The most commonly specified sheet materials are self-adhering rubberized asphalt membranes. These 60-mil-thick membranes are composed of rubberized asphalt laminated to a waterproof polyethylene film. The asphalt side is incredibly sticky but is covered by a release paper, which you remove during application.
The first time you work with this material, it'll drive you crazy because it sticks to everything. But you'll be surprised how fast you can move with it once you develop a rhythm. It takes two people, one on top to smooth it out and stick it down, the other on the bottom to pull off the paper. You'll have many details to learn about surface preparation, priming, patching, joint treatment, terminations, lap joints, penetrations, and corners.
Because they're so sticky, these membranes can be pretty unforgiving. Once a piece is down, you won't get it back up again at least not in reusable condition. However, the system allows easy repairs of holes, fishmouths, puckers, and wrinkles. You'll patch holes or damaged areas with a piece of membrane placed right over the first layer. With a fish-mouth or wrinkle, all you do is slit the raised area, press it down flat, and cover it with a patch.
A chief advantage of sheet membranes is their consistent thickness. Because they're manufactured to exacting tolerances, you can be sure of the 60-mil coverage. These membranes also have good elongation.
A higher in-place cost is one of the main disadvantages of sheets. The cost of the material itself is likely to be greater on a square-foot basis than the liquid membranes. Labor cost is also higher, because of all the cutting, handling, reinforcing, and detailing you have to go through during installation.
Not everyone agrees, however, that sheet membranes lessen the quality-control risk. A rubberized liquid forms a continuous, seamless coating, whereas a sheet membrane results in many seams, with the potential for a poor seal. If you're applying these materials, make sure your lap joints are tight and properly detailed. Make correct use of the manufacturers mastic or other accessories. For example, one manufacturer requires you to apply a bead of mastic to every lap joint within 12 inches of a corner when using its product.