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Flat Roof Overview: Types and Options

For the most, part flat roofs are used on industrial or commercial properties but there are also residential architectural styles that incorporate a flat roof.  If you are considering buying a home that has an existing flat roof or if you are planning a new construction (commercial or residential) and want to know the pros and cons of installing a flat roof, then read on.

In this extensive flat roof guide, we will look at the main questions and concerns that homeowners and businesses have about this type of roof. We also have a separate guide about flat roof repair or replacement of a flat roof and a further guide that looks in more depth at the different flat roof materials available to help you decide on the best flat roof option for your home.

Why Build a Flat Roof?

Flat roofs are cheaper to build because far less material and labor is used. And they accommodate HVAC systems much better than sloped roofing.

A flat roof also creates usable space for a balcony or terrace. Green building designs often include a flat roof with solar panels covered in vegetation to cool the home and lower energy costs.

Several architectural styles incorporate flat roofs including Prairie, popularized by Frank Lloyd Wright, Art Deco, Spanish Revival and Modern functionalist design.

You can get insurance for flat roofs from some national insurance companies and specialty providers. Expect to pay higher costs and have fewer options for companies. Insurers will require an inspection to verify that the roof is properly constructed, is in good condition and has a functioning drainage system before issuing a policy.

Flat Roof Essential Facts

A flat roof lasts 20 to 30 years when properly built and maintained.

You should consider replacing a flat roof when there is noticeable deterioration in the material. Visible wear, cracking or disintegration are sure signs, as are leaks not the result of storm damage or other obvious cause.

Flat roof cost for new construction and replacement varies significantly. Basic flat roof installation averages $3.50 to $6.00 per square foot. Mid-grade flat roofs cost $5.50 to $8.50 per square foot, and top-quality systems are $8.00 to $11.00 per square foot.

A replacement roof typically costs 10% to 20% more due to the time-consuming task of removing the old roof first.

Flat Roof Replacement – What you need to know

All flat roof options include these basic construction steps:

  • Frame the roof, typically from wood trusses for residential construction and steel girders or beams for commercial buildings
  • Install the materials that will give the roof enough slope or crown to allow water to flow to the drains
  • Insulate the roof
  • Install the parts of the drain system that are below the roof deck
  • Cover the trusses or beams with plywood, O.S.B. or similar material to form the roof deck, and cut openings for drains
  • Flash the roof
  • Install the roofing materials – See types of flat roof below

There are several essential components to a flat roof that ensure its performance.

Insulation prevents heat loss in winter and heat gain in summer. Rigid or spray foam, fiberglass batts, blown-in cellulose are common types.  Then the roof material is installed. It can be a single layer of sprayed-on material or membrane, overlapping layers or up to five layers sandwiched together above the roof deck.

Though called a flat roof, it must have slope to promote drainage. The code for flat roof pitch is 2%, or ¼” per foot. There are three common drain types. If the roof has a curb or ledge around it, then drains called scuppers are used. Openings are cut at low spots, often in corners, and scuppers are run through them to downspouts. Gutters are common on roofs with no curb. Very large flat roofs might have drains in the interior that carry water down pipes through the building and away.

Flashing is also a vital part of leak prevention. Its purpose is to form a waterproof barrier wherever the roof meets curbs, walls, vent stacks, ducts, skylights and chimneys. Your roofing contractor has several flashing types and techniques to match the right one to your specific roof requirements.

When there is dead space – space not heated and cooled – beneath the roof, then it must be vented. A build-up of heat and moisture causes a range of problems from roof deterioration to mold and rot. Several types of roof vents are installed to allow airflow that removes excess heat and moisture.

While not essential, skylights can be installed on flat roofs. Most are fixed, or non-opening, units. When properly flashed and installed, the risk of leaks is eliminated.

A type of flat roofing system called a built-up roof, or BUR, includes a top layer of gravel, small stone or stone chips. The primary purpose is to reflect UV solar rays, which cause deterioration in roofing materials. The gravel also protects the roof from damage when walked on.

For more on flat roof replacement visit our cost and options page.

Different Types of Flat Roofs

Below is a brief rundown of the main flat roof types. You can read a much more detailed report on the pros and cons of each in our flat roof materials guide.

Built-Up Roof (BUR) 

As the name implies, a BUR is constructed alternating layers of sheet roofing material and a bonding agent, usually asphalt. A reflective final layer of stone or gravel is added to prevent sun damage.

Modified Bitumen (Mod bit) 

This is another layered roof. Bitumen, a thick petroleum byproduct, is modified with polymers and formed into sheets. An overlapping base layer plus a cap sheet covered in reflective granules are installed using either hot or cold technique.

Single Layer Membrane – PVC, EPDM, TPO

These are three types of flat roof material applied in one layer, often over rigid foam insulation. Various adhesives are used to install them. TPO and PVC membrane roofs have seams. EPDM is seamless, and single sheets can cover 8,000 square feet or more.

Silicone Spray Coating

Spray-on roofing requires minimal preparation, and it forms a waterproof seal over the entire roof when properly applied.

Flat Roof Systems: Which Option is Best for your Home

You have flat roof options best suited to where the roof will be installed.

Flat roof over the kitchen or an addition:

TPO, EPDM and PVC reflective roofing help prevent heat from transferring into your home, raising the temperature and your air conditioning bills. Silicone spray roofing is a good choice for existing homes. Cost is higher, but preparation work in minimal.

Flat roof on garage:

Modified bitumen is an affordable option for garages, protecting the roof and keeping the inside of the garage a little cooler. Spray-on roof is also an option.

Flat roof balcony or patio:

PVC stands up to foot traffic and is a cool roof material ideal for balcony deck.

Flat roof for solar panels:

PV panels lose efficiency in extreme heat, so a cool roof is necessary. The best flat roof options for solar panels are mod bit and built-up roof because they reduce heat absorption and are affordable.

Flat Roof Drainage

Standing water is a major threat to a flat roof and to your home below. A water-tight roof and proper drainage prevent leaks and water damage. Drainage is ensured by properly sloping the roof and having the right drain type in place for your roof’s design. Then, the drains must be maintained and kept clear, so they protect your home and help extend the life of the roof. Your roof contractor will install a drainage system best suited to your flat roof design.