Roof Edge Drainage, What is roof edge drainage

Roof Edge Drainage speaks to what is roof edge drainage and explain roof edge drainage.

It discuuses considerations for typical details using built up roof membranes but the same principals may be used for single ply membranes.

Roof Edge for a Conventional Built Up Roof

The membrane consists of four plies of felt bedded in asphalt with a gravel ballast. The base flashing is composed of two additional plies of felt that seal the edge of the membrane and reinforce it where it bends over the curb. The curb directs water toward interior drains or scuppers rather than allowing it to spill over the edge. The exposed vertical face of the metal roof edge is called a fascia.


Cover plate at joints in the roof edge.

Metal roof edge in 10 ft (3 m) maximum lengths.

Base flashing.

Wood curb.


Roof membrane.



Proprietary Roof Edge System

With this system the perforated metal strip is fastened to the roof with a mastic adhesive that oozes through the perforations to create a tighter bond. When the adhesive has hardened, a galvanized steel curb is fastened in place with the tabs of perforated metal and an aluminum roof edge is hooked on, with a backup piece at the end joints as shown to prevent leakage.

Lastly, the roof edge and the membrane are locked in place simultaneously by installing a clamping strip that engages the hook on the top of the aluminum roof edge. The clamping strip is held in place by screws that pass through the edge of the membrane into the galvanized curb, at the top.


To detail a scupper it is understood that the curb is discontinued to allow the water to spill off the roof into a gutter and downspout. Additional layers of felt, called stripping, seal around the sheet metal components. Most roofs use interior drains as their primary means of drainage, with scuppers more frequently used as secondary drainage to limit ponding in the case of a clog in the primary drain.


Metal roof edge.

A notched metal angle keeps the aggregate on the roof.

A sheet metal gutter and downspout catch and drain water.


Roof membrane.



Building Separation Joint

With this joint large differential between the adjoining parts of the structure can be tolerated. This joint allows this due to the ability of the flexible joint cover to adjust to movement without tearing. High curbs keep standing water away from the edge of the membrane which is sealed with a two ply base flashing.


Flexible waterproof expansion joint cover.

Vapor retarder.

Compressible insulation.

Wood curb and cant.

Division in building structure.

Base flashing.

Area Divider

An area divider is a component designed to allow for some movement only in the membrane itself, not in the entire structure. It is used to subdivide a very large membrane to allow for thermal movement.


Sheet metal flshing.

Wood curb and cant strip.

Base flashing.


A parapet is a low wall that projects above the roof edge with conventional counterflashing and coping. Where the roof membranes turns up onto a vertical surface and are commonly used with bituminous roofing in which the less flexible membranes and multiple plies cannot easily make a sharp right angle bend. Single ply membranes typically do not require cant strips under such circumstances.

A proprietary parapet coping system has a perforated metal channel and is fastened to the masonry with a mastic adhesive. Sections of metal coping are snapped over the channel, with a special pan element beneath the joints to drain leakage.


Cut stone or precast concrete coping.

The coping is held in place by steel dowels set in the wall by the mason and capped by the sheet metal contractor.

A drip groove which prevents water from running back under the coping.

A continuous through wall flashing prevents leakage through joints or cracks in the coping.

A continuous metal counter flashing is interlocked with the through wall flashing.

The turned up edge of the membrane is sealed to the wall with fabric and mastic.

A wood or fiber cant strip eases the bending of the membrane.

Deck, Insulation, Roof membrane and Base flashing.

Roof Drain for Roof Edge Drainage

The usual is a conventional cast iron interior roof drain for a low slope roof with two plies of felt stripping seal around a sheet metal flashing. The drain may also be a single piece model of molded plastic.


Strainer, Strippng.

Copper or lead flashing at least 30 inches (750 mm) square.

Roof membrane above deck with insulation tapered toward the drain.

Drain pipe.

Drain unit which clamps securely to the deck and to the roof membrane.

Plumbing Vent Roof Penetration for Roof Edge Drainage

This and all other edge and penetration for a flat roof use the curb, cant strip and stripping to keep standing water away from the edge of the membrane.


Wood curb and cant strip.

Sheet metal flashing.

A metal draw band which clamps the flashing to the pipe.

The top of the curb is sealed with fabric and mastic.

Base flashing.

Roof Edge Drainage

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