Flat Roof Systems The ASHI Reporter Inspection News & Views from the American Society of Home

Flat Roof Systems The ASHI Reporter Inspection News & Views from the American Society of Home

Flat roofing is an entirely different strategy from steep roofing. While steep roofing is a shedding system, flat roofing is a watertight skin or membrane that is formed over the top of the building. If you think of a steep roof as an umbrella, houses with flat roofs don’t use umbrellas but use wet suits (or to be more accurate, dry suits). Since we have a different approach, there is an entirely different set of materials, installation methods and conditions to look for.


Although we generically call these flat roofs, more correctly they should be described as low-sloped roofs. Roofs should never be flat. They should drain water. The drainage system can be gutters and downspouts, centrally located roof drains and/or scuppers (drainage openings through walls, including parapet walls).

Water ponding is a common problem with flat roofs

The functions of a flat roof

The perfect flat roof would –

  1. keep water out
  • last the lifetime of the building
  • not pose any fire threat (during or after installation)
  • be strong enough to walk on, move equipment across and be suitable for attaching equipment to
  • control heat loss and heat gain from the building
  • add to the architectural appeal of the building

    No one has come up with the perfect roofing system yet. Every roof system keeps the water out, at least most of the water, most of the time. Most flat roofing systems do well if they last 20 to 30 years. Most will contribute to, rather than control, a fire. Installing asphalt-based systems (including built-up and modified bitumen) creates the risk of starting a fire while being applied.

    Roofs are not durable with respect to foot traffic and equipment traffic. While equipment can be attached to roofs, this creates vulnerable spots. Most roofing does not help much with either heat loss or heat gain. Most flat roofs are expensive and are not considered attractive.

    We will discuss briefly two of the most common flat roof systems used in residential applications: built-up and modified bitumen.

    Built-up roofing

    Built-up roofing has only been around since the mid-1800s. People have known for several centuries that bitumens have waterproofing and adhesive qualities.

    However, it wasn’t until the last half of the 19th century that asphalt was in excess supply (because of the asphalt products used in making roads).

    Organic felts

    The base of an asphalt built-up roof is the felts. These felts are fibrous materials saturated with asphalt (or coal-tar pitch in older systems). The felts traditionally were made from byproducts of the paper, wood and cloth manufacturing industries. These organic (rag or cellulose) felts have been common.

    Felt and asphalt not enough

    Roofing membranes made up of felts and mopped-in hot asphalt will not last anywhere near 20 years without additional protection. Asphalt is a good water-proofing material, but it is susceptible to rapid deterioration when exposed to the ultraviolet rays of the sun. The volatiles are boiled out of the asphalt, leaving it brittle and weak.

    Consequently, built-up roof membranes must be protected from ultraviolet light. The traditional protection, called aggregate, may be asbestos, marble, rock slag, gravel or crushed stone. This gravel (as it’s usually called) is embedded in a flood coat of hot asphalt that has been mopped onto the top of the membrane. The flood coat of asphalt holds the gravel and also is a waterproofing layer. It is the primary defense for the felts below. None of the felts should penetrate the flood coat since exposed felts will wick water down into the roofing membrane.

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