Metal Canopies and Water Retention

[phamENG]

Quick question - anyone familiar with the ability of metal canopies (typically aluminum) to hold water? I just did a design of one for a fabricator and considered rain with the primary blocked, meaning the water has to build up and overflow the sides. It wound up with a much tighter support spacing than he has ever had to do before, and was understandably concerned that I'd messed up. He sent me some example calcs which either used steel instead of aluminum or flat out didn't consider rain at all as justification. He contends that the water will seep out through the seams. This seems more than a little dubious to me. Anybody have any insights?

This is the general style: Link

 

[bones206]

Considering the design rainfall event was changed to a higher intensity 15 minute design rain event in ASCE 7-16, I can't imagine a roof could drain itself through seam leakage fast enough to overcome a 15 minute downpour. Even a 1 hour storm would be highly dubious IMO.

I don't think canopies have a special code exemption from the secondary drain requirement, but I'm not %100 sure. Having a secondary drain would be the easiest way to reduce the design rain load, wouldn't it?

 

[phamENG]

From an analysis perspective, yes, but from a fabrication and appearance perspective probably not. This shop isn't big enough to custom fab everything, so he buys the parts form a supplier and puts them together in custom configurations. None of them (that I'm aware of) are set up for secondaries. Of course, it could be as simple as a rectangular cut in the side gutter...something to think about.

 

[dhengr]

PhamENG:
In the lower sketch “Architectural Canopy: Perspective Assembly,” in the lower left, their note “Multiple drainage options available via Gutter Facia”; ask them to show you the options. The primary one should be out of the bottom of the gutter, and down, on the right back corner. The overflow/secondary outlet should be on the left back corner half way up to the bottom of the self mating soffit pans. The overflow should drain directly into the lobby, that will give them some incentive to keep the main drain clean.

 

[bones206]

Bottom line is you have to consider rain loads by code, and you have to obey the laws of physics. These kind of canopies fail all the time and it's probably because they either didn't have secondary drains or weren't designed for rain/snow... or both. The fabricator's seam leakage argument just doesn't hold water .

 

[Aesur]

In my early career I worked with a competitor of theirs specializing in aluminum canopies. I don't recall it being standard of care in the aluminum canopy industry to design for a rain event, even though the majority of these canopies are installed in the South East where rain is of a concern. In fact typically the specifications produced by the project Architect specified to only design for roof loading consisting of Dead and Roof Live only - I believe it was spec section 10530 (rarely this was included in division 5 which is typically reviewed and edited by the EOR).

The warranties and proposals were typically written in such a way that their was no liability for the manufacturer if they didn't clean the gutters often and I can say with confidence that these canopies are rarely, if ever, cleaned. I believe the standard of care was scuppers every 20' or so for overhead supported canopies and drains through the posts at about the same spacing for walkway canopies; note that the majority were smaller in size and therefore only had one scupper. As these are mostly deferred submittals, typically seen as "architectural features" I don't recall ever seeing these even submitted to the jurisdictions with only the project architect reviewing and approving them.

The corners were typically either shop welded or field installed using an "L" bracket with a few screws and lots of caulk. I doubt they would "leak" at a sufficient rate to dissipate the water. I suspect if the gutter filled (3 to 4" of water typically) then it would simply overflow under the cap and pan decking. The canopy you linked appears to use "flat pan" decking which would probably fill up to either 6 or 8" of water depending on gutter depth (possibly more if using a decorative façade around the perimeter). The decking was rarely caulked so you may see some water seepage through the stiffeners (ribs) in the decking, however these stiffeners are tiny and wouldn't allow much of the water to seep through.

As as practicing engineer I went after a few of these types of canopies and quickly determined the fees aren't high enough without large quantities of "reuse" on these items to be worth my time. I typically came up with tighter spacing than wanted because I actually applied wind coefficients (Cf factors) to the base wind pressures, which other engineers rarely did at the time, this created a good deal of friction with clients and I got tired of the battle. I would recommend paying special attention to the connections to the building, the canopy engineer is typically responsible for designing the connection, however it used to be rare that the wall was actually verified for such loading or connection type (ie hollow cells with epoxy and screens in lieu of solid grouted, single 2x wall studs, etc..).

 

[phamENG]

dhengr - I like the overflow into the lobby idea. The link was just for concept - these guys build the same type/style. They don't add secondary drains. Fine with me, but I'll design accordingly.

bones206 - I see what you did there....and I agree.

Aesur - thanks for the valuable information. The fees on these are quite small, but fortunately this guy does enough to make the re-use worth my while. This is the first that cantilevers off the wall, so we're working through it together for the first time and he was shocked at the design I came up with.

Thanks everyone. I'll push on insisting that it be designed for a rain event...my impression is that he won't be happy, but he's a sensible guy and is actually willing to hear reason. And if he finds somebody else, then so be it.

 

[hokie66]

While liquid water may find its way out, hail won't. I have seen a few roofs to fail with hail loading, combined with rainfall which can't get out because the hail blocked the drains.