Sloping Gable Roof

[SteelPE]

I have a small project that consists of designing a set of stairs. Above the stairs the clients wants a canopy (see attached). The client is cost conscious and want to use a system of steel beams and columns to support a wood framed roof. The problem I have is in regards to the wood framed roof. I don't see why the roof can't be done out of wood, however I am having a hard time figuring out exactly how the roof rafters will be placed on the sloping part of the roof. Will the rafters be placed normal to the sloping steel (ie parallel to the shown standing seam lines on the sloping section) or will the rafters be placed normal to gravity (ie the same way the standing seam is shown on the flat areas)?

I have been thinking about this for a week and I can't come up with the correct answer. Now it's on my schedule to complete and I have to do something.

 

[DaveAtkins]

Normally, rafters go with the slope. Therefore, you need plywood to span perpendicular to the rafters, to give support to the standing seam roof.

There are exceptions. You could use wood purlins which span perpendicular to the main wood framing--then you could space the main wood framing further apart. The standing seam roof could attach directly to the purlins--provided it is not a true standing seam roof, but corrugated steel roof panels which have the ability to act as a diaphragm.

DaveAtkins

 

[SteelPE]

So you are saying that the wood rafters will be normal to the sloping steel then?

We plan on putting plywood under the standing seam for attachment. What is currently shown is the architects all steel scheme but the owner wants to use wood vs steel.

 

[dik]

Why wouldn't you have the rafters c/w collar ties(?) running at right angles to the main framing with 2x4 'purlins' on the rafters with pre-finished metal roofing secured to the purlins.

Why plywood? Can you have a metal roof supplier provide the design, details, and drawings?

Dik

 

[SteelPE]

I just love plywood that why

I was actually thinking that plywood was just an easier engineer out. Also, if the roof rafters are at right angles to the sloping steel won't they impose a lateral load that would need to be resisted by something? Since I can't really rationalize that aspect in my head, at least I know the plywood has a diaphragm capacity that can more than likely resist this possible lateral load.

 

[Jerehmy]

Rafters normal to the steel beam. Having the rafters vertical regardless of beam slope would be a pain in the butt to frame, especially cutting the birds mouth at the bottom of the rafter. Installing the plywood would also be a pain.

 

[dik]

Installing the plywood would also be a pain.

and pricey, I guess the client has a little more money than he's letting on... the prefinished roof supplier can provide sealed drawings, if necessary.

Have you considered a fabric?

Dik

 

[SteelPE]

This whole structure is more of an afterthought on a larger addition the client is doing. The contractor and I both think adding the canopy is overboard but the client wants it. I have a feeling the client is going to want it until they get the price. Unfortunately I have to design it before they make that decision.

 

[dhengr]

SteelPE:
Some random thoughts on the matter:
1. There is some verbiage in the IBC about max. number of risers btwn. landing, check that out.
2. Modify your elevation to show all of the standing seams running vert., straight up/down, plumb, and see what that looks like. You would cut the ends of a sheet at an angle to match the ridge and eave lines, on the sloped roof sections. The tin guys might be able to bend the hip ridges (valleys?) in one sheet of roof material, btwn. the standing seam edges, and then trim part of the top and bot. of that sheet to fit the main ridge and eave profiles.
3. Show us a section through the stair and the gable roof. Do you have a 1’ + high conc. curb on each side of the conc. stairs?
4. Stand all of the rafters (little trusses?) vertical or plumb, 2’ o/c. There is no need for a birds mouth if the truss is bearing on its bot. chord (ceiling jst.?) You must rip a slope on the top edge of each rafter to match the roof plane, and you must cut the top end for the slope, giving a vert. bearing seat cut, (plumb cut) if you make trusses. If you just use rafters to a ridge board, the top end has a vert. seat cut and a bevel. The bot. bearings are made up of 2 – 2x’s bolted atop the eave steel beams. The top 2x has horiz. bearing surfaces cut into it every 2’ o/c, that is a 2 to 1 triangular notch for the bearing surface. Then, that top sill pl. is nailed and bolted to the lower sill pl. and bolted to the stl, beam through the lower sill pl.

 

[KootK]

Surprisingly difficult detailing problem for a minor structure. My suggestion:

1) At the end of each diagonal run, install a single, steel rafter bent/gable thing framed along the roof slope, perpendicular to the diagonal roof beams.

2) Install a wood ridge beam between the peaks of the two steel bents.

3) Attach some wood plates to the sloping beams to which you'll fasten the rafter heels. Rafters installed along the slope rather than vertical

4) Get some blocking between the rafter heel even if it's just 2x on the flat.

I'm torn as to whether or not I'd want the ridge beam to be steel too.

This way, your steel is the primary stability element for all things but the lion's share of the roof surface is still wood framed.

I imagine this thing is exposed structure from the underside?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JLNJ]

It looks to be a 12' rise and most building codes let you go 12' without a landing (I'm not sure what residential rules are or if they're in play here).

I suggest eliminating the mid-landing to save a significant amount of cost and nasty detailing where the roof flattens out.

 

[SteelPE]

Just an FYI, I am not in charge of the architectural layout of the stairs. The drawings I attached were those that were prepared by the architect. So stair heights/runs landing locations where provided by the architect.