Surface-mounted exterior fixtures (electrical meter bases,
shutters, dryer vents, hose bibs, etc.) should always be
fastened to mounting blocks (Figure 6).
6. Subcontractors can create problems when they
fasten exterior fixtures directly through the siding
panels, because the fasteners will prevent the siding
panels from expanding and contracting freely. Use
mounting blocks for all surface-mounted
The rationale is simple: Any item that is fastened directly
through a siding panel will lock the panel in place,
short-circuiting the nailing slots that allow the panel to
expand and contract.
Most suppliers keep a number of prefabricated mounting
blocks in stock. Larger items (meter bases, for example) can be
mounted on a 11/2-inch-thick piece of treated lumber.
No matter how careful you are when allowing for expansion
and contraction, a subcontractor can undo the best-laid plans
of a quality siding installation in less than ten minutes.
Plumbers, electricians, and the phone company installers should
mount all of their devices or equipment on mounting blocks.
The general contractor is typically responsible for
providing the siding installer with the type and location of
required mounting blocks, but in many cases, the evil deeds are
done long after the project is completed. It's always a good
idea to explain to both the general contractor and the
homeowner what can happen if items are fastened directly
through the siding. It may not always prevent problems, but at
least you'll be able to say "I told you so."
It's essential that vinyl and aluminum products be
allowed to expand and contract freely. The photos
illustrate what happens if movement is restricted.
A sliding panel that is not pulled up snug in the
locking flange may come loose when warm temperatures cause
yhe panel to expand (left). A panel installed in hot
weather with the nails driven too toghtly will tear when
cold weather causes the panel to contract (right).
To reduce the chance of problems, follow these
• Think of vinyl and aluminum products as being hung,
not fastened, in place. To prevent nails from restricting
panel movement, installation literature specifies a
1/32-inch clearance between the nail head and the siding
panel. I like to tell installation rookies that they should
be able to slip the hook end of their tape measure between
the nail head and the siding panel. All it takes is one
overdriven nail to create problems.
• Adjust the recommended gapping (typically 1/4 inch)
to allow for current temperatures. In extremely cold
weather, use a slightly larger gap, since cold panels will
be on the short side of the expansion and contraction
cycle. Conversely, it doesn't hurt to tighten up the gaps
in very hot weather.
•Be sure to place all nails in the center of the
nailing slot. A nail shank that bottoms out in the nail
slot of an expanding or contracting panel will restrict
•Pull each panel up snug against the locking hem of
the previous panel, but do not overtighten. Panels that are
pulled too tight can tear when they contract, just as
loosely fit panels can pop loose as they expand.
•Fascia runs should never turn the corner. Stop all
runs at inside and outside corners and slip a small corner
insert behind the fascia to mask the subfascia.