How to Flash TPO Walls, Curbs and Corners: Heights, Corner Patches and Terminations

Step by Step

1. 1Step 1: Prepare the wall or curb surface

   Flash to a sound substrate. On wood-framed walls this is exterior-grade plywood or a cover board. On concrete or masonry, it is a primed, clean, dry surface. Sharp edges, protruding fasteners and mortar ridges cut membrane and are removed. Check that the wall is wide enough to carry the flashing, the termination and whatever covers them.

   Install a cant strip at the base where the manufacturer or designer calls for one, so the membrane does not make a sharp 90 degree fold at the roof to wall junction. A cant softens the bend, and it keeps the membrane from being stressed by thermal movement.

2. 2Step 2: Set the flashing height

   The widely used minimum height for wall and curb flashing is 8 inches above the top of the membrane in the field, measured from where water will actually stand. Some codes, manufacturers and designers require more, and curbs on roofs with snow loads, drifting or low slopes often do. Confirm the requirement on the project documents.

   Plan what covers the top edge before you start. The termination is always covered: by counter flashing set into a reglet, by the wall cladding, by coping, or by an overlapping metal flashing. A termination bar left uncovered in the weather is not a finished detail.

3. 3Step 3: Adhere the flashing to the vertical surface

   Cut a strip of flashing membrane about the height plus at least 2 to 3 inches extra at the base, so it laps onto the field sheet. Flashing is usually unreinforced or reinforced TPO as specified, and is adhered to the wall with the approved adhesive, following the same tack and flash-off rules as in how to install fully adhered TPO.

   Roll the membrane up the wall from the bottom to the top, broom or hand roll it tight and keep it plumb. Keep adhesive out of the weld zone at the base, where the flashing will be welded to the field sheet. A flashing adhered over an adhesive-coated lap will not fuse.

4. 4Step 4: Weld the flashing to the field sheet

   The wall flashing is lapped onto the field membrane at the base. Clean both surfaces, test the welder on scrap and weld the lap with a steady pass, as described in how to hot-air weld TPO seams.

   Weld all the way to the end of each piece of flashing. A short unwelded length at the end is a common channel for water, and ends of flashing pieces are where patches and overlaps are needed. Probe the full seam when it has cooled.

5. 5Step 5: Form the inside and outside corners

   An inside corner is where two walls meet at the roof, or where a wall meets a curb. Weld a pre-molded inside corner over the base and up both walls, and weld the vertical flashing pieces to the tabs. Pre-molded corners are the standard because hand-forming membrane into a tight inside corner leaves folds.

   An outside corner is where a wall or curb turns away, for example at the corner of a curb. Use a pre-molded outside corner, or fold the membrane with a formed patch. All corner patches should have rounded edges, because a sharp corner lifts. Weld every edge, then probe. If a preformed piece for your size is not available, the manufacturer can specify a field-formed detail.

   Watch out: Do not fold flashing membrane and leave the fold unwelded. A sharp, unwelded fold at an outside corner is a channel that collects water and is the most common flat roof corner leak.

6. 6Step 6: Cover every T-joint with a round patch

   T-joints happen wherever three layers of membrane meet in a step, such as where a vertical flashing seam lands on a horizontal field seam. The step leaves a tiny channel beside the welded edge that a normal weld cannot close. A patch, typically 3 inches or larger in diameter depending on the manufacturer, covers the step.

   Clean the area, round the corners of the patch with scissors, and weld all the way around the edge. Do not try to trowel sealant into a T-joint in place of a patch. A patch is a weld, and sealant is not.

7. 7Step 7: Terminate the top edge with a bar and sealant

   A typical termination bar is aluminum, about 1 inch wide, fastened through pre-punched slots at roughly 6 to 8 inches on centre into sound substrate, with the top of the membrane pulled tight beneath it. The bar holds the membrane against sag and peeling, and stops wind getting behind it.

   Apply a continuous bead of manufacturer-approved sealant along the top edge of the membrane and over the bar, tooled smooth. Sealant here is a secondary barrier. The wall cladding, counter flashing or coping above is the primary one, and it is set over the top so water runs over it and not into it. See how to install counter flashing.

8. 8Step 8: Probe and inspect every detail

   Go back to every hand weld and probe it along the edge. Look for lifted edges at the tops of corners, for wrinkles in the flashing that have bridged the wall, and for any spot where the membrane has pulled away from the wall between fasteners.

   Curbs get extra attention: check that water can leave the upslope side of the curb and is not trapped behind it, and that the curb flashing has a proper counter flashing or cap. See how to flash a curb on a flat roof.

Wall, Curb and Parapet Differences

Movement and Wind

Common Mistakes

• Flashing too low. Less than 8 inches puts the top edge where splash, snow and standing water reach it.
• Unwelded T-joints. They leave a channel at the step between layers that leaks at the first heavy rain.
• Sharp folds at corners. Hand-folded membrane with unwelded creases collects water and opens under movement. Use pre-molded corners.
• Sealant in place of a weld. Caulked laps and patches crack within a few seasons and have no lap to fall back on.
• No termination bar or no cover. An unpinned top edge sags and lets wind and water behind the membrane.
• Adhesive on the weld zone. It prevents fusion at the base of the flashing and leaves a cold seam.