Wood Roof Truss 1

[dcarr82775]

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.

 

[msquared48]

Just make sure you have considered ponding, the tendency to clog with leaves and ice, and the fact that standing water does cause the membrane to leak with time.

An overflow helps, but will not solve the problems produced.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dhengr]

That certainly could be designed and done, but don’t they usually do this same thing with sloped rigid insulation under the roofing? This may be a more familiar construction method and less expensive. This could cause some sheathing and roof framing problems a bldg. interconnections. Unless they are truly symmetrical you could have some erection orientation problems, mark one end or some such. There will likely be a significant extra at the truss plant because every vertical and diagonal is different and the sloped top chord means they have to change their assembly table and pressing frame to fit that top slope.

 

[dcarr82775]

The architect doesn't want to do tapered insulation. They say the costs have risen too high, so they want to overbuild for the slope. The tapered trusses provide the slope in one direction with overbuild making up the slope in the other direction. So the trusses will be pretty consistent, although I agree there may well be additional cost for that condition. A 'kinked' top chord just helps eliminate some of the overbuild.

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.

 

[MiketheEngineer]

Not really an engineering problem - assuming loads, spans, etc fall into place and ponding is addressed!!!

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!!!!

 

[spats]

Don't let the architect talk you into overbuilding the slope.

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."

 

[hokie66]

ABSA! (Another Bloody Stupid Architect)

 

[ToadJones]

Flat roofs work really great in places where it never rains or snows.

 

[dcarr82775]

Both the owner and architect love the flat roof. I won some other battles, but not this one.

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.

 

[Teguci]

So here are the two options as I see it (see sketch). The kinked truss seems a little more complicated.

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.

 

[spats]

So two layers of decking is cheaper than tapered insulation. How thick will your blocking be at the high points, and how long will the screws have to be to properly anchor the blocking? Will the blocking tend to split? What is the length of fasteners required to properly attach the upper sheathing? Is it attached to just the blocking or down into the trusses? The upper sheathing and blocking is subjected to wind uplift forces.

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!

 

[dcarr82775]

Overbuilds are done all the time on pitched roofs (granted smaller areas but the same problems still exist). They are not that complicated. In this area many buildings get an overbuild type of cold roof which is essentially the same thing. Like anything else, it has to be detailed properly. I see no structural problems with the overbuild. It was the odd truss configuration that had me wondering.

 

[spats]

Knock yourself out.

 

[ToadJones]

talk them into a parapet to hide a sloped roof.

 

[MiketheEngineer]

GOOD LUCK!!

 

[dhengr]

Dcarr:
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.