is. Just curious though, why are you using treated rafters? Are the purlins treated also?

I thought treated rafters were a good idea ... Is it not common to use them? The salesman at Lowes said he would use treated if it was just hardware cloth on the walls. The purlins will not be treated, but I was going to seal the ends of them. Is it a mistake to use treated rafters or just unnecessary?
I used 2X6's, untreated with hurricane straps, not birdmouthed. Treated is tricky because of the corrosion factor on fasteners (nails, screws) and unneccessary as the joists will not be exposed to the elements. Once you have everything tied together (i.e. joists, roofing deck, shingles or whatever) I do not think any amount of snow that we get in the lower 48 can/will collapse a chicken coop/run, unless of course the entire structure is not built properly. I did put mine 16" on center. I am not a fan of the 24" on center thing. Just my .02.


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