Wood Handbook, Wood as an Engineering Material

Prevent moisture-related problems in siding by using rain-
screen design (that is, by furring out the siding 9 to 19 mm
(3/8 to 3/4 in.) from the sheathing–house wrap)
(Fig. 16–15).

Seal all end-grain surfaces with oil-alkyd primer or WRP.

Ensure adequate space (approximately 50 mm (2 in.))

between siding and roof shingles in places where a side-wall

intersects a roof. Siding and panel products above z-flashing

need to be placed approximately 9 to 12 mm (3/8 to 1/2 in.)

above the flashing to form a drip edge (Fig. 16–16).

When installing siding or shingles, ensure that the spacing is

commensurate with the MC of the wood and the anticipated

MC during the service life. Figure 16–17 shows shingles

that were spaced too closely and buckled in service.

Avoid inside–outside beams and joists. For example, a

second-floor floor joist that penetrates a wall to form a porch

rafter is destined to have moisture problems and subsequent

decay and finish failure. This type of wall penetration is dif-

ficult to seal to avoid air movement. Air carries water vapor

that condenses in the space between floors or the porch

ceiling.

Compliance of VOC Finishes with Pollution

Regulations

Volatile organic compounds (VOCs) are organic chemicals

in finishes that evaporate as finishes dry and cure. VOCs are

air pollutants, and the amount that evaporates for a given

amount of solids (such as binder and pigments) in finishes is

regulated. Under the 1990 New Clean Air Act, the U.S. En-

vironmental Protection Agency (EPA) required paint compa-

nies to decrease the amount of VOCs in their finishes.

Traditional solvent-borne wood finishes containing mineral

spirits are no longer available, including oil-based semi-

transparent stains, oil- and oil-alkyd-based primers and

top-coats, solvent-borne water repellents, and solvent-borne

water-repellent preservatives. Solvent-borne finishes are still

available, but the solvent systems are more complex than

mineral spirits. Prior to VOC regulations, penetrating fin-

ishes, such as semitransparent stains, had low solids content

Chapter 16 Finishing of Wood

Figure 16–15. Demonstration of sid-

ing installation over a secondary

drainage plane (rain screen) showing

wall studs, sheathing, water-resistive

barrier (WRB), furring strips, and in-

terleaved WRB at the butt joint. Note

that the butt joint is centered directly

over the furring strip and the underly-

ing stud and the end grain has been

sealed.

Figure 16–16. Demonstration of proper and improper
z-flashing installation: (top) siding installed with a 9-mm
(3/8-in.) gap between the z-flashing and siding to allow
water to drain off the siding; (bottom) siding installed
without a gap, which gives easy access for water ab-
sorption into the siding and thus shows extractives
staining.

Figure 16–17. Shingles installed with insufficient

gap later warped when they expanded after getting

wet.