Adding dormer to attic truss

[Chris]

In the project photos there is a garage with very similar truss design to what I ordered only incorporating three dormers. My attic trusses I just ordered are 8.4/12 and 32' wide 12 feet to peak. They are designed L630 with 42/10/10 loads 24"oc. Other than doubling trusses on each side of the opening can anyone explain how to do the rest..site links or any imformation appreciated. I will need to order more trusses right away if I think I can add one or two dormers.

[mjpliv]

When the truss plant manufactures roof trusses for a project they are designed for specific load cases. In most cases, in a simple roof, the loads are applied uniformly to the top and bottom cords. The software they are using then determines (based on the manufacturers pre-determined preferences and lumber inventory) the size of the top and bottom cords as well as the size and location of the interior webs. The software then calculates the loads placed on each joint and determines the size and gauge of truss plate to be used to hold the whole shebang together. Changing the actual loads on a roof truss by altering their intended installation (such as doubleing up a couple of trusses to make a dormer space) has effectively nullified the engineering on those trusses.

That being said I am sure that more than one manufacturer accomplishes a dormer space by building a couple of extra trusses and getting the framers to nail them together (I have seen it done). This works in theory but only if you are dealing with uniform loads.

If you knowingly alter a truss or install a truss in an application that it was not designed for then you must be prepared to assume full responsibility for the integrity of the structure. Play it safe, if you want to change the design of the roof, go back to the truss manufacturer and have it redesigned. You may have to buy a few extra trusses but it is worth it for a little peace of mind.

[grumpydasmurf]

Well that brings up a question, on a remodel dormer... I mean a dormer installed some time after the building has been built... What could be done?

We've installed plenty of dormers but none on a building with trusses.

I would assume a double reinforced header would need to be installed to transfer the load of all the cut trusses. I would also assume that the side rafters would need to be sistered to carry the load of the new header. A third assumption would be that a knee wall would definetly need to be built at those sistered rafters to further support the load.

I'm no engineer so I wonder if this would be true?

[mjpliv]

The rule of thumb I use for every rafter you remove - sister one up to the rafters that will frame in the dormer. So if you want 4' centers you would need a 2 ply rafter to carry the opening. For a 6' opening use a three ply - and so on. Same holds true for the "trimmer" holding back the cut rafters behind the dormer opening. The "trimmer must be stiff enough to transfer the lateral roof load from the ends of the cut rafters to the multi-ply rafters and then to the walls below.

If you are building an "A" dormer, built the bearing sidewalls on top of the muti-ply rafters to transfer the load of the dormer roof to the wall plate through the rafters and not the floor. Any walls built below the multi-ply rafters are non-bearing and are there so you will have something to insulate.

Be carfefull about "pony walls" in a stick framed attic space. In most areas these are not load bearing. If they are load bearing there should be a rafter cut (notch) in the rafters as they cross the pony wall. A bearing pony wall will generally be no farther than 3' from a bearing wall below and the ceiling joists that it sits on should be 2" larger than what is required for its span without the pony wall being there. All bearing pony walls run 90 degree to the ceiling joists.

In most cases that I have seen, the "pony walls" were actually there to help support the ceiling joists below rather than support the roof above.

Sorry about being so long winded :oops: