Making Rafters for Storage Buildings

I would put ceiling joists at least 4 feet on center to hold the walls together.

collar ties would probably be good enough, but to be on the safe side I would install the joists.

oilcan13

I have seen self-supporting roofs in old hay barns that were built many years ago, and they are truley a work of calculated art. The walls must be self-supporting for this type of roof to work, and the roof joists are quite special. The ones that I remember form the "Gambrel" type of roof that is often seen on old north American barns that were not built for loose hay storage. I apologize for the long link: http://books.google.com/books?id=xWg2AAAAMAAJ&pg=PA115&lpg=PA115&dq=self+supporting+barn+roof+joists&source=web&ots=Rwi5G64lyM&sig=1SW1Th0tD9LLZjJmRm0asiL8-rc&hl=en&sa=X&oi=book_result&resnum=6&ct=result#PPA118,M1

But here it is. One can clearly see how the roof joists are constructed so as to be self-supporting. But again, the walls are self-supporting for this type of construction. With an 8/12 pitch you should be steep enough to reduce the outward loading of the walls if you can determine a way to fasten the opposing joists at the spine in such a way that the natural sag of the roof joists does not overpower the walls and push them outward. I do not think that you can achieve (easily, at least) a self-supporting roof that is open all the way to the spine unless you have some very special roof joists.

For the purpose of safety and building longevity, I advise you to install the cross members. And hey, you can make an attic in that type of roof if you keep the loading light. That would require very strong joists, as your span of 20'-0" is quite great without a centre support-especially with 2" x 4" wall construction.

Good Luck!

Ing. Robert Forbus

HI, ammeekaj!

Tie the rafters just below the ridge and cross-brace them or place plain braces at staggered intervals up and down and at the base of all the rafters for the total length of the shed. That gives you your open roof.

If the rafters are fastened well at the beaks and ridge, then your only remaining problem will be bracing the roof to prevent lateral collapse.

Consider braces at each end such as members on a 20^o (70^o) angle out from base of the ridge supports upward and inward to the ridge, and tie them back to the ridge supports in a triangular pattern.

And then pray that a tornado never hits your shed.

That should allow you to have your (modified) cathedral ceiling shed.

Mark

The thing about a gable roof is that you must keep the side walls from bowing out. If they are restrained by whatever method the roof cannot sag.There are several ways to do that. External bracing on the side walls will give you what you want. The others all involve forming triangle of appropriate size. Collar ties are second choice to ceiling joists. A tension cable at the midpoint will hold the walls but will do little to address sway. My own sneaky method is kind of a hybrid. I torch cut steel plate in an 'L' shape at the same pitch as the roof with the leg width the same as the rafter depth. The leg length shoukld be 4X the width. They are multiple thru-bolted to the rafter ends at the ridge. In this case you may not have a ridge board but instead you will have solid blocking. This acts much the same a collar tie but but doesn't have the same leverage so expect and account for rafter sagging and stiffen accordingly. Additionally to that I use as long of an overhang as possible and return the overhang to the wall with a structural member and use steel plates to rigidly fasten it to the wall. This is a great place to add a full size 3/4" plywood gusset. Also it is critical to use good steel brackets to fasten the rafter to the wall plate.

In essence you have used 2 small triangles to equal the large one the collar tie would give you. With roof framing it is critical especially in a case like this that all joints are very strong and very stiff. Use gussett plates whenever possible to support joints. Use solid blocking between structural members at and near supports and suitable intervals to prevent twisting of rafters. Use metal plates with many smaller fasteners whenever possible. This method will allow for a completely open ceiling. I can send you a drawing if you need it.

I have a 22 x 28 building on my property and uses engineered 2 x 4 trusses for the roof. There are no supporting members in the center of the room.

I forgot to add, that they are on 24 inch centers.