Hip Roof
Hip Roof
(OP)
Hi There:
I am sizing some hip rafters for a house.
The roof is a rectangle 34 ft. x 38 ft. with four symmetric hips and a 4 ft. long ridge. It has a 12:12 pitch.
The attic floor is fully sheathed with 3/4" plywood and the rafters sit on a plate on top of the plywood.
The house location is central NC, so snow loads are not huge.
I typically design my hips as beams and support them with a post down to bearing walls. However, in this case, since the attic floor will prevent rafter spread in all directions and the roof is pretty steep, would you guys thing there is enough folded plate action going on to preclude me from having to support the hips on posts or design them as beams?
Thanks in advance.
I am sizing some hip rafters for a house.
The roof is a rectangle 34 ft. x 38 ft. with four symmetric hips and a 4 ft. long ridge. It has a 12:12 pitch.
The attic floor is fully sheathed with 3/4" plywood and the rafters sit on a plate on top of the plywood.
The house location is central NC, so snow loads are not huge.
I typically design my hips as beams and support them with a post down to bearing walls. However, in this case, since the attic floor will prevent rafter spread in all directions and the roof is pretty steep, would you guys thing there is enough folded plate action going on to preclude me from having to support the hips on posts or design them as beams?
Thanks in advance.






RE: Hip Roof
The jack rafters will be tied at the bottom (although you will need to detail something as only ties in one direction can be continuous) then at the top the force would be transferred into the diaphragm (you may need some blocking here to transfer this force into the diaphragm??) I think ilevel (or weyerhaeuser? the guys who manufacture LVL's) might be able to help some with this.
EIT
www.HowToEngineer.com
RE: Hip Roof
FWIW, in my 20 years looking at houses, I have only seen problems with hip roofs if they were low sloped. The high sloped ones, even if they are really cut up and have no floor diaphragm seem to work just fine.
RE: Hip Roof
If it is a rectangle, then to get no columns, I either use a glulam or collar tie truss at the hip intersection or close to it. That works too.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Hip Roof
I feel like the thrust is resolved at the base of all the rafters (due to the floor diaphragm) and not just at the hip corner. As such there should not be a thrust load at the hip. Thoughts?
FWIW, I have done the tension ring thing too on the past but on much smaller roofs with no floor deck.
RE: Hip Roof
In the ones I have done, the Architect wanted an open vaulted ceiling, so there were no deiling joists to act as collar ties. With the hip sitting on top of a corner column, the only way to verify through simple calculations to take the lateral thrust of the hip wats to provide a ring beam of sorts with a substantial corner connection. If the roof diaphragm is deep enough, you may be able to justify the roof diaphragm acting as a deep beam, but I think that in order to accomplish this, you may get more spreading in the walls and cracking of the sheetrock at certain locations. I just prefer the corner connection as it is a more direct and verifiable solution.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Hip Roof
I hope not. The only way you will have any folded plate action is if the floor diaphragm or the connection of the rafters to the floor diaphragm fails. With a properly designed floor diaphragm the roof will be three hinge arches and modification of one. It should work fine. Be sure to take a careful look at your connections (to floor diaphragm and ridge) and the tension forces in the floor diaphragm.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Hip Roof
RE: Hip Roof
I agree with you about the vaulted ceiling, but with a floor diaphragm, it is another story.
@woodman88 -thanks. With a 12:12 roof and our small snow loads, the loads in the floor diaphragm are pretty small
@NAC521 - Do you do this without a floor diaphragm and just the sheetrock ceiling? W/o the floor diaphragm, you are introducing the the rafter thrust from the ladder framing into the ceiling diaphragm as a tension force.
RE: Hip Roof
I guess I don't understand the layout here. Can you poast a sketch?
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Hip Roof
RE: Hip Roof
For the case of a vaulted ceiling do you span the top plates horizontally between perpendicular walls? I would think that you wouldn't be able to get very far and may need to use an interior wall as a 'support' for the top plates, no? That does seam like a tough connection to make; steel angle?
EIT
www.HowToEngineer.com
RE: Hip Roof
As such, the thrust of each rafters goes directly into the floor diaphragm.
@nac521 - Nice - that is the way i have been doing it as well when the attic can be sheathed
@RFreund - that is a tough connection and you would likely need 2x6 exterior walls to get enough horizontal beam action and not have the interior walls too far apart and have them be continuous across the building. There are many ways to skin a cat on a vaulted hip roof, but most of them only work on smaller roofs. I have done a larger vaulted hip roof before, but it used steel bents made from W10x77 i-beams.
RE: Hip Roof
1. How is this standing? Is there some way to use a 'scissor' type truss for the hip framing?
2. How do you ventilate the hip roof (space between the jack rafters)if it is stick framed?
I want to ask him if I can go up in the attic and even check the walls for plumb.
I will follow up with a sketch on a couple ideas.
EIT
www.HowToEngineer.com
RE: Hip Roof
Is there anyway that the roof sheathing on the hip side starts to act as a beam spanning from hip to hip? Meaning that as the wall (say West wall on the attached sketch) starts to deflect outward due to the thrust form the jack rafters, the roof sheathing actually starts to prevent the rafters from 'sliding down'?
EIT
www.HowToEngineer.com
RE: Hip Roof
EIT
www.HowToEngineer.com
RE: Hip Roof
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Hip Roof
The greater the slope of the roof diaphragm, the greater the contribution tro supporting the hip...some.
But look to the obvious for the solution here. The hips are more likely than not supported by a structural member, truss, collar tie, beam, whatever, at the intersection of the two hips, or very close to it.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Hip Roof
@Garth - See attachment, is this the layout you are referring to? It makes sense for most hips that I have seen, but these would be oddly shaped trusses as the ceiling is vaulted below. But maybe they are oddly shaped scissor truss with a flat top cord for a portion...
EIT
www.HowToEngineer.com
RE: Hip Roof
But you should talk to a local truss manufacturer and/or their engineer about this.
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Hip Roof
EIT
www.HowToEngineer.com