How Do You Prescriptively Size A Hip Rafter on Hip Style Roof?

[wwhitney]

Given that Steveray was discussing insulation not fitting within the rafter cavity, I figured he was speaking directly to either no-attic or conditioned attic spaces.

The accompanying diagram he posted includes ceiling joists, and the thread has been about conventionally framed roofs, so I was assuming ceiling joists are present.

[Aside: Table N1102.1.3 refers to "ceilings" and doesn't mention attics. Which at first glance would suggest that prescriptively you can not choose to insulate the roof deck in lieu of the ceiling joists. But the first sentence of N1102.2.1 refers to insulation "in the ceiling or attic"--does that passing reference give permission to insulate the roof deck instead of the ceiling joists, or would such a choice require going to the "Total UA" alternative? Since the roof deck area is bigger, the heat loss will be greater if the roof deck is insulated to the level called out for the ceiling joists, so requiring the Total UA alternative for a conditioned attic makes sense to me.]

Where I disagree with you is in that the requirement is R-49 over 100% including at wall top plates. It does not say anything about it being ok to "squish" the insulation at the eaves.

Not following you. For Zone 5A, Table N1102.2.3 calls out R-60 ceiling insulation. Then N1102.2.1 amends that by saying if you provide R-49 uncompressed over the full ceiling including wall plates, that suffices instead of R-60. The clear implication is that the R-60 requirement allows squishing at the wall plates.

Cheers, Wayne

 

[wwhitney]

The Canadian code requires the hip to be one size larger than the rafters, so for 2x6 rafters the hip would be a 2x8.

I believe that is what the R802.4.3 requirement works out to for most cases. There may be unusual cases such as very steep roofs where it ends up being more that just one size up, I haven't checked.

Cheers, Wayne

 

[mark handler]

STYROFOAM extruded polystyrene insulation is rated at an R-value of 5.0 per inch at 75°F.

 

[steveray]

Not trying to hijack guys....

 

[jar546]

Not trying to hijack guys....

Two words.

Thread Drift

 

[Energystar]

The load on a hip or valley is linear. It is a simple tapered load as shown due to the tapered tributary areas.


The reactions are about 1/3 at the bottom of a hip and 2/3 at the top. A valley is the reverse. There is a rule of thumb that says to use one size larger than the rafters but double it. So, 2x6 rafters would need a double 2x8 hip. (I hate rules of thumb). Even if one confines the length of the hip to readily available lengths, say 16', the load can still be exceeded. This example is a 6/12 pitch roof with max rafter span of 10'-5". The hip is just under 16' and it just fails with a double 2x8 according to Forte. (20 lb. live/12 lb. dead)

Even so, this is a grossly simplified look at the problem. There is a lot more going on here than just a simple tapered load on an angled beam. It would take a solid modeling program and FEA to figure it out, but I would assume it would perform better than the software says.

 

[Energystar]

Here is the chart from the 2018 WFCM.