Parts of a Flat Roof – Parapet, Membrane, Flashing, Drain

All 13 parts on the diagram

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Start here: 6 main parts 6

Parapet The part of an outside wall that rises above the roof.
Coping A cap on top of a wall that sheds water.
Roof drain A drain set into the roof surface, under a strainer.
Roof joist One of the level timbers that carry a flat roof.
Roof deck The flat layer of boards or panels under the roofing.
Roof membrane The sheet covering that makes the roof waterproof.

The roof from above 1

Scupper An opening through the parapet that lets water drain off.

The roof cut through 6

Vapor barrier (vapor retarder) A sheet between the deck and the roof insulation.
Roof insulation Boards of insulation laid under the roof membrane.
Gravel It holds the roofing down and shields it from the sun.
Cant strip A sloped strip in the corner that eases the roofing's turn upward.
Base flashing A strip of roofing carried up the wall from the roof.
Counterflashing Metal let into the wall that laps over the base flashing.

How this list was compiled: Read 2026-09-30; a name is kept only where two of these use it. (1) Canada Mortgage and Housing Corporation, Canadian Wood-Frame House Construction, https://archive.org/details/CANADIANWOODFRAMEHOUSECONSTRUCTION : built-up roofs (layers of roofing felt sealed with tar or asphalt, covered with gravel, which is ballast against wind uplift and shields from the sun); the roofing, mopped to the cant strip, turns up the wall; membrane flashing (base flashing) and the counter-flashing that laps over it; roof membrane; roof joists; copings. (2) L. O. Anderson and O. C. Heyer, Wood-Frame House Construction, US Department of Agriculture, https://archive.org/details/WoodFrameHouseConstruction : roof joists for flat roofs laid level; a parapet wall carried above the roof; built-up roofs over a wood deck, covered with gravel; counterflashing. (3) US Naval Education and Training Command, Builder 3 & 2, https://archive.org/details/ERIC_ED210471 : built-up roofing; roof decking; a vapor barrier between the decking and the roof insulation; counterflashing. (4) US National Bureau of Standards, A Glossary of Housing Terms, https://archive.org/details/glossaryofhousin91subc : coping (a capping at the top of a wall, serving to shed water); parapet wall (the part of an exterior wall extending above the roof line). (5) Unified Facilities Guide Specifications UFGS-07 52 00 and UFGS-07 60 00, https://www.wbdg.org/FFC/DOD/UFGS/UFGS%2007%2052%2000.pdf and https://www.wbdg.org/FFC/DOD/UFGS/UFGS%2007%2060%2000.pdf : roof deck, vapor retarder, roof insulation below the membrane, cant strips, base flashing, counterflashing inserted in reglets, scuppers, copings, roof drains. (6) Wikipedia: Flat roof (any sheet used to cover a flat roof is usually known as a membrane; roof drains or scuppers remove ponding water; gravel), Parapet (a wall carried up past the roof's edge), Scupper (an opening in the side walls that lets water drain and not pool within the walls of a building), Coping (architecture), Membrane roofing. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. Licence: own work

How it works

A flat roof is built in layers, and most of its named parts sit at the edge, where the roofing meets a wall. Tap a part to see its name.

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From above

The parapet is the wall carried up past the roof, and the coping caps it. Rain leaves through a roof drain set in the surface or through a scupper, an opening in the parapet.

Cut through

Roof joists carry the roof deck. On the deck lie a vapor barrier, the roof insulation and the roof membrane, which keeps the water out. Gravel on top holds the roofing down and shades it. In the corner a cant strip eases the turn up the wall; base flashing climbs the wall and counterflashing laps over its top edge.

This is a built-up roof with a parapet. A flat roof with no parapet ends in a metal edge called a gravel stop.

Questions and answers

What are the parts of a flat roof called?

From the bottom up: roof joists, roof deck, vapor barrier, insulation, roof membrane and often gravel. At the edge are the parapet and its coping, the cant strip, base flashing and counterflashing. Roof drains and scuppers take the water away.

What is the wall around a flat roof called?

A parapet. It is the part of an outside wall that rises above the roof line. The cap along its top is the coping, which sheds water off the wall.

What is a scupper on a flat roof?

An opening through the parapet at roof level. It lets rainwater drain off the roof instead of pooling inside the walls, often into a downspout on the outside.

What is the difference between base flashing and counterflashing?

Base flashing is roofing turned up the wall from the roof. Counterflashing is metal let into the wall above it that laps down over the top of the base flashing.