Residential Roof Framing and Wind Uplift Ties per NBCC part 9

[pbc825]

Hello all,

Our firm doesn't primarily practice in residential projects, but we are working in the loss/insurance/lawyer industry more and more. We were hired by a single family dwelling home-owner to perform a structural assessment for an addition that was nearly complete. They were having "trouble" with the contractor they had hired for the work, and had since hired a new contractor. I found a few deficiencies, but what we're working through now is the roof truss framing. The new trusses were piled on top of the old roof framing and the new trusses were not tied down for wind loads. Our analysis suggests that the dead weight of the roof does not counter the wind uplift in the 0.9D+1.4W load case. During a meeting with someone who I believe to be more familiar with residential portion of the National Building Code of Canada, 2010 (Part 9) mentioned wind uplift ties are not required as per section 9. I read through section 9 in search of this clause and did not find it. Can anyone help with this?

Also, if there is an exception for ties, why is that? My analysis suggests ties would be put to good use, so why would NBC blindly exclude them?

Thanks in advance.

 

[jayrod12]

Technically Part 9 specifically clause 9.23.3.4 specifies that roof rafters and trusses can be connected to the plates with 3- 3 1/2" toe nails. This is ludicrous of course and no engineer in their right mind allows it normally. Check out 9.23.3.4.3) however as if q>=0.8kPa then they must use connectors.

 

[pbc825]

Thanks for your help Jayrod.

There are a number of ludicrous things happening in this project. One of the most ludicrous though is that my analysis clearly indicates ties are required and because clause 9.23.3.4 does NOT require them, the other party is off the hook for installation. Good thing we're not lawyers

 

[TLHS]

It's not that they're off the hook. It sounds like they're legitimately not code required. You can't expect a residential contractor to be doing a Part 4 analysis, or hold them to the results of one. They're doing what the code tells them to do.

Roofs don't seem to be flying off regularly, so for whatever reason, the Part 9 construction requirements must be conservative to a reasonable level within their limited scope.