Flat Roof-to-Wall Flashings

There is an almost endless number of flashing situations that may arise and there are almost as many different approaches to detailing these situations. The most elegant solutions typically are not used on single-family homes. Very often, the flashing details that we see are a “minimum possible” rather than an ideal solution.

Flashings on flat roofs are critical—just as critical as they are on steep roofs. Most roof leaks are at the flashings. In this article, we’ll take a closer look at flat roof-to-wall flashings.

It’s common for flat roofs to terminate against walls. For example, a one-story garage with a flat roof may be attached to a two-story house. There are several ways to address this detail; we’ll describe some of the most common approaches.

Typically, a base flashing and counter flashing are used. If the membrane is asphalt (built-up or modified bitumen), a cant strip is recommended. This wood or fiberboard triangular piece (typically 3 inches horizontal by 3 inches tall) is used to allow the membrane to make two 45° turns rather than one 90° turn to reduce the risk of the lap joints separating or opening at the upturn. 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 it is often the roof membrane material itself. Typically, it is provided with protection against sunlight. The base flashing extends onto the flat portion of the membrane and is sealed to it. The flashing extends over the cant strip and up the wall surface, approximately 8 to 14 inches above the roof.

The top of the base flashing is protected with a counter flashing. Typically, this is a piece of metal that is let into a reglet (a horizontal slot cut into the wall or mortar joint). 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.

The reglet can be as little as 1 inch deep. In rare cases, it extends all the way through the wall, although this would typically be two separate pieces of metal. 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, re-roofing or both. The bottom part of the counter flashing also can be replaced without having to disturb the part that is embedded in the wall. We do not see this approach regularly on single-family homes.

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 they should be weathertight.

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 may be a common practice; however, there are at least a few weaknesses to this approach. First, in our experience, the flexible sealant (caulking) at the joints is relatively watertight for only a short period. Second, the mechanical fasteners (nails or screws) are potential leakage points. 

Modified bitumen, EPDM and PVC/TPO may be flashed in a similar way, although cant strips are not required with EPDM and PVC/TPO. It is common to have flexible tubing laid in at the roof-wall intersection to allow for some differential movement between the roof and the wall.

PVC/TPO and EPDM membranes shrink considerably under some circumstances. Thus, it is imperative 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 sometimes are installed before the membrane, attached to the roof perimeter and the vertical surface. Then the roof membrane is adhered to this strip.

As discussed with perimeter flashings, the base flashings for PVC/TPO, EPDM and modified bitumen typically are just more of the roof membrane materials.

Flashing at Parapet Walls
Parapet walls are similar to taller walls, with a couple of minor exceptions. If the parapet wall is relatively short, the base flashing may extend up and over the top of the parapet wall and drape over the outside face. The counter flashing often is extended to form a coping or cap for the top of the parapet wall. The outside face of the coping also forms the drip edge flashing. Coping also can be stone, concrete or terra cotta.

Another possible difference with parapet walls is the presence of through-wall flashings. It is recognized that parapet walls are exposed to wind-driven rains from both sides and that these walls get considerably wetter than conventional exterior walls. Because most walls are porous, it is recognized that water will enter the wall system.

High-quality parapet walls may feature through-wall flashings just above the counter flashings. Furthermore, through-wall flashings sometimes are provided just below the coping to ensure that any water that gets into the top of the wall cannot find its way down into the building. Note: In general, these high-quality details won’t often be found residentially.

Good quality roofing work often includes a metal cladding on masonry or concrete parapet walls on the inner or roof side of the parapet. The entire height of the parapet wall is protected from wetting with metal. With single-ply roofing systems, the parapet is protected by the roof membrane itself, extended up the wall. This minimizes the need for through-wall flashings. It also may prolong the life of the masonry. Where this is seen, you usually are looking at high-quality work.

Inspection Tips
When inspecting a roof, more time should be spent looking at the flashings than at the field of the roof. It is helpful to think of yourself as wind-driven water and see where you might be able to get into the roof through flashing details. Some defects are obvious; others are invisible.

The excessive use of roofing cement on flashings typically suggests that a leak repair has been attempted. Often, this type of low-quality repair will work only in the short term, until expansion and contraction of building materials results in gaps and cracks.

Like any other part of the roof inspection, your flashing inspection is not complete until you’ve looked at the roof from below. Only from this perspective can you confirm any suspicions about weak flashings or discover problems that were not visible from above.

For those of you who use moisture meters, remember that a wet reading is conclusive, but a dry reading only means that it hasn’t leaked lately. Stains, crumbling or discoloration may mean an active leak, even if the moisture meter shows a dry reading during the inspection.

As with many systems, some flashings that are installed poorly do not leak. In other cases, flashings that appear to be first class do leak. In situations for which you can find no evidence of leakage, but you can tell that the flashing work is clearly low-quality, you should tell the client exactly that. You should prepare the client to expect problems with the flashing so that you do not receive a call back about this issue.

Leaks in flat roofs can be difficult to find because moisture often will show up on the interior, some distance from the failure point in the roof. Water may travel horizontally along plastic vapor barriers, for example, before coming through ceilings.

More details about flat-roof flashings are included in the ASHI@Home Training Program (https://www.homeinspector.org/ASHI-HOME-Training-System).

Carson Dunlop – Consulting engineering firm devoted to home inspection since 1978. www.carsondunlop.com

For More Information on Flat Roofs, Visit:

Flat-Roof Systems: http://www.ashireporter.org/HomeInspection/Articles/Flat-Roof-Systems/2448

Flat-Roof Inspection, with a Focus on Modified Bitumen: http://www.ashireporter.org/HomeInspection/Articles/Flat-Roof-Inspection-with-a-Focus-on-Modified-Bitumen/1849

Flat-Roof Drainage: http://www.ashireporter.org/HomeInspection/Articles/Flat-Roof-Drainage/2554

When Flat Doesn’t Equal Safe: http://www.ashireporter.org/HomeInspection/Articles/When-Flat-Doesn-t-Equal-Safe/485

Safety Concerns Up on the Roof: http://www.ashireporter.org/HomeInspection/Articles/Safety-Concerns-Up-on-the-Roof/14775