Flashing
CSI Division 07, Thermal & Moisture Protection
Thin sheet material, such as metal, rubberized asphalt, or plastic, installed at any point in a building's exterior envelope where two different materials or planes meet (roof-to-wall intersections, window and door openings, wall penetrations), specifically to direct water away from the joint instead of letting it collect and find its way inside.
Why it matters
Nearly every serious water intrusion problem in a building traces back to a flashing detail that was missing, installed in the wrong sequence, or installed backward. Flashing works almost entirely on the principle of layering pieces so water always flows down and out over the next piece below it, and getting that sequence wrong defeats the whole system even if every individual piece looks correctly installed.
On a real project
At a window opening, flashing is installed at the sill first, then the window itself, then flashing at the jambs and finally the head, each piece overlapping the one below it like shingles, so water draining down the wall always sheds over the next flashing piece rather than behind it.
Who this matters most to
A Carpenter or Roofer installs flashing at the specific location and has to follow the correct lapping sequence exactly, since flashing installed out of order can direct water inward rather than outward. A Quality Control Manager inspects flashing details at windows, roof intersections, and other critical junctions before they're covered by siding, roofing, or other finishes.
Where this goes wrong
A crew installs window flashing out of the correct sequence: the siding above the window gets installed first and tucked behind the head flashing instead of the flashing lapping over it, and the mistake isn't visible once the siding is finished. Water that runs down the wall during a storm gets directed behind the flashing instead of over it, leaking into the wall cavity around that window for years before the resulting damage becomes visible inside.