Wood Roof Truss
Wood Roof Truss
(OP)
We have an apartment project with a flat roof and internal drains. The roof structure will consist of engineering wood trusses (24" minimum depth). Anyone see a problem calling for wood trusses where the top chord is pitched down to the middle (kinked if you will)? Seems like it should work fine is designed properly.






RE: Wood Roof Truss
An overflow helps, but will not solve the problems produced.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Wood Roof Truss
RE: Wood Roof Truss
We are in SD right now, and once the owner gets contractors on board in a couple weeks they may have their own ideas of how to get the slope.
I hadn't thought of ponding, but a good point. I will certainly look at it regardless of how the slope is achieved.
RE: Wood Roof Truss
BUT IT WILL LEAK - SOONER OR LATER!!!!!!!!!!!!!
Many years ago I spent a number of months designing new pitched roofs for one of our military academies (they liked airplanes) that was only about 20 years old. They had like 200-300 buildings with flat roofs. Guess why - THEY LEAKED!!!
NEVER FORGET THIS - FLAT ROOFS WILL ALWAYS LEAK!!!!
RE: Wood Roof Truss
I assume you'll likely have more than one internal drain. The problem with this scheme is that you would have to construct a series of triangular-shaped roof planes directed down to each drain. This would require the plywood roof decking to be cut on the bias where sloping planes meet. In order to support the decking, and complete the roof diaphragm, there would need to be solid blocking between trusses along the bias. All trusses would need to have variable depth blocking along their length, varying not only from truss to truss, but along the length of the trusses as well. It is my opinion that this would be a construction nightmare! They'll never get it right, and the strength of the roof diaphragm will be greatly compromised.
I recently received the same request from an architect on a high-end residence. I talked him out of it. The architect responded after some thought: "I must agree with you that the complexity of the roof planes on this roof will not work well with ripped blocking and we are going to revise our drawings to tapered insulation."
RE: Wood Roof Truss
RE: Wood Roof Truss
RE: Wood Roof Truss
Spats,
The roof diaphragm will be directly attached to the trusses, and then the 2x overbuild and decking above that. The diaphragm is there as good as any pitched roof.
RE: Wood Roof Truss
A leaking roof falls fully on the architect and roofing contractor. Significant warrantees are usually provided against leaking by the contractor. If those guys have a problem let them deal with it.
RE: Wood Roof Truss
Justify it however you want. It's still a bad idea. If the architect insists, I would insist he assume full responsibility for everything above the lower deck diaphragm... then he'll change his mind!
RE: Wood Roof Truss
RE: Wood Roof Truss
RE: Wood Roof Truss
RE: Wood Roof Truss
RE: Wood Roof Truss
Most overbuilt cold roofs are vented so that if & when they do leak, they also have some chance of drying out. And, there is often some form of roofing on the lower plane of sheathing so it will drain water. But, they are also generally two parallel planes so the overbuild framing is fairly clean and simple; unlike your's where all the blocking for the overbuilt sheathing will be of varying depths, plus the trusses having such varying web members, and their costs, whatever. It isn't that it can't be done, but it ain't gona be cheap or easy either. I'd do some serious cost comparisons to see that the sloped insulation is truly more expensive and less complicated.
I would pay particular attention to flashings, venting and vapor barriers in these built-up roof areas, so you're not just growing hoarfrost and mushrooms in that space. Then, I would put this all in a registered letter for the file and to the Arch. and client, so that you are on record that they are responsible for this approach if & when things do go south.