How to Flash a Curb on a Flat Roof: Height, Crickets, Membrane Upstands, Corners and Counter Flashing
Curbs hold up equipment, hatches and skylights, and they stand in the way of water. Raise them, build a cricket behind them, and flash every corner.
- Professional
- 3 to 8 hours per curb
- Updated
The short answer
Flash a flat roof curb by building it at least 8 inches above the finished roof surface, adding a cricket behind any curb wider than about 30 inches, running membrane up all four sides as an upstand, forming each corner with a molded or welded corner piece, terminating the top edge under counter flashing or a cap, and leaving room around the curb to work and probe.
Anything that sticks up through a flat roof, such as a rooftop HVAC unit, a roof hatch, a large vent or a skylight, sits on a curb: a short box framed above the roof surface. The curb lifts the opening above the water running across the roof, and the membrane is turned up its sides so water cannot get under the equipment. It is the standard way to handle any penetration too large for a pipe boot.
Curbs are also obstructions. A wide curb stands across the path of water, and water runs up against its upslope side and stops. Without a cricket to send it around, a pond forms against the curb and sits against the flashing, which is where many curbs fail. The flashing has to be built for a full-time wet environment at the base.
This guide covers curb height, crickets, membrane upstands, corners and terminations. The membrane details are shown for TPO in how to flash TPO walls, curbs and corners. For skylights on a flat roof, see how to install a skylight on a flat roof.
Tools
- Tape measure, level and chalk line
- Hot-air welder or torch, depending on the membrane
- Silicone roller
- Screw gun
- Caulking gun
- Hook blade utility knife and scissors
- Metal snips and hand seamer
- Rounded-tip probe
Materials
- Curb framing, typically treated lumber or manufactured curb, with cant strip
- Plywood or cover board cladding for the curb sides
- Membrane flashing in the roof system
- Pre-molded inside and outside corners where available
- Termination bar or counter flashing
- Cricket boards of tapered insulation
- Sealant compatible with the membrane
- Metal cap flashing
Safety first
- Curbs are often near open hatches, mechanical openings and edges. Tie off, keep hatch covers closed and guard openings before working beside them.
- Equipment on a curb can be heavy or energized. Coordinate with the trade responsible for it, and lock out power before flashing near an electrical or gas unit.
- Heat from welders and torches near equipment and fuel lines is a fire and burn risk. Keep a fire extinguisher close and keep hot work away from vents and exhausts.
Step by Step
Step 1: Confirm the curb height and construction
The widely used minimum height for a curb is 8 inches above the finished roof surface, measured from where water will stand. Some designs, manufacturers and codes require more, and curbs on roofs with snow drifting or low slope often do. Confirm the requirement on the project documents.
The curb is framed to the roof structure, not just to the insulation, and it is flat, plumb and square. Cover the sides with exterior plywood or cover board, and install a cant strip at the base so the membrane does not make a sharp 90 degree fold. Keep curbs about 12 inches from walls and from each other so there is room to work and probe.
Step 2: Build a cricket behind the curb
A cricket is a small, sloped ridge of tapered insulation that splits water and sends it around an obstruction. On the upslope side of a wide curb, water builds up against the flashing with nowhere to go. As a common rule, curbs wider than about 30 inches measured across the slope need a cricket, but confirm the requirement with the designer.
Crickets are typically built at about 1/2 inch per foot. The cricket edges must meet the roof surface cleanly so the membrane lies flat over it. See how to install tapered insulation and crickets.
Step 3: Install the field membrane up to the curb
Lay the field membrane as normal, cut neatly at the curb base. Do not try to wrap the curb in the field sheet, because the field sheet is fastened or adhered to the deck and cannot stretch around the curb without folds. The upstand is a separate flashing piece, welded, torched or adhered to the field.
Keep the field membrane at least 2 to 3 inches out from the curb face where the flashing will lap onto it, and keep the area clean, dry and free of adhesive where a weld will be made.
Step 4: Adhere the membrane upstand on each side
Cut a piece of flashing membrane for each side, or one long piece, with extra at the base for the lap and enough at the top to terminate. Apply adhesive to the curb side and the back of the membrane, let it flash off, and roll it on without wrinkles, as in how to install fully adhered TPO. Bonding on a vertical surface needs care, so work in sections.
Weld, torch or adhere the lap at the base to the field membrane, at least 1-1/2 inches wide for single-ply, and probe every edge. Keep the flashing relaxed at the cant.
Step 5: Form the corners
Curb corners are outside corners, and the membrane must go around them without a crease. Pre-molded outside corners are made for this job. If none fits, form the corner with a patch of flashing membrane cut to shape, with rounded edges, and weld every edge.
A sharp fold left unwelded at an outside corner is the most common curb leak. Every point where three layers meet needs a patch, typically 3 inches or larger with rounded corners. See how to hot-air weld TPO seams for testing and probing.
Watch out: Do not cut a curb flashing to fit and leave the corner open, expecting sealant to close it. Sealant is not a substitute for a welded or torched corner.
Step 6: Terminate the top edge and add counter flashing
Terminate the membrane at the top of the curb, or at least 8 inches above the roof surface, with a bar fastened about every 6 to 8 inches and a bead of sealant. The bar holds the membrane against the curb so wind and sag cannot open it.
A metal cap or counter flashing then covers the termination. The cap should overlap the membrane by at least 3 inches on each face and be sloped away from the equipment. The equipment's own flange or skirt often serves as the cover, so check the manufacturer's detail. See how to install counter flashing.
Step 7: Probe, water test and check drainage
After everything has cooled, probe the lap at the base, the corners and every patch. Mark and fix any spot that lifts. Then hose the roof above the curb and watch: water should reach the cricket and run around both sides toward the drains.
If water stops anywhere beside the curb, the cricket or the slope needs adjusting. Do it now. A curb that holds water will leak at the base sooner or later. See how to plan flat roof drainage.
Different Curbs, Same Rules
- HVAC and equipment curbs: usually manufactured to a standard height, with a flange or skirt from the unit that covers the termination.
- Roof hatch curbs: often part of the hatch assembly, and flashing runs up to the hatch frame rather than the curb cap.
- Skylight curbs: framed to the skylight's dimensions, with flashing that may finish under the skylight frame. See the skylight guides.
- Duct and vent curbs: need space around them for pipes and wiring, and the flashing must not block service access.
Wind and Movement
Equipment on a curb vibrates and moves as it runs, and the roof and curb move at different rates with temperature. Flashing has to be relaxed at the base and fastened only at the top. A taut membrane tears at the base cant, and a loose one sags and lets wind in.
In the Fraser Valley, curbs on roofs exposed to winter storms take wind from all directions. Check the termination bars and caps after big wind events for any lifting or loose fasteners.
Common Mistakes
- Curb too low. Less than 8 inches above the finished surface puts the flashing where splash, snow and standing water reach it.
- No cricket behind a wide curb. Water ponds against the flashing and the base leaks.
- Unwelded or unpatched corners. Sharp folds and open T-joints are the usual leak points.
- Flashing pulled tight. Taut membrane tears at the cant when the roof and curb move.
- No termination bar or cap. Loose top edges sag and let wind and water behind the membrane.
- Curb too close to a wall. There is no room to weld and probe the flashing, and repairs become impossible.
When to Call a Professional
Curbs carry equipment, openings into the building and often electrical and mechanical services. The flashing has to work with the equipment's own detail and with the membrane manufacturer's specification, and a mistake leaks into a mechanical space. It is professional work.
Call a roofer if water is staining the ceiling near a curb, if flashing is lifting or pulling away from a curb, or if water ponds beside the curb after rain. Those roofs usually need a cricket or a corrected upstand.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Flat Roofing and Torch On service or the towns we cover.
How to Flash a Curb on a Flat Roof: Questions
How high should a roof curb be?
The common minimum is 8 inches above the finished roof surface, measured from where water will stand. Some manufacturers, designers and codes require more, especially on low slopes or where snow drifts. The membrane should be flashed up to at least that height and terminated under a cap or counter flashing.
Do you need a cricket behind a roof curb?
Usually yes, for wide curbs. A cricket on the upslope side sends water around the curb instead of letting it pond against the flashing. A common rule of thumb is to add one when the curb is wider than about 30 inches across the slope, but the designer or manufacturer should confirm the requirement for the specific roof.
How do you flash the corners of a curb?
Use pre-molded outside corners where available, or form the corner from a patch of flashing membrane with rounded edges, and weld or torch every edge. Cover every T-joint with a round patch, then probe the welds. A sharp fold left unwelded at an outside corner is the most common curb leak.
What is counter flashing on a roof curb?
Counter flashing is a metal cap or skirt that covers the top edge of the membrane flashing, so water runs over it and not behind the membrane. On a curb, it is often a cap over the top or the flange of the equipment itself. It should overlap the membrane by at least 3 inches and slope away from the unit.
Related Guides
All Flat Roof Details guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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