Today we’re talking flat residential roofing, and specifically, how to get them to stop leaking.
Now, why would anyone choose to have a flat roof? Well, like the lawn dart (banned in 1988, after having skewered more individuals than those wounded at the Battle of Grunwald in 1410), and clacker ball toys (banned in 1985, after it was discovered that with vigorous clacking, the resulting explosion was second only to the M67 fragmentation grenade) it seemed like a good idea at the time. Theoretically, flat roofs aren’t a bad idea, and are normally the least complicated way of covering a carport or back porch. Plus, the material cost of flat roof joists are often cheaper than a peaked, or truss system of roof. And, with a little added engineering, a flat roof can be turned into an outdoor terrace, providing a bird’s eye view of the neighborhood.
Having living space on the roof can also be quite private, unless you live by the airport, and provide a unique type of entertaining area for guests. Flat roofs rarely leak because they can’t disperse water properly. After all, flat roofs are like a table top, allowing precipitation to simply flow over the edge into an eavestrough type of system. So, even snow, which melts eventually, shouldn’t be an issue. However, in our part of the world, we get freezing rain, followed by a 24 hour thaw, then 20 centimeters of snow, with a 10 day severe cold spell to wrap up a typical January month end. When that happens, the snow melt buried under the exterior crust will pool in the middle, usually for several weeks, providing a true test of your flat roofing membrane. Eventually, and after years of pooling, the water will make its way through.
So, how does a homeowner with a flat roof avoid the inevitable? By using the best in materials, and by providing adequate drainage. First, a flat roof, or even one that is slightly sloped, requires a solid plywood base. So, if you’re repairing or replacing an existing roof, remove all asphalt or granular roofing materials that are presently on the roof.
Never apply modern day products over existing materials. One, you’re leaving two Volkswagen Jetta’s worth of material weight on the roof, which will only lead to roof sag, more water pooling, and eventual leakage. And two, the planks or plywood under this existing roof could be in lousy condition, or even close to rotting if the leaking has been ignored for some time. Left unchecked, adding a couple of tons of new roofing material to a flat roof with a weakened joist system, could make things really uncomfortable for the fellow in the top bunk when that first heavy snowfall hits.
With the old materials cleaned off the roof, check the condition of the underlay material. If you discover the underlay to be a series of 1×6 or 1×8 planks, replace the ones that have cracked or rotted, then cover the planks with a ½ inch thick plywood sheeting. Roofing materials, whether it be steel, asphalt shingles, or flat rubber membranes, absolutely require plywood as an underlay.
Next, you’ll be applying a two part roll roofing product such as the Henry Bakor Duratac system. The system consists of first installing what’s referred to as a base sheet, which is a rubberized, self-sticking, 39”x65 ft. roll-on membrane that gets applied directly to the plywood. If applying the base sheet in late fall or early spring, first apply a primer to the plywood to help adhesion.
Next, apply the cap sheet, which is essentially the same type of self-sticking roll as the base sheet, except the cap sheet has a granular surface to effectively defend against the elements. Further keys to a successful flat roof application? Use a heavy roller to effectively seal the membranes to the plywood, and each other. Plus, avoid leakages due to pooling by properly flashing around chimneys, plumbing stacks, and everywhere the roof meets a ledge or wall.