01 May 1999
constructions will also use thermal insulation. Vapor retarders are sometimes required to protect
the roofing system from attack by interior moisture.
a. Built-up Roofing (BUR). BUR Consists of multiple reinforcements such as asphalt treated
glass or organic felt laminated together with hot-applied bitumen (asphalt or coal tar pitch) or cold
adhesives (figure 2-4). Surfacings include aggregate, coatings, capsheets, and sprayed roofing
granules. A typical system includes thermal insulation and may include a vapor retarder.
b. Polymer Modified Bitumen. MB consists of reinforcing sheets factory-coated with polymer
modified bitumen. They may be laminated in the field using hot bitumen, heat fusion, or by cold
adhesives (figure 2-5). Surfacings include capsheets with mineral granules, metal foil, and field
c. Elastomeric (Single-Ply) Membranes. Elastomeric membranes consist of a factory-
produced sheet generally of EPDM rubber with seams field-sealed with adhesive or tape (figure
2-6). Sheets are unsurfaced unless ballast is used. Elastomers are vulcanized (thermoset), and
usually non-reinforced except when used in mechanically fastened systems. A fleece-backed
sheet is also available for fully adhered systems when it is desired to use hot bitumen as an
adhesive (e.g., for re-covering an asphalt-contaminated deck or old BUR).
d. Weldable Thermoplastic Membranes. These membranes consist of a sheet of reinforced
thermoplastic material such as PVC or TPO. Sheets are unsurfaced or ballasted. Seams are
generally heat fused although solvent welding and adhesive bonding are also possible (figure 2-
e. Structural Standing Seam Metal Roofing. SSSMR consists of metal panels with raised
seams more than 37 mm (1-1/2 in.) high (figure 2-8). Sealants are utilized at side seams and
endlaps to provide waterproofing. Most are considered hydrostatic, resisting standing snow and
occasional ponding. Caution: ridges and valleys of a SSSMR may not be as watertight as the
f. Sprayed-in-Place Polyurethane Foam. SPF is a thermoset rigid foam, field-formed by the
reaction of liquid components (in the presence of a foaming agent) sprayed onto the substrate.
SPF is protected by liquid-applied elastomeric coatings or an application of loose gravel (on
slopes < 4%).
g. Components of Membrane Roofing Systems (figure 2-9).
(1) The deck supports roofing loads and is selected to conform to fire resistant design
classifications. Not all systems require a deck (e.g., structural standing seam metal roofing).
(2) A vapor retarder protects the insulation against moisture vapor attack from the
warm, high vapor pressure side of the roof assembly. Not all buildings require a vapor retarder
(See General Considerations for Roofs, CRREL Misc. Paper 3443 and CEGS 07220).
An air barrier prevents air movement (infiltration or exfiltration) through the roofing
components beneath the insulation. It also furnishes support and a smooth, continuous substrate
for the membrane.
(5) The membrane is intended to keep water out of the components below (as well as
out of the building). The membrane system affects fire resistance.