I’ve been a Los Angeles roofing contractor for well over a decade and in that time I think I’ve had more questions or confusions concerning flat roofing than any other roofing. The two most commonly used and least expensive flat roof types are known as torchdown and hot tar.
Hot tar is just that, it is hot tar, or more correctly stated, hot asphalt. It starts as a 100 lb keg (slightly larger than a 5 gal water bottle), gets chopped up and tossed into a large hot kettle to melt. This is then literally mopped out onto a membrane to form a waterproof barrier.
Hot tar is also referred to technically as BUR, or Built Up Roofing. We say “built up” because usually you apply a few membranes with the hot tar and build up a system.
Ordinarily the only common thing between torch and hot tar is the base sheet which is a grey, fiberglass, lightly sanded membrane that is nailed in place with nails that have a large metal washer built into them to prevent the membrane from tearing and helps hold it in place. This membrane is called a fiberglass basesheet, or just “basesheet.” Roofers refer to it as 28lb – its weight per 100 square foot, the standard in roofing materials.
Torchdown is the same principle, except that the hot tar is built into the membrane itself and it is melted with a flaming torch. Torchdown is rubberized and in that sense, is a better product than hot tar but better is a relative term here.
Which is better? It really depends on the application. With torchdown, you get just one shot to get it right since there is just the one membrane. With hot tar, there are at least two membranes and therefore a modicum of redundancy.
There are two kinds of hot tar roofs. Capsheet (the smooth white membrane called capsheet – the most common sort) and gravel or “rock”. If your roof is dead flat, then you really must use hot tar for its redundancy and in fact, it should be gravel (rock). The reason is fairly simple. A gravel roof is finished different than a smooth white hot tar roof, known as capsheet. With gravel, the final process is to flood a layer of the hot tar with the rock, making a solid, seamless surface which is more impervious to pooling water.
In Los Feliz area of Los Angeles, flat roofs tend to last longer than cities with harsher weather, but it is important to have your roof inspected regularly to see if the time has come to replace your roof.
