As an architect who has worked with the roofing industry
since 1977, I often inspect leaking roofs. I have found that
poor flashing details - at penetrations, at roof edges, or
where a roof changes planes - are much more likely to be the
cause of a leak than the roofing itself. Similarly, many leaks
are caused by the use of insufficient or inadequate fasteners
for sheathing, flashing, or roofing, and by the attempt to
substitute caulk or roofing cement for flashing.
The cost of installing the roof on a new building usually
amounts to less than 5% of the total construction cost. Yet
some lawyers estimate that 60% to 80% of construction lawsuits
involve roof failures. Often, it's a small leak that quickly
grows into a big, expensive problem.
The following problems are among the most common I
encounter. In every case, the leak could have been avoided had
the installer used and paid more attention to detail.
FastenersProblem: No support at sheathing edges.
plywood sheathing on this flat roof was installed without
blocking under the edges. If someone walks on the roof, the
roofing may crack when the sheathing flexes at the plywood
Use H-clips or solid
blocking at plywood edges, or use tongue-and-groove
plywood roof sheathing.
Smooth-shank nails don't
hold. Smooth-shank common nails used to fasten roof
sheathing can work themselves loose over the years, especially
if a roof deck is walked on. Rising nail heads can eventually
poke through the roofing.
Use ring-shank nails for roof
Flashing is incorrectly
fastened. There are three problems with the coping
flashing on this parapet wall:
The flashing has been
fastened on the horizontal face, instead of the
The flashing has been
nailed instead of screwed.
No allowance has been
made for expansion and contraction of the metal
flashing, nor for differential movement between the metal
flashing and the wood nailer, which expand at different
An exposed fastener on the horizontal surface of the
flashing is a potential leakage point. If flashing is attached
with nails instead of screws, the last hammer blow can leave a
concave dimple in the flashing, encouraging ponding around the
If long runs of flashing are installed without allowing for
expansion and contraction, the moving flashing can work nails
back and forth until they loosen.
Attach flashing on a vertical surface rather than a
Use hex-head screws with neoprene washers rather
than nails if it is necessary to use exposed fasteners.Install fasteners in slotted holes
, which allow
the flashing to move with changes in temperature.