Installation of Underlayment and Primary Flashing
This web page is a great source for training new roofers as well as
refreshing the experienced roofer. Time and again I have heard from
contractors “It sure would be nice if I could get all my crews to install
roofing materials in the same way.”
The purpose of all roof flashing including Skylight Flashing is to keep as much of the moisture
as possible on the surface of the roofing material.
Step Flashing is laced over each course of Asphalt Shingles, Cedar Shakes and
Shingles, and most Alternative Steep-Sloped roofing materials no more than 1″
above the weather exposure of the roofing being installed. In other words,
no more than 1″ above the butt of the roofing material of the next course.
With Tile, the Step Flashing is exposed and placed over the surface of each
course starting at the butt of one course and extending 3″ up under the Tile of
the next course. Plus Tile requires a secondary flashing roofed into the
underlayment, which is not discussed here.
Wrapping the skylight, chimney, and walls, etc. with an Ice and Water
Membrane instead of the standard underlayment is always a good idea and may be
required in some areas.
Raising the skylight curb may be required to properly flash and roof around.
The easiest way is to do this is to add one or two layers of 2″ x 2″ lumber to
the top of the existing curb. This also changes the look of the interior
side of the skylight. To hide the additional skylight framing simply
install 1″ x 4″ or larger trim boards over the edge of the existing drywall.
In most cases this will also improve the appearance of any interior skylight shaft.
Remember, after the sky light is set on the curb, to always
fasten down the skylight. Use corrosion resistant screws.
There are many ways to install the primary flashing and roofing materials
around skylights. Some wrap corners, some don’t. Some counter flash,
some don’t, etc. The following are drawings of one tried and true method
presented in step-by-step installation procedures from dry-in to setting a
standard 2′ x 4′ skylight on a curb.
We do not guarantee that by following these installation procedures that a
leak will not occur, but it will greatly reduce that chance and other roofing
problems that frequently occur at skylights. Many of the procedures
illustrated here are the same for head walls, rake walls, and chimneys.
The first four drawings illustrate an easy way to dry-in around skylights.
Notice the front and back of the skylight are treated independently of the rolls
of underlayment as they are installed across the roof. Using these
separate pieces of underlayment saves the hassle of trying to cut full rolls
around the skylight and makes for a more water tight installation.