Base Flashing / Curb Flashing — Waterproofing Vertical Transitions on Flat Roofs

What Is Base Flashing?

Base flashing is the primary waterproofing layer at any roof-to-vertical-surface transition on a flat or low-slope roof. It overlaps the roofing membrane horizontally and turns up the vertical surface — a wall, parapet, HVAC curb, skylight curb, roof hatch, or any other penetration through the roof plane. Base flashing is the "first line of defense" at these transitions; counterflashing (or cap flashing) is installed above it to seal the top edge.

The NRCA (National Roofing Contractors Association) requires base flashing to extend at least 8" above the finished roofing surface. This 8" minimum accounts for the potential ponding water depth and provides adequate coverage above the membrane stripping-in area.

Types of Base Flashing Applications

Wall Base Flashing

At roof-to-wall transitions (interior parapets, mechanical room walls, stairwell walls), base flashing laps 4" minimum onto the roof membrane and turns up the wall 8" minimum. The top edge is sealed with counterflashing inserted into a reglet. The base flashing is mechanically fastened through the vertical leg into the wall substrate before the counterflashing is installed.

HVAC Curb Flashing

HVAC curbs are prefabricated wood or metal frames that support rooftop mechanical units. Curb flashing is installed on all four sides of the curb, with specific lap sequencing: the downslope (low side) piece is installed first; the side pieces lap over the downslope piece; the upslope (high side) piece laps over the side pieces. This ensures water from the upslope side can't undercut the flashing joints.

Skylight and Roof Hatch Curb Flashing

Identical to HVAC curb flashing in principle. Skylight curb flashing must be compatible with the glazing system and is often specified in a heavier aluminum to resist thermal movement from the solar gain on the skylight above.

Profile Geometry

Standard base flashing is a two-leg L-profile:

• Horizontal leg — lies on the roof membrane and extends at least 4" (typically 6") from the base of the vertical surface. The membrane is stripped in over this leg with stripping-in tape and sealant.
• Vertical leg — rises up the wall or curb face at least 8" above the finished roofing surface. Fastened mechanically to the vertical substrate.

Some base flashing profiles add a locked top edge or a receiver for counterflashing at the top of the vertical leg. This is specified when the project requires the counterflashing to be pre-engineered with a compatible base flashing system.

Materials

How Trimgy Handles Base Flashing

Base flashing is drawn on Trimgy as a two-leg L-profile: horizontal leg length and vertical leg height. The 1/4" grid makes it easy to specify the exact dimensions without rounding errors. For HVAC curb flashing on a job with multiple curbs, draw the profile once and order the total footage for all four curb sides combined (a 24"×36" curb needs approximately 10 LF per curb).

Commercial roofing contractors who re-flash curbs during re-roofing projects benefit from Trimgy's ability to order exactly the footage needed without a shop minimum. Get a price, order the exact quantity, and the profile ships to the job site.