I have taken quite a bit of ribbing over our decision to build a house with a flat roof.
“You’ll end up with a swimming pool over your head!”
“It will leak, and then you’ll be sorry.”
“It will collapse the first winter under the snow loads.”
Ironically, most of these conversations take place in shopping malls and offices that have flat roofs.
Truth is, flat roofs are not flat. They have very subtle slopes that allow the water, whether rain or melted snow water, to flow off. And the technology is proven. Flat roofs are by far the most common roof seen in commercial buildings, although not so with residential buildings in this part of the country. Why is that? Maybe it’s aesthetics, maybe it’s the relative simplicity of building a sloped roof, or maybe it’s just habit.
In our case, my wife and I chose a flat roof for aesthetics. We want a low, modern building with horizontal lines. A flat roof adds greatly to that look. Our architect had no problem with our request. In fact, most of his work is designing commercial buildings, so he was in his zone. That our roof trusses and deck would be wood rather than steel really only mattered when it came to the number of trusses and their depth. The engineering is well established either way.
My wife and I don’t like the look of eavestroughs, so our roof design is a series of inverted slopes with large capacity roof drains in the valleys, eight in total. They are all connected to large downpipes that run vertically inside the building through an architectural void feature, and then under the foundations and away from the house. The result is a parapet of only 12”, and no awkward-looking eaves.
The roof deck itself is covered in a commercial-grade material used on major big box and chain restaurant roofs all over North America. It took a week to install and seal the seams, fit the corners, and install mushroom caps and boots for the various stacks and vents that, like the roof drains, run through the architectural void space to the basement.
We had essentially two insulation and deck-shaping options, both common with commercial roof structures: either install shaped rigid foam insulation above the deck and under the membrane to form drainage valleys, or spray foam insulation under the roof deck, using angled roof trusses so that the deck itself would form the drainage valleys. We chose the latter, given the added benefit of spraying insulation foam into all the nooks and crannies around the roof trusses and roof deck edges. The costs were about equal either way.
Is a flat roof cheaper than a conventional sloped roof? I really doubt it when compared to an asphalt tile roof with typical slopes and dormers. But compared to the same sloped roof with, say, cedar shakes? Probably pretty close. I call our roof the ‘leather seat’ option; it’s special, and makes our roof line very unique and appealing.
My chief carpenter (and long-time friend) thinks my flat roof is ‘Roofer Madness.’ So for a bit of fun, I’ll invite him over in February and see how nervous he gets. One of us will be wrong!