Flat roofs should always have a slight pitch for quick drainage of rainwater, but sunken areas that create puddles can appear from time to time. When these sunken areas appear, it's important to examine the problem carefully and determine if it is related to structural damage. You can easily raise these sunken areas with a range of liquid and solid roofing materials, depending on the material of your existing roof. Options include roof putty, felt or asphalt mixes.
A variety of liquid patching materials are appropriate for fixing low spots on flat roofs. Roof patch putty can be applied, using a trowel, to fill low areas. You can also use a general epoxy concrete filler and apply mastic on top, using a trowel. Mastic is a flexible sealant that adheres to almost any material. If there is gravel on the roof, move it aside to reach a flat surface for these kinds of applications. Flexible mastic, applied with a trowel, may be used on its own to fill and resurface an entire roof. Always review the manufacturer guidelines for the best application of these products.
Another method for fixing low spots is to apply new roofing plies to the problem spot. Old-style flat roofs were often designed with pieces of asphalt felt paper sandwiched between tar. In these kinds of felt roofs, it is possible to fill low spots with two or three layers of felt, applied with tar or another bitumen. Use the type of felt most compatible with your roof, selecting from asphalt-infused felt, polyester felt, tar-saturated felt or another kind. Use asphalt, gravel or cap sheet on top of the felt, if necessary, and follow manufacturer instructions for the particular material. Newer flat roofs, on the other hand, are often made of rubber. You can use the same rubber roofing material by cutting it to contour it to the size of the problem area, then bonding it to the roof with bitumen.
Another option for filling low spots is to use asphaltic fills. These are mixtures of sand, perlite or other lightweight aggregates, combined with asphaltic material. You can apply these fills in combination with a liquid asphalt roof coating that protects and waterproofs the roof. This type of application is ideal for flat roofs made of asphalt, metal or concrete. Cover the application with gravel or tar to fit the existing roof composition.
Causes and Warnings
The most important step when fixing dips in a flat roof is to determine the cause of the problem. Weakened sheathing, rotting joists and sagging insulation are some possible causes. If structural damage or decomposition is to blame, it may be best to tear out the section of roofing and repair the problem before filling the hole. The appearance of puddles on the roof could also be caused by drainage problems, so make sure outlets are cleared and drainage systems are functional. Never use tile to fill low spots in flat roofs. Tile is designed for sloping roofs and will not adequately move water from the problem spot.