Which modified bitumen sheet is modified with a plastic and is most commonly installed by torch?
- SBS modified bitumen
- EPDM membrane
- Type IV ply sheet
- APP modified bitumen
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D. APP modified bitumen
APP (atactic polypropylene) gives the bitumen plastic-like properties and a higher softening point suited to torch application. SBS is rubber-modified and is usually mopped, set in cold adhesive, self-adhered, or torched only when made for it.
Reference: NRCA Roofing Manual: Membrane Roof Systems
The SBS modifier in a modified bitumen sheet gives the asphalt:
- a hard, plastic-like surface finish
- rubber-like stretch and recovery
- resistance to cooking greases
- the ability to be solvent welded
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B. rubber-like stretch and recovery
Styrene-butadiene-styrene is a synthetic rubber, so SBS sheets stay flexible in cold weather and stretch and recover with movement. Plastic-like behavior is the mark of APP products.
Reference: NRCA Roofing Manual: Membrane Roof Systems
Field seams in EPDM membrane are made with:
- seam tape or adhesive on primed laps
- a hot-air welder and silicone roller
- hot asphalt mopped between the laps
- an open-flame torch run along the lap
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A. seam tape or adhesive on primed laps
EPDM is a thermoset (cured) rubber that will not melt and fuse, so laps are joined with splice tape or adhesive after cleaning and priming. Hot-air welding works only on thermoplastics such as TPO and PVC.
Reference: NRCA Roofing Manual: Membrane Roof Systems