The area directly below the roof deck must be ventilated in order to minimize the moisture buildup. An excessive moisture condition can result in deterioration of the roof deck, a lowering of the thermal resistance of the insulation and eventually even damage to the interior of the structure.
Frequently, the builder omits provisions for ventilation, so when performing a flat roof inspection be sure to check for ventilation openings. These openings might be in the form of roof vents – vertical pipes protruding through the roof deck – or open areas in the side of the building, just below the roof. Roof vents are typically shielded from the rain by a cover, but they should not be confused with the plumbing vent stacks, which also protrude through the roof deck. If there are no ventilation openings, note the condition on your worksheet and consider installing some.
A flat roofing system must have a watertight covering, not one that just sheds water, to protect the area below the roof from water penetration. The most commonly used flat roof coverings are built-up, single-ply, roll roofing and metal. When performing a flat roof inspection, look for ponded water, because unless the roof was specifically designed to hold standing water as an energy conservation method, its presence is a potential problem. Ponded water can become a breeding place for insects, and can also favor the growth of vegetation and fungi. The roots of plants growing on the roof can puncture a asphalt covering, while if the ponded water freezes it can delaminate the roof covering. Moreover, the temperature difference of the wet and dry areas on a randomly ponded roof results in differential expansion that might cause warping and cracking of the roof cover. If you find ponded water or signs of past ponding, note the location in your worksheet as an area that should be drained.
If properly designed, a flat roof should have provisions for drainage. Two basic drainage designs are used in a flat roof: the perimeter system and the interior drainage system. In the perimeter system, water drains from the interior portions of the roof, and then collects in gutters or scuppers located along the perimeter, allowing it to flow into downspouts or to drip off the roof. In the interior drainage system, drains are located in the roof itself and are connected to downspouts that run through the interior of the structure. During your roof inspection, look for one of these drainage systems. The interior roof drains are often clogged with debris and are sometimes set higher than the surrounding area, which results in ponding. Check for cracks around the joints between the roof drain and the roof covering.
Also, during a flat roof inspection all roof-mounted structures and projections such as hatch covers, skylights, chimneys, vent stacks and other should be checked as well.
The key to avoiding problems and costly repairs is regular inspection and proper roof maintenance. Flat roofs should be checked at least three times a year, and even more often if you have a lot of trees nearby, because leaves and debris can clog drains. During the winter, perform a flat roof inspection after heavy snowfalls, particularly during a thaw and make sure ice or snow are not blocking drains causing a buildup of water.
To extend the life of your roof system, consider applying reflective aluminum paint, which prevents sunlight from breaking down the roofing material and heating up the building. Once the flat roof reaches the end of its life expectancy, seek a qualified contractor to strip and replace the roof.
The four most common types of flat roof systems, in order of increasing durability and roofing prices, are roll asphalt, single-ply membrane, multiple-ply or build-up, and flat-seamed metal. The roofing cost ranges from $2 per square foot for roll asphalt or single-ply roofing applied on an existing roof to $20 per square foot or more for new metal roofs.