Why do most businesses have flat roofs?
January 28, 2015 12:55 PM   Subscribe

It seems like wherever I go, businesses have flat roofs, while houses have sloped ones. I've always heard bad things about flat roofing, but presumably they have some sort of reasoning for not going with the sloped.

I've thought maybe they need to walk on it more frequently, but I can't really imagine why that would be. Or maybe they don't want any excess height. Even the little stores that are actually smaller than the typical house seem to have stuck by this trend, though.

Basically I'm stumped.
posted by Trifling to Society & Culture (15 answers total) 5 users marked this as a favorite
Reddit discussion of this here. iLTerrible's answer about halfway down seems to be the most specific.
posted by Etrigan at 1:10 PM on January 28, 2015

Best answer: Couple of things.

1. They're cheaper.

2. You can put things like AC units on top of them, saving real estate below for parking and square footage.

3. You can put standing water abatement thingies on a business and no one will care, so pipes and channels through holes. A neighborhood would care, other business would be less concerned.

4. There are rarely attics in businesses.
posted by Ruthless Bunny at 1:12 PM on January 28, 2015 [3 favorites]

5. Modernist aesthetics. The exception that proves the rule: businesses who wish to signal tradition/nostalgia/domesticity, e.g. Comfort Inn.
posted by feral_goldfish at 1:30 PM on January 28, 2015

I think it's mainly for easy installation and replacement of air conditioning units and the like. Roofing is expensive and mainly there for aesthetic reasons. Really cheap modern housing uses flat roofs.
posted by deathpanels at 1:38 PM on January 28, 2015 [1 favorite]

The decision to build a pitched roof instead of flat in climates with little to no snow is aesthetic. People like the more traditional sloped roof for homes, even when it's not the best application for their climate. Businesses are far more likely to skip an aesthetic choice that trades out cost and convenience in favor of a certain look.
posted by quince at 1:44 PM on January 28, 2015

It's a matter of scale. A central peaked roof on, say, a Safeway, would enclose a huge volume of air since it would scale with the cube of the linear dimensions. That's a lot of extra space to heat, extra materials to buy in construction, and it is not useful space to that store.
Flat roofs are not exactly flat - they need a few degrees of slope to drain, but that's subtle enough that it's still easy to walk around up there and fix the roof, you can put AC units on the roof without them sliding off, etc.
posted by w0mbat at 2:15 PM on January 28, 2015 [2 favorites]

Also, for bigger businesses (think: malls) all of the structure you see inside is basically built like an interior shell. The structure you see from the outside (especially on malls built in the 80s and 90s) is the outer shell from which the interior space is suspended/secured/etc. This gives them a lot more capacity to renovate interior designs and structures with changing fashions without having to do very expensive work on the superstructure.

(My partner is a commercial real estate developer and we were just talking about this last night--specifically, suspended/dropped ceilings).
posted by late afternoon dreaming hotel at 2:36 PM on January 28, 2015 [2 favorites]

Related to esthetics is cladding choice. Roof tiles, thatch, shingles, metal, torch on membranes, etc. all have different minimum slopes. Some of the more durable choices (eg: the standing seam metal on my roof which carries a 50 warranty and should last much longer) can only be installed down to 1.5:12.
posted by Mitheral at 3:34 PM on January 28, 2015

One of the search terms you want is "wide span roof," and one of the places you want to search is in the history of church and mosque architecture; the development of the ability to enclose huge numbers of people in roofed areas WITHOUT lots pillars is really important to large-assembly buildings, and they all go nuts when steel girders start letting you do that!

The basic issue is that flat roofs are cheaper (and if you pitch them even slightly the water runs off fine) and sometimes stronger. But yeah, mostly cheaper to build, and less interior area to heat/cool.
posted by Eyebrows McGee at 5:10 PM on January 28, 2015

It seems like wherever I go...

Perhaps you could tell us where you go?

This makes a difference to the answer.
posted by pompomtom at 6:35 PM on January 28, 2015

Best answer: One of the search terms you want is "wide span roof,"

Long span. That designation usually covers stuff like open-web steel joists, trusses, or pre-cast concrete T shapes, etc.

Most people here are hitting on a reason or two, but it's really a combination. The wikipedia link for the benefits of flat roofs doesn't really go very far in describing all the things that contribute to flat roofs being cheaper (at least in the US and Canada), and quince has a pretty good point that you could just as easily ask why more houses don't have flat roofs (and the answer is pretty much looks).

First, a flat roof generally has a slope of less than 1.5 :12 (i.e. it gains 1.5 vertical units for every 12 horizontal ones). As mitheral notes, 1.5:12 is basically the minimum slope for standing seam metal roofing. All roofs will fail at some point, and a flat roof will typically be guaranteed for 20 years or so, but flat roofs have a few more failure modes than pitched roofs. Most commercial flat roofs have a minimum slope of .25:12. So, reasons for flat roofs:

1. Cheaper materials. Not just in terms of the roof itself, but also for what supports it. If you have a sloped roof, you will quite obviously have a larger amount of material forming the angle of a pitched roof than you do forming the relatively straight line of the hypotenuse of that angle. When you start dealing with things like steel and concrete because you're going with a long span structure so you can have more open floor area, that makes a huge difference. That also means less material for the roof itself. And since flat roofs generally aren't meant to be visible, they're only designed for function and don't need to look pretty, so they can be pretty cheap. Plus, they come in rolls instead of on pallets, so shipping a load to cover a certain amount of roof is easier.

2. Cheaper labor. Sloped roof materials are typically nailed in one piece at a time. With a metal roof that's not so bad since the pieces cover a decent amount of territory, but it's pretty time consuming with a two-piece mission tile roof. For a single-ply flat roof, you just put glue on the deck, unroll the 5' wide membrane and run a glorified hair dryer over the seams. Done.

3. Aesthetics. This is what quince was getting at, but people build sloped roof houses because that's the way it's normally done. Tract home developers build them because they sell better. Some codes allow taller overall building heights with sloped roofs, where you measure the height at the center of the slope instead of the top. Some communities with CC&Rs or development rules mandates sloped roofs of a certain pitch or material. Then there's height: for smaller buildings like bank branches it's not a big deal, but for a Home Depot a sloped roof would be immense. Even just the regular little shops in a strip mall are around 60' deep, which would make for an 8' tall roof. If you're going for maximum clear span on the interior, you're looking at structural members that are 3'-5' deep. That's a lot of space for nothing.

4. Function. It's really easy to just put all the HVAC stuff on top of a commercial structure and then run all the systems in the space between the roof deck and the ceiling, or just out in the open if it's a warehouse type place. Any new tenant can move everything around up there (lights, speakers, HVAC) and reconfigure the whole thing really quickly just by putting in a new ceiling grid.

As far as the failure modes, any roof most easily fails by water penetrating the seams. On a flat roof, the danger is that there's more potential for water to just be sitting there (ponding) than there is on a sloped roof, where gravity does half the work of shedding water and there has to be some serious wind for water to migrate uphill under the shingles. Ponding on a flat roof can occur from insufficient slope in the design, contractor screw-ups, or deflection of wood structural members. Ponding on a sloped is pretty tough to do unless you have ice dams. Plus, flat roofs have drains or scuppers, where there are seals or flashings that can fail or be improperly installed. Any penetration in the roof membrane is a potential leak, and a flat roof will just have more penetrations.
posted by LionIndex at 8:23 PM on January 28, 2015 [3 favorites]

Forgot a big one (but generally it's a more recent concern): fire protection. Concealed spaces in buildings over a certain size require a bunch of additional construction, including installation of fire sprinklers and firestops, which are not necessarily required in residential attics.
posted by LionIndex at 9:02 PM on January 28, 2015

w0mbat: A central peaked roof on, say, a Safeway, would enclose a huge volume of air since it would scale with the cube of the linear dimensions. That's a lot of extra space to heat, extra materials to buy in construction, and it is not useful space to that store.
False assumption. A sawtooth roof only adds 1/2 the height of the peaks times the footprint of the building. THESE are the sort of roofs you do in fact find on many large buildings where esthetics are not an overriding concern (i.e., not churches or museums).

I've long believed that such flat roofs are only economical in the short run. One company I worked at paid to have the roof reasphalted... three times... in one year... and still had to have large plastic sheets funneling the roof leak down into large trash cans for regular disposal.
posted by IAmBroom at 10:08 AM on January 29, 2015

Sawtooths are mostly what you find in industrial uses because leakages aren't such a big deal (no finishes to ruin) or you really want natural light. With a sawtooth, you're taking one of the weaker points of a flat roof - the intersection between the roof plane and a vertical surface - multiplied it however many times, and put it right over the center of your interior space.
posted by LionIndex at 11:57 AM on January 29, 2015 [1 favorite]

Sawtooth roofs are very prone to leaking in the valleys and they also require an interior row of posts along the valley to support the roof weight concentrated in the valleys. A flat roof can accommodate a more open span by placing roof joists closer together.
posted by JackFlash at 12:26 PM on January 29, 2015 [1 favorite]

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