Anatomy of a Commercial Roof: Terms You Need to Know
By High Country Roofing · Family-owned commercial roofers in Emmett, Idaho
A commercial roof is an assembly of layers, not a single material. Knowing the parts makes an inspection report or a bid much easier to follow.
The layers
- Deck. The structural base, usually steel, wood or concrete. If the deck is rusted or rotted, nothing on top will fix it.
- Insulation. Board insulation, often polyiso, adds R-value. Wet insulation loses its value and usually has to be replaced.
- Membrane. The waterproof layer: TPO, PVC, EPDM, built-up, modified bitumen or a coating.
The details where leaks start
- Seam. Where two sheets of membrane join. TPO and PVC seams are heat-welded; EPDM seams use tape or adhesive.
- Flashing. The waterproofing at walls, curbs, edges and penetrations, where the roof changes direction.
- Penetration. Anything that goes through the roof: pipes, vents, HVAC supports.
- Pipe boot. The rubber or membrane collar sealing a pipe penetration.
- Parapet. A wall that extends above the roof edge. The membrane has to run up and over it correctly.
- Curb. The raised frame that rooftop units sit on.
Drainage
- Drain. An opening in the roof that carries water down through the building.
- Scupper. An opening through a parapet that lets water off the roof edge.
- Ponding. Water still standing 48 hours after rain or snowmelt. It speeds up wear and adds weight.
- Tapered insulation. Insulation cut with a slope to move water toward drains.
Have a report or a bid you want explained? Call (208) 251-8203, or request a free roof inspection and we'll walk you through your roof.
Common questions
What is the difference between the deck and the membrane?
The deck is the structure that holds the roof up. The membrane is the waterproof layer on top, usually with insulation between them.
Why does flashing leak even when the roof looks fine?
Flashings sit where the roof meets walls and equipment, which is where the most movement happens. A separated flashing can leak while the rest of the membrane is sound.
Want a roofer's eyes on your roof?
Free inspection and photo report. No cost, no obligation.