Design for roof slopes greater than 12/12

[TimeToPlay]

Issue#1
I have to design a roof with not only uneven slope but also the slopes are very steep. I have looked TJI, PSL, etc. products and the manufactuer does not have product that can support such steep slopes. Does anyone know if there are any wood products and connections can support this design?

Issue#2
The roof design will have a ridge beam that has a max span of 30 ft with intermediate wall supports. Ideally, the rafter will be supported by the ridge beam and the wall. Because the roofs are steep and uneven, I suspect that there will be
thrust loads on the wall. Any thoughts on this?

I would like to appreciate any help on this matter. Thanks!

 

[1503-44]

The steeper the slope, thrust loads from dead and live roof loads are smaller than in flatter slopes. The disadvantage is the height dramatically increases wind loads, which are transferred to the top of the wall. It is usually advantageous to tie the walls together to share load, or at least the sloped beams about 1/2 way up. Lower generally preferable.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[XR250]

If you have an appropriately sized and supported structural ridge beam, there is no thrust on the walls caused by gravity loads.
Due to the extremely steep pitch though I could see some snap through buckling of the wall at the hinge happening from wind loads but that is very unlikely.
I would be framing the rafters out of dimensional lumber.

 

[jhnblgr]

Have to design it like a wall and beam. Check bending, axial and shear.

 

[XR250]

Have to design it like a wall and beam. Check bending, axial and shear.

Can you elaborate? I do not understand your suggestion.

 

[1503-44]

No vertical thrust on walls assumes no significant deflection and spreading of beams at the wall anchors. A cross tie will help that case.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Goatenstein]

How would one calculate the wind load on rafters this steep? ASCE7 only addresses roofs up to 45 degrees.

 

[XR250]

I would use wall pressures.

 

[jhnblgr]

Look up combined bending and axial loads in NDS. The assumption of rafters is axial loads are minimal. But this is a steep sloped roof and this assumption doesn’t hold true or safe. Wind load will have an impact on design.

 

[TimeToPlay]

Thank you for your inputs

Yes, this is almost seems like designing for another wall taking wind loads!

Right now I have a 7"x18" parallam PSL as the ridge beam spaning across the 30 ft span, braced by parallam PSL rafters @16" o.c. and supported by two columns at the ends.

My concern with using a cross tie (collar tie?) is that will transfer loadings from one rafter to another rafter. This loading will be like a point load creating extra moments at the rafters, isnt it?

 

[1503-44]

Yes. That would be true. If rafters will not take the bending moment, a cross tie directly from wall to wall would be needed. I have also seen that done with tension cables, usually as a repair for wall kickout after excessive spread is observed, but tension cables would not allow you to use both walls to resist wind shears. If you need both walls for that, a tie beam would do that.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[RontheRedneck]

A couple of practical thoughts -

LVLs can be used as rafters. They don't face the same limitations as I-joists, and long lengths are no a problem.

A 7"x18" parallam PSL as the ridge beam was mentioned. Make sure what you're specifying is readily available. Large beams like that can be difficult to source.

 

[1503-44]

Especially long ones.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[phuduhudu]

Also with such steep roofs you may find that the roof itself if it has any kind of sheathing will act as a deep beam diaphragm spanning gable to gable or onto internal cross walls and your parallam beam may not end up doing much. There are a couple of other threads going on at the moment about roof plane diaphragms which I am following closely. You may not want to design it as a diaphragm but it may behave like one whether you design it to or not and if it doesn't work then something in the diaphragm needs to give until the load settles back onto the ridge beam and you get corresponding eaves spread. The other problem with this very steep roof arrangement is that this makes the roof diaphragms less able to resist horizontal wind loads at the eaves which are now also more significant due to the steep roof. As others have said it is now more like an extra storey of wall so you need some lateral restraint at the eaves.