A cant strip — also called an angle fillet — is a triangular section batten laid into the internal angle between a roof deck and an upstand. It means the waterproofing does not have to turn a sharp 90-degree corner, but rises in two gentle bends of roughly 45 degrees.
Why it matters
Bitumen membranes, EPDM and single-ply sheets are tough, but they cope badly with a sharp internal angle. There the sheet is permanently stressed while the substrate moves with temperature change and settlement. That is exactly where splits appear — and the angle between roof and upstand happens to be where the water collects. The cant strip takes the stress out of the detail and stops water sitting in a seam.
You will find one at roof edges, against parapets and upstands, and around dormers, chimneys and rooflight kerbs.
Fitting and points to watch
- Size. Sides of roughly 50 x 50 mm up to 100 x 100 mm are usual. The stiffer the membrane, the larger the fillet can be
- Material. Softwood is the most common, but stone wool, rigid foam and PIR versions exist. On an insulated deck these non-combustible or load-bearing options are more practical than timber
- Fixing. Nail, screw or bond it depending on the substrate. A loose fillet shifts and drags the membrane with it
- Sequence. The fillet sits on the insulation or deck, under the waterproofing. In a two-layer system the base sheet already dresses over it
- Moisture. Do not use soaking wet timber: it moves as it dries and can pull the sheet away
- Common omission. A missing cant strip is one of the most frequent causes of leaks on self-built flat roofs
Related terms
- Flat roof
- Bitumen roof
- Roof upstand
- Parapet
- EPDM
- Roof covering
- Rooflight kerb
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