United States
Hong KongFlashing and waterproofing
Flashing directs water away at every penetration and transition.
- Dampproofing resists moisture; waterproofing resists water under hydrostatic pressure.
- The two are not interchangeable.
On this page5 sections
Water gets in at interruptions, not through the middle of a wall. Flashing is how the interruptions are managed.
Where flashing goes
- Roof penetrations - vents, pipes, chimneys
- Roof-to-wall transitions
- Window and door openings
- The base of walls
- Changes of material or plane
Every one of those is a place where two things meet and water finds the gap.
The principle
Shingle fashion: each layer overlaps the one below so water runs onto the surface beneath rather than behind it.
Reverse-lapped flashing directs water into the assembly instead of out of it. It looks finished and it fails immediately.
It is invisible once cladding is on and it produces a leak that seems to come from nowhere. Questions describe water appearing below a window and ask for the likely cause.
Dampproofing against waterproofing
Roof flashing details
At a sloped roof that meets a vertical wall, step flashing is installed in successive pieces, woven with the shingle courses so each piece sends water onto the course below. At a masonry wall, base flashing turns up the wall and counterflashing covers its upper edge; the counterflashing sheds water over the base flashing instead of letting it enter behind it.
At the lower end of a roof-to-wall intersection, a kickout flashing projects water away from the wall and into the gutter. Without it, runoff can travel behind the cladding and soak the wall. In valleys, the flashing or valley liner is installed beneath the roof covering and detailed to shed water toward the eave; the specific material and exposure depend on the roof system, manufacturer instructions, and adopted code.
Every upper piece must discharge onto the face of the lower piece. Step flashing follows each shingle course, counterflashing covers the upturned base flashing, and valley flashing drains toward the eave. A reversed lap sends water into the assembly.
| Dampproofing | Waterproofing | |
|---|---|---|
| Resists | Moisture and water vapor | Water under hydrostatic pressure |
| Used where | No standing water expected | Below the water table, or where water will stand |
| Cost | Lower | Higher |
A basement below the water table needs waterproofing. Dampproofing there will fail, and the question is often framed exactly that way.
Drainage first
The best waterproofing is water that never arrives. Grading away from the building, functioning gutters and foundation drains do more than any membrane. Water finds gaps. Grade it away.
Common questions
What is the flashing principle?
Shingle fashion, each layer overlapping the one below so water runs onto the surface beneath rather than behind it.
What is reverse lapping?
Flashing installed so it directs water into the assembly. It is invisible once clad and fails immediately.
What is the difference between dampproofing and waterproofing?
Dampproofing resists moisture. Waterproofing resists water under hydrostatic pressure.
Which does a basement below the water table need?
Waterproofing. Dampproofing will fail there.
What matters more than the membrane?
Drainage. Grading, gutters and foundation drains stop water arriving in the first place.