Jeffery, you’ll see that untreated wood can be in contact with concrete. See Item 2.
2009 IRC SECTION R317
PROTECTION OF WOOD AND WOOD
BASED PRODUCTS AGAINST DECAY
R317.1 Location required. Protection of wood and wood
based products from decay shall be provided in the following
locations by the use of naturally durable wood or wood that is
preservative-treated in accordance with AWPA U1 for the species,
product, preservative and end use. Preservatives shall be
listed in Section 4 of AWPA U1.1. Wood joists or the bottom of a wood structural floor
when closer than 18 inches (457 mm) or wood girders
when closer than 12 inches (305 mm) to the exposed
ground in crawl spaces or unexcavated area located
within the periphery of the building foundation.
All wood framing members that rest on concrete or
masonry exterior foundation walls and are less than 8
inches (203 mm) from the exposed ground.
Sills and sleepers on a concrete or masonry slab that is in
direct contact with the ground unless separated from
such slab by an impervious moisture barrier.
The ends of wood girders entering exterior masonry or
concrete walls having clearances of less than 1/2 inch
(12.7 mm) on tops, sides and ends.
Wood siding, sheathing and wall framing on the exterior
of a building having a clearance of less than 6 inches (152
mm) from the ground or less than 2 inches (51 mm) measured
vertically from concrete steps, porch slabs, patio
slabs, and similar horizontal surfaces exposed to the
Wood structural members supporting moisture-permeable
floors or roofs that are exposed to the weather, such
as concrete or masonry slabs, unless separated from such
floors or roofs by an impervious moisture barrier.
Wood furring strips or other wood framing members
attached directly to the interior of exterior masonry walls
or concrete walls below grade except where an approved
vapor retarder is applied between the wall and the furring
strips or framing members.
This section addresses the need for minimum protection
against decay damage for wood members located
in certain locations.
For those portions of a wood-framed structure that
are subject to damage by decay, the code mandates
that the lumber be pressure-preservative treated or be
naturally durable wood, or be of a species of wood
having a natural resistance to decay. Naturally durable
wood by definition is the heartwood of decay-resistant
redwood, cedars, black locust and black walnut.
Item 1: Crawl spaces and unexcavated areas under
a building usually contain moisture-laden air. These
spaces must be ventilated in accordance with Section
R408 to remove as much moisture as possible before
it causes decay. Wood placed a minimum specified
distance above grade in unexcavated under-floor areas
or crawl spaces, as shown in Commentary Figure
R317.1(1), need not be either preservative-treated
wood or wood that is naturally decay-resistant durable
wood. These clearances below floor joists and beams
are deemed to be the minimum necessary to allow adequate
circulation and removal of moisture from the air
and from the wood framing members. Such clearances
apply within the exterior wall line of the building
Item 2: Foundation walls will absorb moisture from
the ground and by capillary action move it to framing
members that are in contact with the foundation. Unless
a minimum clearance of 8 inches (203 mm) is
maintained from the finished grade to wood sills resting
on concrete or masonry exterior foundation walls,
decay-resistant or preservative-treated wood, as
shown in Commentary Figure R317.1(2), must be
used. The 8-inch (203 mm) clearance specified in this
section has been determined to be large enough to
prevent wetting of wood framing members under most
Item 3: Concrete and masonry slabs that are in direct
contact with the earth are very susceptible to
moisture because of absorption of ground water. This
can occur on interior slabs, as well as at the perimeter.
In the case of wood sills or sleepers placed on concrete
or masonry slabs, decay-resistant wood or pressure-
treated wood is required where the slabs are in
direct contact with the ground, as illustrated in Commentary
Figure R317.1(3). Concrete that is fully separated
from the ground by a vapor barrier is not in direct
contact with earth.
Item 4: A minimum 1/2-inch (12.7 mm) clearance
along the top, sides and ends of wood members projecting
into exterior masonry or concrete walls must be
maintained, as illustrated in Commentary Figure
R317.1(4), unless the wood is treated or is of a species
that is naturally decay resistant.
Item 5: Experience has shown that wood siding may
extend below the sill plate to within 6 inches (152 mm)
of the earth without decaying. Commentary Figure
R317.1(5) shows the required minimum 6-inch (152
mm) clearance from the ground for wood siding,
sheathing and wall framing on the exterior of a building.
It should not be in direct contact with the foundation
wall. If the sheathing is located over a concrete
step or slab, the clearance may be reduced to 2 inches
(51 mm) minimum.
Item 6: Concrete or masonry slabs that serve as
roofs or floor systems that are exposed to the weather
are very susceptible to moisture from rain or snow. If
these slabs are supported by wood, the wood must be
decay resistant or must be separated from the slabs by
vapor barriers so there is not direct contact. This is
similar to Item 3.
Item 7: When a basement area is finished, a com-
mon practice is to provide wood furring strips on top of
the concrete or masonry walls for attachment of finishes.
These furring strips must either be of decay-resistant
material or must be separated from the wall by
a vapor barrier. This will prevent the moisture from
moving through the walls and rotting the furring strips,
and even possibly transferring the moisture to the inside