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WAC 246-359-210

Treated wood foundations for wood framed construction.

(1) All lumber and plywood used for wood foundation systems must be pressure treated and bear the grade mark FDN (foundation grade) or better.
(2) Where FDN lumber and plywood is cut or drilled after treatment, the cut surface must be field treated with a preservative that is designated for that purpose.
(3) Hot-dipped zinc-coated steel nails or stainless steel fasteners will be used as fasteners for treated wood foundation walls. Electrogalvanized nails or staples and hot-dipped zinc-coated staples cannot be used.
(4) Treated wood foundations must have composite footings consisting of a minimum two-by-eight lumber footing plate set eighteen inches below finished grade on top of a layer of gravel, coarse sand or crushed stone. The gravel, sand, or crushed stone footing will have a width of not less than sixteen inches and a depth of not less than six inches, and must be placed in firm, undisturbed soil.
(5) The gravel, sand, or crushed stone footing must consist of:
(a) Washed and graded gravel free from organic, clayey or silty soils with a maximum stone size not exceeding three-fourths inch;
(b) Coarse sand free from organic, clayey, or silty soils with a minimum grain size of one-sixteenth inch; or
(c) Crushed stone with a maximum size of one-half inch.
(6) Treated wood foundation walls must be constructed of two-by-six studs at a minimum of sixteen inches on center with a double two-by-six top plate. Cover the studs with a minimum one-half inch thick pressure treated exterior plywood sheathing placed on the exterior of the studs. Treated wood foundation walls will not be greater than forty-eight inches measured from the bottom of the footing plate to the top of the double top plate.
(7) Joints in the footing plate and top plates must be staggered at least one stud space. Framing at locations where openings occur in the wall and floor systems above, and at other points of concentrated loads must have studs added at those points to support the concentrated loads.
(8) Before backfilling, cover the gravel, sand, or crushed stone appearing outside the treated wood foundation wall with strips of six-mil thick polyethylene sheeting, Type 30 felt, or equivalent material with adjacent strips lapped to provide for water seepage while preventing excessive infiltration of fine soils.
(9) Backfill on the outside to eight inches or more below the top of the treated wood foundation walls. Backfill on the inside of the treated wood foundation walls (crawl space) a minimum depth of six inches above the top of the footing plate.
[Statutory Authority: RCW 70.114A.081. WSR 99-03-065, ยง 246-359-210, filed 1/18/99, effective 2/18/99.]
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