Smart Flat Roofs: The Craft of Parapet Detailing
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BUILDblog covers the technical side of flat roofs which, as it turns out, aren't so flat.
[Photo by Art Grice]
Most of us architects love flat roofs. The composition made by the long horizontal eave lines just makes for good design. But the thing is, flat roofs don’t work very well unless they’re detailed correctly –especially here in the rainy Pacific Northwest. Even “flat” roofs need to have a bit of slope to them -typically somewhere between 1/4” and 1” of rise per every 12” of length. The slight angle gives the water somewhere to go; preventing puddling and leaking. The trouble with these low slopes is that the angle isn’t enough to look intentional (like the deliberate look of a shed roof) but it’s too much to appear flat –it’s just sloped enough to look… well, crooked. And for those of you watching from the cheap seats crooked ≠ good design.
[Photo by BUILD LLC]
More often than not, on a “flat” roof structure you’re not really seeing the roof; you’re seeing a parapet (a wall or vertical element below the roof that is extended up to continue past the roof). The parapet contains the sloped roof and frees up the architecture to have a perfectly flat profile. Naturally, there are many different types of parapets for all the different situations, materials, construction methods and climates out there. For today’s post we’re going to give you the skinny on 3 parapet designs we’ve found to be successful on our designs here in the northwest.
[Photo by Art Grice]
The first of which is probably the most common. It is used at a single-ply roof membrane sloped ¼” per foot to a gutter like on the Bainbridge Residence that we designed and built (above). At the highest point of the sloped roof, the parapet is about 10” tall, at the lowest point, the parapet is about 14” tall. The parapet itself is standard wood framing with an applied siding (in this case T&G 1×4 cedar). The assembly is straight-forward but what complicates matters is that the roof needs to be vented. Like any roof with batt insulation, the venting is an important part of the system. Normally you would find the venting at the soffit (the underside of the roof overhang) but that’s not possible with the parapet. Instead the air is vented through the roof to the parapet walls –the parapet then incorporates a continuous strip vent beneath the cap flashing. The cap flashing requires a gap between the siding large enough to allow for air movement to and from the vent. While it’s a circuitous route, the air is working 24-7 and we’ve found it to be a good system so long as the parapet walls don’t get too tall.
The drawing below shows how the parapet wall meets with the sloped roof, note that the flashing extends up the parapet wall a minimum of 8”. The parapet wall needs to be tall enough for both the base flashing and the cap flashing to work together and provide the required venting.
[Photo by Swiss Pearl]
The second example was designed for a rainscreen application and was used on the Medina Residence (above). It’s similar to the applied siding detail in the first example, with a couple of important differences. Since the rainscreen system uses a waterproof membrane behind the siding, the cap flashing at the parapet wall doesn’t need to cover the rainscreen panels at the face. This is a nice detail because it keeps a crisp, clean edge profile at the top of the parapet wall. It was important for this design to be as clean and minimal as possible –even a 2 or 3 inch flashing return at the top would have looked visually intrusive. As long as the cap flashing is sloped to the roof, the additional water that gets behind the rainscreen panels is negligible.
The third and last example is perhaps the most complicated. The Davidson Residence (above) uses a built-up “flat” roof deck at the center portion of the house (the portion with the guardrails). Simply put –we built a wood deck over a sloped roof system. The mechanics are, however, much more intricate. The rainscreen siding covers a number of items: the various layers of structure, the required venting for each structural bay, and the internal gutter & downspout system. We wanted the look of an unencumbered simple box at the exterior, which we achieved –but there’s a lot going on behind the scenes. One of the advantages of using a rainscreen at this location is that the vertical downspouts can reside behind the rainscreen panels but outside of the waterproof membrane. It cleans things up visually without jeopardizing the envelope of the house. The decking on top also acts much like a rainscreen in that it allows water to pass through the reveals between boards; the water is then directed to a system of gutters and downspouts. Standing on top, all you see is decking.
There’s plenty of different ways to detail parapets and these are just a few. Let us know if you’ve got some successful methods in your own bag of tricks.