In the art of designing low-slope membrane roofing systems, the industry has been bogged down by troublesome details that require frequent repair. The most egregious of these are metal roof edge details.
While we boast about 20- or 30-year warranties for our roof systems, we carefully exclude the ubiquitous gravel stop edges from the warranty. One reason for this is that roofing material manufacturers in general don’t produce or market the roof edge metal systems. While manufacturers’ roofing manuals may contain drawings showing such features, there is invariably a disclaimer, such as “This detail will require more than average maintenance.” According to the National Roofing Contractors Association (NRCA), it’s best to avoid flashing details that require rigid metal flanges to be embedded or sandwiched into the roof membrane whenever possible.
One “fix” that is sometimes recommended is elevating the gravel stop edge above the plane of the roof and pitching roof water away from the building edge towards interior roof drains. This won’t solve the metal movement and consequential splitting of the stripping plies, but at least water is not running into the splits, which rots the supporting wood nailers and eventually leaks into the building interior.
The typical sequence involves installing the roof membrane and embedded metal and topping it with stripping plies. In the real world, these stripping plies crack, especially if a metal cap is used to join fascia edging together. Just walking the perimeter of a building and visually examining the fascia metal will let you know if there is such a problem.
When metal is embedded, the edge metal also serves as a gravel stop, so it will be necessary to spud (chip) back the gravel, remove the torn stripping plies, and install new stripping, consisting either of ply felts, glass mesh, or (more recently) modified bitumen. After a couple of years, we can expect to do this again and again.
This is helped somewhat when flashing membrane – something more flexible and tougher than the roof membrane itself – covers the roofing plies and extends down the exterior blocking. The flange of the metal fascia is placed on top of the installed membrane, rather than being sandwiched. This flexible flashing at least achieves the goal of directing the water to the outside the building instead of the interior.