It is common for flat roofs to terminate against walls that are higher than the roof. For example, a one-story garage with a flat roof may be attached to a two-story house. A base flashing and counter flashing are typically used. If the membrane is asphalt (built-up or modified bitumen), a cant strip is typically used. This wood or fiberboard triangular piece (typically 3 inches horizontal and 3 inches tall) is used to allow the membrane to make two 45 turns rather than one 90 turn. This makes the membrane less susceptible to cracking. The roof membrane typically extends onto the cant strip, but not up onto the vertical wall surface.
A base flashing may be one of several materials, but is often the roof membrane material itself. It is also often reinforced and provided with something to give it additional protection against sunlight. It extends out onto the flat portion of the membrane and is sealed to it there. It extends up over the cant strip onto the vertical wall surface, 8 to 14 inches above the roof.
The watertight joint for the roof system is not the top of the base flashing. The top of the base flashing is protected with a counter flashing. This is typically a piece of metal let into a reglet (a horizontal slot cut into the wall). The metal extends down over the top of the base flashing. In some areas, it is common for the counter flashing to cover most of the base flashing. In other areas, this is not done.
Sometimes the reglet may be considerably deeper than one inch and in rare cases may extend all the way through the wall. This through-wall flashing detail suggests high quality work.
High quality work includes a two-piece counter flashing where the top part is permanently set into the wall, and the larger bottom part of the counter flashing can be removed to facilitate repairs and/or re-roofing. The bottom part of the counter flashing can also be replaced without having to disturb the part that is imbedded in the wall.
Good practice dictates that the bottom edge of the counter flashing is turned under to form a hem. This eliminates exposed sharp edges and makes the bottom of the flashing more rigid.
The counter flashing should be made up of pieces of metal no longer than 10 feet. Joints between adjacent pieces of counter flashing should allow for expansion and contraction, but should be weather-tight.
Differential movement between the flat roof and the wall can pull apart the best-installed flashing details. In residential construction, serious differential movement is not usually an issue.
The elegant solution is to build an 8- or 10-inch-tall wood wall resting on the roof against (but not connected to) the higher house wall. The cant strip, membrane and base flashing are laid in place, with the base flashing running up the mini wall, but not onto the main wall. The counter flashing is then attached to the main wall, extending out over the mini wall and down well past the top of the base flashing. With this detail, there is no hard connection between the flat roof and the wall. Considerable differential movement can take place without causing a problem. You normally won't know whether this was done.
An inferior alternative to letting the counter flashing into a reglet is to attach the counter flashing to the face of the wall with mechanical fasteners and to simply seal the top of the metal counter flashing with sealant.
This is a very common practice, but there are at least two weaknesses to this approach. 1. The watertightness of the roof relies on a flexible sealant (caulking) between the top of the metal counter flashing and the wall. These joints are relatively watertight for only a short period. 2. The second weakness is that the mechanical fasteners (nails or screws) are potential leakage points.
Mod bit, EPDM and PVC may be flashed in a similar way, although cant strips are not used with EPDM and PVC. It's common to have a flexible tubing laid in at the roof/wall intersection to allow for some differential movement between the roof and the wall.
PVC and EPDM membranes shrink considerably under some circumstances. It has recently been recognized that it is important to secure the membrane well at the perimeters. This can be done in a number of ways, but the flashing material should not be expected to hold the roof membrane in place. Fastening strips or termination bars may be used to pinch the membrane down to the roof deck or into the bottom of the wall before the base and counter flashings are installed. Strips of the roofing material itself are sometimes installed before the membrane, attached to the roof perimeter and the vertical surface. The roof membrane is then attached to this strip. The base flashings for PVC, EPDM and modified bitumen are typically just more of the roof membrane materials.