How to Build a Roof on a Commercial Building

Install structural metal decking. This material looks like corrugated plastic sheeting, often seen on shed roofs, but is made from bent sheets of metal. The corrugation adds strength and rigidity to the material. It is installed by welding to the structural beams or joists that hold up the roof. Spot welds at 12-inch increments are usually adequate.

Install layers of rigid foam insulation. These layers are made up of sheets of rigid foam that not only insulate the space below, but also help structure the roof assembly. The foam layers are held to the deck and to each other with construction adhesive. The roof membrane that caps the system will actually hold the foam down; the construction adhesive used in this step is simply to ensure the boards stay in place during installation.

Taper the layers of foam using a specialized sander designed for this task. Normally, the foam will be tapered so that it slopes toward roof drains spaced at intervals across the roof surface. This will ensure proper drainage.

Install a single-ply roofing membrane. This rubber sheeting material is the outer layer of the roof assembly that provides the watertight seal as well as the durable and UV resistant outer later. It is attached using roofing adhesive applied in a continuous layer to the foam and then to the roofing membrane. Squeeze out all air pockets in the membrane using a heavy roller to ensure the membrane fully adheres to the insulation.

Install the finish materials at all edge conditions. Whether there is a parapet or high wall at the roof edge with coping, which is a metal cap that seals the parapet, or a simple gravel guard or fascia system, you have to install something that finishes the roof at the edges and provides a seal for the ends of the membrane. The membrane will typically wrap under the metal cap, and mechanically attach on the outer edge. This ensures that if wind-blown moisture was to get under the metal cap, it still cannot penetrate below the roof membrane.