Is a flat roof in the US south a bad idea?
June 7, 2012 5:32 PM   Subscribe

Flat roofs in the south: is there a major problem with having one on a house in Houston, Texas? Snow load is not an issue, and every commercial building has one. The specific context would be a garage with a flat roof with an outdoor seating area on the roof for nice evenings.
posted by BeeDo to Home & Garden (9 answers total) 3 users marked this as a favorite
My main concern would be water pooling and rotting whatever material is being used.

And the pain it would be to pull all the furniture inside during hurricane season.
posted by politikitty at 5:44 PM on June 7, 2012 [3 favorites]

My main concern would be water pooling and rotting whatever material is being used.

Yeah, but that's an issue with flat roofs anywhere and the solution is just to build a "flat" roof with adequate slope so that ponding doesn't happen. Generally, flat roofs have a minimum slope of 1/4" of rise for every 12" of run (depending on your location, this may be mandated by code) - that includes the slopes of the valleys formed by whatever crickets are needed to direct water towards a roof drain. If you're just draining off one side into a gutter, crickets aren't a problem.

Anyway, commercial buildings have flat roofs because they're cheap, and the occupants of commercial buildings change frequently enough and have different enough needs for heating and AC that it makes more sense for those types of buildings to not spend too much on a more permanent roof. Depending on the material used, the manufacturer of a flat roofing system will provide a warranty for anywhere from 10 to 20 years. Even so, it's much more difficult to have a leak-free flat roof than one with a more traditional 3:12 or 4:12 slope no matter the location.

Ultimately, flat roofs are built everywhere, so there's no real reason to not build one specifically in the south. They usually don't occur on residential structures because they can impart a "modern" look that many people don't want for their house. I don't see it being a problem in Houston, but if you have a Homeowner's Association, they may have rules about how shallow your roof can be pitched.
posted by LionIndex at 5:57 PM on June 7, 2012

Most of the commercial flat roof buildings have a tapered drainage system surrounded by a parapet wall eliminating any "ponding" areas. If your intent is to use the space as an entertainment area, you need to incorporate a fall protection system, and this can be as simple as lining the roof edge with potted plants,with a large sign that says "Always walk forward, and don't hurt my dark star ceanothus."
posted by vozworth at 6:51 PM on June 7, 2012

It's not a flat roof that you want in this application as roofing isn't designed to be walked on and certianly not for point loads like those introduced by chairs. Instead you want a roof deck.

It's pretty easy to get a waterproof roof deck (for as much as anything in housing is waterproof). My house (in Canada mind you but it does get into the high 30s for weeks at a time here and we get both rain and snow) has Duradek or similar over a suspended slab. My MIL's place has the same over plywood decking. Both are at least 10 and probably 15 years old and the only real problem is some grungy dirt type stain on my deck. I've only lived here 5 years and there were renters in before I bought and I attribute the staining to their lack of maintenance. My MIL's deck looks fine.

This is also a pretty common treatment for balconies in condos and apartments.

In fact we're hoping to double the size of our deck (with plywood not suspended slab) and I plan to just tie into the existing deck.
posted by Mitheral at 9:17 PM on June 7, 2012 [1 favorite]

Yes to all of the above.

With the addition that Houston's average annual rainfall is 50-55 inches a year. That's a lot of strain on a flat roof.

I'd also caution you that most of the flat roof homes in Houston were built in the 1950's and 1960's - and may be for sale by original owner, as an estate. They may or may not have done any work on the roof to keep it up. (We're in Meyerland, lots of mid-century homes here.)

If you're looking to buy a house with a flat roof, get it thoroughly inspected by someone experienced with flat roof homes. There's a mid-century enthusiast group on Houston Architectural Forum that would be happy to help you find one.

As for the furniture, if you're outside the loop, just be aware that you may have concerned, conservative homeowner's association people getting all up in your business. Every neighborhood has a Mrs. Kravitz, but the ones in Texas are particularly empowered through their associations. Inside the loop, unless you're talking about a condo, neighbors are less empowered because there are less rules for older neighborhoods.
posted by pomegranate at 4:50 AM on June 8, 2012 [1 favorite]

As a Houstonian I have the same question as you. It would be nice to have a flat roof, to enjoy the evening breezes.

As I understand it, this is a structural engineering issue. A flat roof has to be stronger, to support the weight of the rain. If it sags in one area the water will gather there, causing more sagging. Ultimately, it may fail. Proper drainage is needed. I have seen flat roofs on houses in my area. In addition to drains, they have scuppers to direct overflow down the outside of the house. The other problem you will have is that the roof fabric must be able to withstand the abuse of chair and table legs, people stomping around, and even a dozen party guests dancing like fools. On some commercial buildings, where there is foot traffic, I have seen the roof covered with concrete pavers, to resist the abrasion of people and furniture. This will add much more weight, of course. Finally, you will have to consider the question of keeping your guests from falling off of the roof.

You will have to have this roof engineered by an experienced professional. Clearly, it can be done, because you can find examples around town.
posted by Midnight Skulker at 7:21 AM on June 8, 2012

Had the same question after visiting the Restoration Hardware on Westheimer, which has a gorgeous roof deck. Looking out over the Highland Village area, I thought, "Why don't more people do this here! Especially in places like Rice Military where lots are so small and backyards are nonexistent!"

Then, in looking at properties on HAR, I finally noticed one townhouse (off Alabama, near Dunlavy) which was built to incorporate a GORGEOUS roof deck into the structure. I don't think that there is any reason why that CAN'T happen here, but I can think of a lot of reasons it wasn't done historically. Long story short: we built out, not up.

Ranch and bungalow style houses were most popular here inside the loop for most of the building history of the city. Roof decks just don't really work with those styles, and only might have worked with some mid-century adobe-inspired examples. But why would anyone do that when you have a backyard? And folks looking for a "view" wouldn't be satisfied with that anyway. Enter: the suburbs, where the modern ideal of home size was born. Now that the loop is being re-settled and rebuilt, home buyers have brought that sensibility with them, and have gone vertical in order to accommodate larger square footage. (see: Rice Military) Contrast that to Philadelphia, which has been building vertically (but not too vertically! lest one inspire the wrath of Billy Penn!) for hundreds of years, out of necessity. EVERYONE has a roof deck because a lack of outdoor space inspired people long ago to seek a way to create a new outdoor space. The roof is the most logical first stop once you've - quickly - exhausted your ground level options.
posted by jph at 7:38 AM on June 8, 2012

Here's that townhouse. It's got an option pending, but you might consider looking up the builder, saying that you saw their work and would like to talk to someone about the issues associated with building roof decks in Houston. I'm sure they'd be happy to talk to a potential client.
posted by jph at 7:40 AM on June 8, 2012

Response by poster: Thanks everyone, very informative.
posted by BeeDo at 12:14 PM on June 8, 2012

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