There's plenty of material available about how to flash windows
and patio doors, but most of it seems to cover flanged units.
Nearly every house I build, though, includes at least one
"flangeless" unit — typically a standard entrance door
with an applied brick-mold casing. The tricky part about
flashing cased units is sealing the area where the back of the
casing meets the housewrap. And door units present an added
challenge: Since there's no sill flange, wind-driven rain has a
straight shot at penetrating the area immediately below the
In this article, I'll explain how I use self-healing membrane
to fabricate flashing flanges for the jambs and heads of cased
units, and a flexible membrane to form a one-piece sill
Prepping the Opening
The days of X-cutting housewrap openings are long gone. The
proper method is first to make a level cut at the door head
(1), followed by two 45-degree cuts at the corners (2),
creating a head flap that is folded up and out of the way (3).
An inverted Y-cut is used to prepare the remainder of the
As with most flashing details, I start at the lowest point and
work my way uphill, making sure each layer overlaps the
previous one and avoiding any reverse laps. In the case of an
entrance door, this means I start with the sill flashing.
Self-Healing Sill Pan
To prevent moisture from working its way under the door
threshold to the framing, I fabricate a sill pan using Tyvek
FlexWrap, a flexible self-healing membrane that can be
stretched to form seamless sill corners.
In the past, I've tried both metal and plastic sill pans, but
the upturned rear flange always made for nasty trim details,
because the rear flange never fit snug against the back of the
threshold. Also, I always felt that any fasteners I drove
through those pans were potential leaks. Plus the fabrication
lead times for metal pans were a pain.
I cut a length of FlexWrap one foot longer than the
rough-opening width, and with the piece upside-down on my
sawhorses, I measure in 1/2 inch from the edge and carefully
score the release paper with my utility knife (4). I make sure
I've got a brand-new blade in the knife, and I score only the
paper, being careful to avoid cutting into the membrane itself.
(Before I developed the feel required to cut freehand, I found
that I could create an effective blade-depth stop by pinching
the end of the knife blade with a pair of small vise
After I score the release paper, I fold the FlexWrap back on
itself, creasing the cut and finishing off any areas where I
may not have cut completely through the paper (5).
I snap a line on the subfloor 1/2 inch behind where the
interior edge of the threshold will rest; this marks the back
(or inside) edge of the sill-pan material. I pull the release
paper off the FlexWrap, leaving the 1/2-inch strip I scored
(6). I center the membrane above the door opening (7) and lower
it into place, making sure the back edge lands on my layout
I use a J-roller to apply pressure and ensure a good bond
between the flashing and the housewrap (9). At the outside
corners, I carefully push the flashing out to form a seamless
corner (10). The flashing has a memory, so to prevent it from
curling back I drive a cap nail (11) at the outer edge; that
holds it in place until the adhesive achieves its full grab (24
to 48 hours).
Finally, I smooth the vertical portion of the flashing against
the housewrap, and again use the J-roller to press things
tightly in place (12).
At this point, the opening is ready for the door. My next step
is to apply peel-and-stick membrane (I typically use DuPont
StraightFlash) to the back of the door casing and frame,
creating my own sealed flanges to prevent moisture from working
its way behind the brick molding. I stick the membrane to both
the back of the casing and the side of the door jamb.