A Trademark of Modern Design, Here’s What You Need to Know
If you’re moving to a home of modern design from one that is classic or traditional, you’re likely unfamiliar with one of the trademark components of a modern dwelling – the apparently flat roof.
A flat roof – and this article is defining that term very broadly – enables designers the ability to create linear forms including box shapes and generous roof overhangs. At a recent Lindal webinar, Michael Harris delineated the options:
“One of the ideas that someone had was that we should be able to put a number of different roofs on the same building, so that would have low-pitch roofs, steep roofs, butterfly roofs, mono-pitched roofs, flat roofs and flat roofs that are lower or higher.”
Harris was trained in architecture at MIT and has contributed numerous designs to Lindal. He also formerly served as the company’s CEO.
Flat Is Where It’s At
For simplicity, this article will lump every type of roof on a modern home that isn’t gabled (a roof with multiple pitches and slopes and generally covered in shingles or tile) as “flat” even though it may have a very noticeable slope.
In some cases, the roof line will angle appreciably front to back or side to side with no peak in the middle (more formally known as skillion). Some designs feature a seemingly flat roof over a small and lower section in the middle of the home, a breezeway for instance, with symmetrical wings on each side, sloping either outward or inward. These are usually termed butterfly but multiple roofing sections that slope in the same direction are known as sawtooth.
Other designs seem totally horizontal and thus trigger concerns of rooftop puddle formations from which water may seep into the home after a heavy rain.
But what may look flat from the curb is actually at least a 2 percent slope which means that every foot is a quarter-inch higher than the last. That’s about the minimum standard of most building codes.
More Than Style
Beyond style appeal, flat roofs can serve several useful purposes.
- Since building codes limit house height, a flat roof can provide more generous room height beneath ceilings. This also avoids sloped walls crowding inward on finished attics.
- A flat roof with easy access and guardrails can be used as outdoor living space, possibly a sun deck or a vantage point to enjoy scenery.
- Lots of sunlight makes for good gardening on a flat roof. The soil also acts as an insulation blanket that will improve the thermal performance of the home.
- HVAC, satellite dishes, and other eye sores can be stashed on the roof particularly if shielded from ground view. This is one of the reasons you see flat roofs on a lot of commercial buildings.
- If you’re opting for solar panels, they are easier to install, and easier to hide, on a flat roof.
- Flat roofs have better insulation for several reasons: the ceiling beams are thicker since they are the actual roof; many flat roof designs include an additional layer of rigid foam insulation; and there is no attic to store heat. Additionally, in an area with snowfall, a covering of snow on the roof provides insulation.
- You can walk on flat roof – to make repairs or for whatever reason – without fear that should your knees buckle or a foot slip you’ll be plunged into a gymnastics event for which you’re likely ill prepared.
Materials Matter
Unless a builder is a modern home specialist, the builder probably lacks familiarity with the materials and methods a flat-top modern home requires. Poor craftsmanship is likely to spring water leaks.
A flat roof is much like a tilted deck topped with layers of weather proofing, water sealant, surface material and insulation. Instead of shingles or tile, roofers will install a choice of materials, most known by its initials:
- EPDM (ethylene propylene diene terpolymer); the rubber used to make innertubes, available in three thicknesses.
- PVC (polyvinyl chloride and vinyl); the white surface of the membrane reflects much of the ultraviolet light keeping the home cooler.
- TPO (thermoplastic polyolefin); similar to PVC except the seams are heat welded, rather than glued, making them less susceptible to separation.
- BUR (built up roofing); three or more layers of waterproof material mixed in with hot tar and gravel.
- Modified bitumen: a single-ply membrane that can be applied in multiple layers with a granular surface for easy footing. This is also called a rolled roof; the materials are usually peel-and-stick and easy to apply.
Spray-on coatings of polyurethane, acrylic or silicone can also be applied for extended life to these materials.
Standing seam metal, more common on pitched roofs, can also be used on a flat roof.
There are functional, aesthetic, and economic pros and cons to each material that should be explored more deeply than what’s provided in this overview.
Drainage
With a tightly sealed roof, water is channeled into scuppers – an opening into what serves as a gutter system. This may flow into a downspout on the exterior of the home, or into an interior drainage system.
These scuppers are nestled near the parapet which is an extension of the house wall at least a foot above the roof line. They may be higher or have guardrails on top if the roof is going to be used as living space.
There should also be scuppers whose openings are about two inches above the roof line, and flow out the parapet with no downspout – just drip onto the exterior wall. These are overflow scuppers. They enable the homeowner to see if the primary scuppers or downspout is clogged.
Cold or Hot?
Another consideration is where to place insulation. Placing it in the rafters is termed cold roof construction and it requires ventilation to keep condensation or mold from forming on the roof, or excess humidity rotting roof materials. A vapor layer needs to be installed between the insulation layer and the room below to create a barrier.
Warm roof construction involves rigid layers of insulation above the timber rafters or within the roofing materials, so long as it’s beneath the weather-proofing membrane. It is more thermally efficient than a cold roof, but higher in construction cost and adds height to the structure.
Seeing the Light
Its curb appeal is a matter of personal preference, but the non-traditional so-called flat, skillion, butterfly, sawtooth, etc., roof designs popular in modern dwellings can provide a key benefit inside the home – natural light.
By accommodating higher ceilings, there’s room for clerestory windows. Frank Lloyd Wright’s Usonian homes, inspiration for Lindal’s Modern Elements designs, featured clerestory windows (windows near the roof line, high above eye level).
Clerestory windows enable natural sunlight to penetration deep recesses of the home, and operable ones can also be opened to provide ventilation. They also support passive solar heating.
Hopefully, this illuminates the issue of roof design.