Upper span on gambrel roof

[Jackhammer]

Could anyone tell me what the upper span limitations might be on a 16' lower span.

[bkrahmer]

I think you need to clarify a few things. Your question seems a bit vague to me. Also, how do you plan to build the roof? Trusses, rafters, etc?

I built the roof for my gambrel house, 32x37. Very interesting, I could share my experiences if I had a better idea of what you are trying to do.

[Jackhammer]

bkrahmer, sorry for being so vague.

I am building a 16X20 storage shed. I plan to construct a gambrel roof from rafters.

I have my wall studs placed on 16"oc with a double top plate. I also plan to construct my rafters from 2X4 again on 16"oc directly above the wall studs.

Since my walls are 8' I was wanting to keep my roof as low as possible and still have room for a storage loft. I plan to rest 2X8X16 on the double top plate for the floor of the loft.

I wasn't sure if there were any limitations on the spacing of the upper span.

Example: If the upper pitch is 4/12 and the lower pitch is 13/12 there would be a 10' 2 7/8" upper span. Would this be to much of an upper span since I will not be using a ridge beam or purlins?

[Jackhammer]

Would it be much easier to construct a gable roof with a 4/12 pitch and forget about the loft?

[bkrahmer]

Another question: When you haven't finalized a design, why have you ruled out the fact that you won't have a ridge beam?

Also, where are you located? Will you have snow loads?

I also think you are missing a major point about gambrel roof design. Even if you have decided what two pitches you want, the ratio of the lengths determines the height for a given span. If you want to keep the roof as low as possible, you could go with what you have, or you might consider doing a 5/12-16/12. That would give you more room.

If I were building that, this is what I would do: 1. Build the entire first floor. 2. Put on second subfloor. 3. Raise a 2x8 ridge board. 4. Build my rafters in halves, using 2x6's. At the pitch break, I'd cut up 3/4 plywood gussets and glue and nail them. 5. Attach them at the top using Simpson sloped hangers, and at the bottom using L50 angle braces on a 2x6 plate nailed to the floor joists. Come to think of it, it might be better to use a built-up ridge beam of two 2x8's. That way, the nails from the hangers on both sides have less chance of causing problems with each other.

[Jackhammer]

bkrahmer, I thought I had my design finalized but after reviewing some information supplied to me I had second thoughts.

I'm trying to figure out what purpose a ridge beam would have if the 2'X4' rafters were installed directly over the 2'X4' wall studs with each being on 16"oc. Some say I need one while others say there is no need, that it is a waste of lumber on this size building.

I am located in Arkansas and yes at times there would be a snow load but nothing of any significance.

Being a beginner I'm probably missing several major points in this endeavor but hopefully it will turn out as planned with help from pros like yourself.

I wish I had put my walls at 7' rather than the present 8'1" which would have given me more room to work with the height.

As it is I am looking at 9'2" from the ground to the top of the double plate. I am trying to keep the total height of the building at or below 15'.

I understand that the load of the roof will be transferred to the rafters and on to the wall studs, etc. but I am concerned with the stress on the joints of the upper span. I may be needlessly worrying since the joint will have gussets on either side.

Also, the 2'X8'X16' being used for the upper storage floor I assume will help prevent any pushing outward of the walls during heavy roof loads.

[bkrahmer]

I'm flattered that you think that I'm a pro. :) Far from it, but I know a little bit...

I'm not necessarily saying that you need a ridge beam. In my case, I was concerned that a heavy snow load on the upper pitch would be too much for the gussets to handle. In your case, I wouldn't be nearly as worried. Metal roof? I am only thinking that using a ridge board/beam would help a bit in assembling the roof. One-piece gambrel rafters are very unwieldy, because they're extremely top heavy. Building them in halves make them much easier to deal with.

My only suggestion at this point would be to use 2x6's instead of 2x4's. One reason is this span table: http://www.southernpine.com/pdf/table21.pdf. Another is that 2x4's don't give much area to allow a gusset to support them adequately, IMO.