Residential Roof Nailing Roof

[karimom]

We are having some debates at the office regarding the roof rafter and ceiling joist nailing and would like some outside advise.

Essentially, we have a 35' wide house framed to a 1:3 pitch. When calculating the tension reactions through the ceiling joists/ties to prevent wall thrust, we are getting around 14KN ~ 3.1 kips - meaning we would need ~30 nails. However, in the Ontario Building code, the code says we need 3 nails with a length of 3". Either we are not reading the OBC properly or they are assuming that the rest of the house takes some of the thrust, either way it would be good to get some clarification and outside opinions. I want to believe the code, but it's hard when running the calc gives you such a high number.

 

[jayrod12]

Is it ridge beamed for the rafters?

 

[karimom]

No ridge beam, only rafters together with a ridge board

 

[TehMightyEngineer]

Something doesn't sound right; you shouldn't need 30 nails. You forgot option #3 which I would surmise is most likely; there's an error somewhere in your thrust calculations. What's the spacing of your roof rafters? 24" oc?

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[XR250]

I get about 1.4k using 30 psf total @ 16" o.c., so 3.1 k sounds reasonable for 24" o.c. and higher snow load. I would tighten up the rafter spacing.

 

[karimom]

Please remember that we are in northern canada, our snow loads are 40 psf. This is for an existing structure. 30 nails doesn't sound right, which is the reason for the comparison to the code.

 

[Tempgolfer]

I never had to calculate this but can you rely on the roof sheathing to transfer the lateral thrust load into the shear walls?

 

[bob33]

This is covered in Table 9.23.13.8 of current OBC (available on line at no cost). The table requires 8-3" nails with rafters and ceiling joists at 16" centers and a maximum span of 9.8 m. Your span is greater than that so the connection has to be engineered. Good luck.