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10: Flat Roofs for Homes and Commercial Properties
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Flat Roofs for Homes and Commercial Properties

Posted: 10 November 08

Flat Roofs

The roof of any home or commercial property that is nearly level or slightly pitched (below 3/12) is called a low-slope roof. No roof should be completely flat as it must have at least a slight slope for it to drain properly. Problems in low-slope roofs are common and more difficult to diagnose than pitched roof problems because the path of water leakage through flat roofs is often quite hard to trace.

Roof Structure
The roof deck is usually supported with wooden or metal trusses. The decking can be several materials; metal, pre-cast concrete, gypsum, or even wood sheathing. If there is a sagging of the deck, it should be corrected before it worsens. The next component in a flat roof may rigid foam insulation, usually the thickness of the foam varies making more of a pronounced slope. Some roofs have an additional layer of wood sheathing, Fiber board, Perlite roof board, Gypsum board, or glass fiber board called a cover board that the roof membrane is attached too. Then whatever waterproofing material is applied on top either by mechanical fastener, glue, or held down by a ballast.
Here are the most common types of flat roofs.

Asphalt Roll Roofing
Asphalt roll roofing, used since the 1890s, generally consists of one layer of asphalt-saturated organic or fiberglass base felts, applied over roof felt with nails and cold asphalt cement, and usually covered with a granular mineral surface. Seams are typically covered with a roofing compound. Roll asphalt lasts about 10 years.

Built-Up Roof
The traditional hot-tar-and-gravel roof is built from three or more plies of waterproof material alternated with hot tar and ballasted by a layer of smooth river stone or similar material. Once made of tar paper, these types of roofs gradually are using more-advanced materials such as fiberglass membranes. Built up roofs can last between 10 to 30 years but many well maintained and installed systems can last a lot longer.

Modified Bitumen
A single-ply rolled roof similar to ice-and-water shield, but impregnated with a mineral-based wear surface. Torch-down systems involve heating the adhesive as the material is unrolled. Newer peel-and-stick systems are safer and easier. Modified Bitumen roofs life expectancy is 10-20 years.

Rubber Membrane
EPDM (short for ethylene propylene diene monomer) is a true rubber it is usually grouped with and polyvinyl chloride (PVC) sheets. The durable material resembles an inner tube, but it’s engineered to resist damage from sunlight. EPDM and PVC can be mechanically anchored with fasteners, ballasted with stone, or glued. It can come in a variety of colors and finishes. Rubber membrane roof life expectancy is 10-15 years.

Low-slope roofs are expensive to repair, so extra care should be taken in their examination. Here are a few hints.

  1. Inspect the flashing and joints around all roof penetrations, including drains, soil stacks, chimneys, skylights, hatchways, antenna mountings, and other roof-mounted elements. Note if metal flashings show signs of rusting or reanchoring and if asphaltic or rubber flashings are brittle or cracked.
  2. Check parapet wall caps and flashing for signs of damage due to wall movement. Examine all portions of the roof covering. Look for signs of previous repairs that may indicate trouble spots and note the repairs. Look for open joints, sagging flashing, missing counter-flashings, punctures, tearing due to building movement, pitch pockets that require refilling due to shrinkage, missing or protruding fasteners.
  3. Check copings for open joints.
  4. Look for missing ventilator hoods, gutters or downspouts.
  5. Walk entire surface of roof. Look for soft spots, blisters, splits, ridging, bare spots on aggregate-surfaced roofs. If you don’t inspect a portion of the roof note why and document with a picture.
  6. Watch for accumulation of debris that can clog drains or that can be blown about during a storm, causing roof damage.
  7. Pay particular attention to areas adjacent to expansion joints. Look for splitting in the joint covers or tearing due to differential movement.
  8. Any time water pools in an area there is cause for concern and the general rule of thumb is if it is there for longer than 48 hours you have a problem. Sometimes pooling water will discolor a certain area of roof. When water pools in areas, there can be a sediment build-up so if it hasn’t rained in a while you can still analyze some pooling problems.
  1. Flat roofs are frequently used as platforms for mechanical equipment, signs, and braces, where improper mounting and flashing causes leaks. Look at these areas carefully.