Why are there flat roofs?

Okay, I live in the Northeast, where it snows a lot in the winter and rains a lot the rest of the year. Almost every company I have worked for has been in a building with a flat roof. And, every single one of them has had problems with leaky roofs when it rains, or as in has here lately, snows then rains. Worse, many buildings collapse under the weight of the heavy, wet, snow and ice.

Most houses have angled roofs to let the water roll off. In very snowy states like New Hampshire and Vermont, house roofs can take on extreme angles to diminish the chances of heavy snow caving in on itself.

So why not have angled roofs or arched domes on shopping plazas, factories and strip malls? Is it just too expensive?

Enormous weight and structure that would increase the engineering effort and costs. The roof also houses mechanicals on large structures.

In my office, there is a flat roof. One reason is that the construction budget is different from the maintenance budget. The former was so constrained that the offices were built without light switches. This allowed them to avoid a lot of wiring and also use 330 volts (which powered six fluorescent tubes) meaning the wires were only 1/3 the cross-sectional area. Maintenance and operating budgets were in the future and the building had to be built now. Eventually, the folly was realized and light switches were added (at considerable extra expense since the electrical code did not permit 330 volts at a wall switch).

As for roofs, I guess angled roofs, especially over a large area, are considerably more costly than a flat roof, while the maintenance is for the future. But there must be more to it than that. An angled roof over a large building would get very high. My office building is 80’ x 80’ and a peaked roof even at a modest 30 deg would go up an additional 24’ or so.

Out here in the South West most of the houses have flat roofs. My house actually has a flat roof in fact. It’s cheaper to build and maintain I think (you just put down some tar paper and then throw rocks on top of it). Since it doesn’t snow much here we don’t usually have a problem with snow accumulation and such. They do tend to leak after a few years though.

There might also be some aspect of keeping the house a bit cooler too. My house is made of adobe, and the roof and walls are pretty thick and sort of a light brownish tan color. I suppose that by not having an attic it helps to keep the place cooler (and maybe uses less wood to build). I only bought it because I had missed south western architecture in my long exile to the East Coast. :wink:

As for why malls and commercial buildings have flat roofs, I think it’s a cost saving measure. Also, a lot of them have a lot of their HVAC infrastructure on the roof, and that might be more difficult on a pitched or sloped roof than on a flat one.


Flat roofs scale easily, while peaked roofs need to be re-engineered at different sizes. And at a certain width, you need to switch to multiple peaks in a sawtooth pattern… which inevitably has water leaks at the internal troughs.

I’ll second the HVAC equipment. For most commercial buildings the units are either huge, there isn’t enough room to place them on the ground, or both.

Plus, working on a slanted roof is a lot more difficult than a flat roof.

They’re also loud. At our store we do have pitched roofs, but we still have three compressors up there. I’d hate to listen to them if they were on the ground. Not to mention the added risk of someone smacking the forklift/delivery van/plow/shopping cart etc etc etc into them.

ETA it would be a hell of a lot easier to work on them if it was on a flat roof (or the ground) though.

Another pro for flat roofs is that I assume it’s alot easier to deal with water removal since gutters aren’t needed. Without gutters you don’t have to deal with gutter leaks, water dripping between the building and the gutter, ice dams, HVAC repair guys leaning their ladder against said gutter and bending it.

Addressing a couple of points in the OP: Flat roofs drain through the use of roof drains or scuppers. The reason many flat roofs leak is because either a) people are walking where they shouldn’t, or b) penetrations for piping and vents have not been properly sealed. If buildings are collapsing under snow loads, they weren’t properly engineered in the first place. Commercial buildings almost always have flat roofs to accommodate equipment (as noted earlier), and it’s no different up here. Most residential units have pitched roofs here, unless it’s an apartment building that has air-handling equipment.

I worked in commercial buildings with sloped roofs and they leak too. At least on a flat roof they apply a membrane usually. On sloped roofs they rely on overlap to prevent leaks not waterproof seams. During heavy wind driven rains and when snow is on the roof the sloped roofs leak.

Matt_mcl told me that one reason Montreal vernacular architecture is flat-roofed is so that the snow can pile on the roofs and help insulate them during the winter. :slight_smile: Keep in mind that I’m describing the classic Montreal townhouse, which is quite narrow, and wouldn’t have as many span problems with snow load. Don’t know how they seal the roofs though. Tar and tar paper, I presume.

Plus rocks, as xtisme said.

FWIW, the building code in Santa Fe requires flat roofs, because the Historic Preservation Committee wants to avoid a skyline. All the houses must be in earth tones, also. It’s actually pretty neat, when you’re coming up on the town from the south, the subdivisions really blend into the land scape. As for my thoughts on the Historic Preservation Committee, though… that’s a rant for another thread.

And let us not forget ice damming. Lovely thing, that.

I think the easy answer is economics. That and most flat roofs have a drainage pitch.

The difference between the live load for a living/office space (50psf) and a roof built for a good sized snow load (100+ psf) is, at first glance, substantial. But when you consider that the cost for a peaked roof goes up as the square of the span (more or less) it makes sense to build a double-strong top floor.

The upshot for me is that with a pitched roof you pay the for the dead-load of increased structure (weight of trusses, strength of walls to resist thrust loads) during construction, or you can build a double strength top floor and get away with a lower dead load and lower construction cost. Tie that in with easier access to equipment on the roof and it’s an easy choice.

Exactly the opposite is true. The air space in vented attic allows the cieling to stay much cooler than the mostly unvented joist space of a flat roof. I’ve lived in both in Albuquerque, and the pitched roof was warmer in winter and cooler in summer…with only 4" of rock wool insulation in the attic.

One thing to consider: How high would the snow pile up beside a supermarket or movie theater with a pitched roof?

Water removal on a flat roof is harder. A good building has a sloped roof to aid the draininage. On a flat roof the drains are normally along the column line. A flat roof will over time sag some. The greatest sagging occures in the center between the columns. there fore the drains end up at a high point. When I was Chief of the Stanford Emporiun store the birds collect on the roof because of the standing water in the winter. During one rain storm one drain got plogged with feathers, I had 1 to 2 inches of water on the roof, I calculated over 600 tons of water on the roof. All it would have taken was one mild earthquake and all that water would have been on the sales floor.

If the membrane is weak on a sloped roof it may not leak, and if it does it will be a small leak. On a flat roof it can poor through.

flat roofs are dumb and I hate them.