01 May 1999
(a) For mechanically anchored systems both expanded and extruded polystyrene foam
insulation may be used. However, an overlay of rigid fiberboard is recommended to avoid relaxation
of the foam with the loss of screw compression.
(b) Plasticized PVC should not be used in direct physical contact with EPS insulation,
asphalt, coal tar pitches, petroleum products, or where coal tar fumes are present. Approved
separator/slip sheets are recommended.
b. Climate/Weather. Thermoplastic sheets require clean, dry substrates during application.
Excessive wind can make application of the light, flexible sheets difficult. Hot air welding may
compensate for damp or cool membrane conditions. Automatic seam welders that monitor the
temperature of the weld help insure proper welds.
c. Logistics. Weldable thermoplastic rolls are usually not shipped in rolls wider than 2 m (80 in.),
although a few products are custom fabricated into large roof panels. Rolls or panels should be
protected from distortion by following the manufacturer's recommended storage and hoisting
procedures. Welding machines require adequate and consistent voltage. Generators provided by the
contractor may be required rather than using building power.
d. Familiarity with the System and Site. Weldable thermoplastic membranes are understood by
most commercial roofing contractors. Only manufacturer-approved contractors should install these
systems, and all components should be from a single source. Since deck primers and adhesives may
use toxic or flammable solvents, avoid fume intake into the building's air handling system. Heat
welding can also create noxious odors.
e. Life Expectancy. Well constructed weldable thermoplastic systems typically last 15 years or
more. Some flashing maintenance and repairs are to be expected during this period. Past problems
with membrane shrinkage and embrittlement are no longer of great concern.
f. Costs. Weldable systems are cost effective on both initial and life cycle bases. While
materials are more expensive than asphaltic roofing, labor is generally less.
7-2. BUILDING ELEMENTS.
a. Slope. A minimum slope of 2% (1/4 in./ft.) is required.
b. Low-Sloped Roofs. Moisture and vegetation attack can occur where water ponds and roots
grow. Properly welded seams are resistant to moisture attack.
c. Steep Roofs. Mechanically attached and fully adhered systems have been used at all slopes.
Mechanically attached systems should not be used on steep roofs in windy areas.
d. Structural Considerations. Mechanically fastened membranes are subject to wind flutter and
increased fastener density is required at corners and perimeters. The deck must be suitable to receive
fasteners. Anchorage of nailers must meet FM 1-49 (Perimeter Flashing) recommendations.
e. Expansion Joints, Seismic Joints, and Area Dividers. Joints should be located at high points
where practicable and placed on curbs above the water line. Expansion joints should be provided only
at each expansion joint in the structure. Structures need expansion joints at intervals not over 60 m
(200 ft) in length or width. Joints should also be used at changes in deck direction or membrane
materials. If a structure does not contain enough expansion joints it is inappropriate to solve the
problem by just adding expansion joints to the roofing system. Area dividers do not provide for
movement but are useful in subdividing big areas into isolated, smaller areas, each of which can be
roofed (and maintained) separately.