01 May 1999
WELDABLE (THERMOPLASTIC) ROOFING SYSTEMS
(including PVC, CSPE, TPO)
07548 (Polyvinyl Chloride [PVC] Roofing Only)
7-1. OVERVIEW OF WELDABLE SINGLE-PLY SYSTEMS. Thermoplastics are materials that soften
when heated and regain their physical properties upon cooling. They are appealing in a roofing system
because once the seams are properly welded they are waterproof and permanent. Among the earliest
thermoplastic polymers used in membrane roofing were Poly(vinyl)chloride (PVC), Chlorinated
Polyethylene (CPE), and Chlorosulfonated Polyethylene (CSPE or Hypalon). More recently the
weldables have been expanded to include thermoplastic polyolefins (TPO's), ketone ethylene esters
(KEE) and various polymer blends. To date, ASTM has published two thermoplastic single ply material
specifications: D 4434 for PVC material and D5019 for Hypalon CSPE and Polyisobutylene ((PIB).
Two recommended practices, D5036 (Application of Fully Adhered Single-Ply PolyvinylChloride Roof
Sheeting), and D-5082, (Application of Intermittently Attached Single-Ply Poly(vinylchloride Sheet
Roofing) are also available. Other material standards and recommended practices for thermoplastic
roofing are in preparation. During the manufacture of weldable single-ply membranes thermoplastic
polymers are blended with pigments, stabilizing agents, plasticizers, and processing aids, laminated
with reinforcements and formed into machine-width sheets. They are then rolled up as relatively
narrow rolls, or in a few instances, factory joined into large tarpaulins and shipped to the roofing site
ready to be installed. In the field they are unrolled over a suitable substrate, allowed to relax to
eliminate winding stresses, seamed by heat fusion, flashed, and are immediately ready for the
weather. Because of their flexibility they can be mechanically attached or fully adhered. Loose laid
systems, while once quite popular (using unreinforced sheets), are rarely used today. Most of the
weldable thermoplastic roofing systems are compounded to white or other light colors to reduce solar
maintain reflectance and nonchalking pigments for greater durability. Thermoplastics may be
recyclable. Current thermoplastic systems have internal scrim reinforcement. Typical thickness of the
membrane sheet is 1.14 mm (0.045 in. [i.e., 45 mil] although, products range from 0.91 mm (0.036 in.)
to 2.4 mm (0.095 in.). The minimum coating or laminate thickness above the cross points of fabric
should be 0.40 mm (0.016 in.).
a. General. While prefabricated single-ply membranes are waterproof when manufactured they
should be sloped to drain. Water flow should be away from field seams.
(1) Installation. Key steps in installation include hoisting the rolls to the roof, unrolling,
allowing the membrane to relax, and then:
(a) Attaching to the substrate with adhesive, seaming sheets together and flashing, or
(b) Mechanically attaching the membrane to the substrate, seaming and flashing.
(c) Application instructions are provided in industry literature (ASTM D5036, D5082) and
should be verified by field quality assurance.
(d) Seam welds are accomplished with hand or machine hot air welding. Solvent welding
that was popular in the past is no longer common. Welds should be tested periodically during
installation to verify that they are adequate.
(2) Substrates. Suitable substrates for adhered systems include solvent resistant thermal
insulation boards (i.e., not polystyrenes) and most flat decks (plywood, OSB, structural concrete).