cant strip
a beveled strip used under flashings to modify the angle at the point where the roofing or waterproofing membrane meets any vertical element.
cant strip: angled filler that eases roof-to-wall transitions
A cant strip is a beveled or tapered wedge of material installed at the junction where a roof membrane or flashing meets a vertical surface, such as a wall, parapet, or mechanical curb. It serves to create a gradual angle transition rather than allowing the membrane to meet the wall at a sharp 90-degree corner, which would crease, tear, or fail prematurely under thermal movement and weathering.
Cant strips are typically made from wood (often dimensioned 45 degrees at standard sizes like 4 inches by 4 inches), rigid foam insulation, or pre-formed rubber or synthetic materials. The 45-degree angle is the most common profile in pitched roofs and low-slope installations, though steeper angles are sometimes used. On low-slope commercial roofs, cant strips are nearly universal; they are essential to the long-term durability of built-up roofing, single-ply membranes, and modified bitumen systems.
The angle modification prevents the membrane from folding or kinking at the base of the wall, which reduces stress concentration during thermal expansion and contraction cycles. Without a cant strip, the membrane material at that corner is forced to bend sharply, concentrating mechanical strain and accelerating degradation. Wind pressure, foot traffic, and UV exposure all amplify stress at sharp corners; the cant strip distributes these forces over a longer path.
Installation and material considerations
Cant strips must be firmly adhered or mechanically fastened to the roof deck before the waterproofing membrane is applied over them. Wood cant strips are common but can absorb moisture and deteriorate if the membrane is breached; foam and synthetic alternatives resist rot but may compress under heavy loads or in high-temperature environments. The membrane is then brought up and over the cant strip, with flashing or trim secured at the top to direct water away from the wall.
Failure at cant strip junctions typically manifests as membrane cracking, wrinkles, or separation from the flashing. This is often a result of improper installation (inadequate fastening or adhesive), material selection unsuited to thermal movement, or damage from foot traffic or maintenance work. The cant strip itself is sometimes overlooked during inspection, but its condition directly affects the service life of the entire flashing assembly and the underlying roof system.