How to Design a Flat Roof That Won’t Leak

Designing a flat roof that won’t leak requires more than just minimal slope measurements.

Sydney Franklin Sydney Franklin

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The archetypal flat roof, even today, is largely associated with the mid-century modern aesthetic. From the outset, a building is deemed elementally minimalist based on its elongated form and perfectly linear roof. Architects are still attracted to the elegance that flat roofs bring to a piece of architecture, and for good reason. Set a mini McMansion-like suburban home and its steeply pitched roof against a low-lying, glass-clad modernist masterpiece and most design enthusiasts — architects and non-architects alike — would agree that the latter is a purer architectural form.

This said, even with contemporary technology, flat roofs aren’t easy to design with success. If the fundamental job of a roof is to cover a space and prevent water from seeping inside, then the simplest and most elemental roofing design would be heavily slanted. That’s why flat roofs — which are actually slightly pitched — must be detailed and constructed with the utmost precision. Here’s how to do it.

Hufft Projects designed Baulinder Haus in reference to Marcel Breuer’s Snower House in Kansas City; photo via Hufft Projects

Cold or Warm?

First, you have to determine how shallow a slope you can get away with. Flat roofs are typically designed with a pitch of 10 degrees or less. According to the International Code Council, the minimum slope for water runoff is ¼ inch per 12 inches. Some local codes take it a step further allowing a slope of only ⅛ of an inch per foot. When detailing construction documents, it’s imperative that architects specify the exact pitch of their roof design.

Second, you must be aware of the two classifications for flat roof constructions: cold and warm. These types are based on the positioning of the thermal insulation. To keep the interior dry and unaffected by the elements, the warm deck roof method is usually recommended over the cold.

Typical details for a “warm” flat roof (U.K.); drawing via BuildingRegs4Plans

To put together a warm deck roof, it’s important that the insulation be positioned above the structural deck — the layers that make up the ceiling. Zero ventilation space is required, and the construction appears stacked and compact. Above the insulation, a bitumen felt underlayment is torched on to shed water and protect the materials below. This is known as a warm-deck sandwich roof. An inverted warm deck roof can also be installed in which the insulation boards are set on top of the structural deck and waterproof membrane.

The warm deck method keeps the roof deck and its various components at nearly the same temperature as the building’s interior throughout the year. It also adds protection against condensation from the room below as any moisture vapor gravitates upward into the roof’s structure.

Typical details for a “cold” flat roof (U.K.); drawing via BuildingRegs4Plans

The cold flat roof method, however, creates space for ventilation in the form of massive air gaps above the insulation and in between the structural joists and the structural deck. This method is often problematic because it leaves room for the heat of the sun to damage the structural elements because they’re not protected by insulation.

Making It Last

Once you decide which classification is best for your project, you should carefully assess the performance requirements for each element within the roof deck. This may vary depending on climate, expected fluctuations in temperature and typical weather conditions. Regardless of your location, the key to flat roofs is a sound installation process and — even more importantly — the quality and longevity of the waterproof membrane, which commonly takes the form of bitumen felt.

Even the secret to the success of green roofs — which provide an added natural barrier atop the roof’s structural components — is the strength and durability of the waterproof membrane beneath the soil. The green roof simply further protects the membrane from damaging ultraviolet rays and heavy rain over the passage of time.

A typical, flat green roof build-up

Sky Garden House by Guz Architects

We’re just scraping the surface — or peeling back a few layers — of flat roof construction here. We could talk further about integrating tapered roof insulation, the best practices for sealing penetrations or the specifics behind accounting for snow loads. But the takeaway here is that creating a flat roof requires awareness of not only the context and climate you’re building in — it also demands you build knowledge of the process of detailing it for construction.

It doesn’t matter if you’re building a big-box department store in rainy Seattle, a Modernist retreat in sunny Southern California or a ski resort in Aspen, 21st-century flat roofs extend beyond residential design and encompass a wide variety of typologies in different locales. No piece of architecture is naturally resistant to weather, and it’s the architect’s job to make sure a design is equipped with the high-performances materials and considered details it needs to protect the people that inhabit it.

Top image: Mies van der Rohe’s Farnsworth House by Max Gooday

Architects: Want to have your project featured? Showcase your work through Architizer and sign up for our inspirational newsletters.


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