If the re-roofing involves removing the existing roofing New England states require a certain amount of insulation to be installed. Generally the insulation can be directly covered with a waterproofing layer. If people and machines regularly utilize the roof it is advisable to further protect the insulation with a coverboard that is a more dense material than the insulation. Material like DensDeck or Firestone’s High Density ISOGARD or even plywood may be installed. This is also advisable if solar panels are being installed. If the existing roofing or some layers of existing roofing are being roofed over and receive high traffic this material may also be used. There is some thought that dense material directly underneath the waterproofing layer will help prevent punctures.
If a new waterproofing layer is being installed on existing roofing it is typical for a manufacturer to require a separation layer between the old roofing and new waterproofing layer. This requirement can depend on the type of existing roofing being covered. Single ply roofing cannot be laid directly on an asphalt and gravel roof. There are a few choices for this separator layer – 1″ isocyanurate, 1/2″ fan board (extruded polystyrene) and fiberboard are the most common. We do not recommend fiberboard (see insulation section). Certain types of single ply membrane (called fleece-back or felt-back) come with a mesh layer already attached to the bottom of the membrane which can serve as the separator layer as well as strengthen the membrane.