Termite and Decay Zones

Why this matters

The bottom few inches of a house are where wood meets dirt, water and insects, and the code draws careful lines there for a reason. Those lines get erased by trades that are not thinking about termites at all: a landscaper adds a planting bed, a concrete crew pours a patio against the siding, a plumber trenches through the soil a pest company treated, an HVAC tech lets a condensate line drip on the foundation. None of them installed anything wrong in their own scope. Together they turned a protected wall into a buried one, and the damage shows up years later as somebody else's problem.

How the code knows your area's risk

The IRC's climatic and geographic design criteria table in Section R301.2 has a column headed "subject to damage from," and one of its entries is termite. Each jurisdiction fills it in when it adopts the code, using the termite infestation probability map the IRC supplies, which divides the country into four bands: very heavy, moderate to heavy, slight to moderate, and none to slight. Some editions and some local amendments carry a decay entry as well; check whether yours does. How to read the rest of that table is in The Local Design Criteria Table.

The entry matters because several requirements switch on with it. In areas subject to termite damage, the IRC's termite section (R318, as adopted) requires one or more protection methods: chemical soil treatment, a baiting system, pressure-preservative-treated wood, naturally termite-resistant wood, physical barriers, or steel framing. In areas the map rates very heavy, the code also restricts foam plastic insulation installed below grade on the outside of foundations, because termites tunnel through it unseen, under the conditions and exceptions your edition lists.

These are subterranean termites: colonies in the soil that reach wood through mud tubes and hidden paths. Drywood termites, common in some southern coastal areas, live in the wood itself and are not controlled by grade clearance. Formosan termites, an aggressive subterranean species established along parts of the Gulf Coast and elsewhere, make the stakes higher where they occur.

The decay rules apply everywhere

Termites need a map. Decay does not. Wood rots wherever it stays wet long enough, and the IRC's protection-against-decay section (R317.1, as adopted) applies regardless of the termite entry. It requires naturally durable or preservative-treated wood in specific locations, measured from specific points. The ones a field trade is most likely to disturb:

• Framing on a foundation: wood framing members, including sills, that rest on a concrete or masonry exterior foundation and are less than 8 inches from exposed ground.
• Siding and sheathing: wood siding, sheathing and wall framing on the outside of the building with less than 6 inches of clearance from the ground, or less than 2 inches measured vertically from concrete steps, porch slabs, patio slabs and similar horizontal surfaces exposed to the weather.
• Crawlspaces: wood joists or the bottom of a wood structural floor closer than 18 inches, or wood girders closer than 12 inches, to exposed ground in a crawlspace.
• Wood on a slab in direct contact with the ground, unless separated by an impervious moisture barrier.
• Wood in contact with the ground must be treated wood rated for ground contact.

The datum is part of the rule. Each figure is measured from a named point, and getting the point wrong is the usual way a clearance that "looked fine" fails.

  OUTSIDE FACE OF THE WALL, MEASURED VERTICALLY

  wood sill on foundation --+
                            |
  bottom of wood siding ----+--+
                            |  |  siding to ground:
     sill to ground:        |  |  6 in min
     8 in min               |  |
                            |  |  (siding to a patio
                            |  |  slab or step:
                            |  |  2 in min, from the
                            |  |  slab's top surface)
  exposed ground -----------+--+------------------

Most inspectors and pest professionals treat mulch, decorative stone and planting beds as part of the ground, because they hold moisture and give termites cover. The ground is wherever the material against the wall ends.

Treated wood is not one product

Pressure-treated lumber carries an end tag stating its use. Above ground material is not rated for burial or for the wet, ground-contact exposure that rots it. Ground contact material is. Installing an above-ground board as a post base or a sill against soil is a decay failure waiting to happen, even though it is "treated."

Two details travel with treated wood. Cutting it exposes untreated wood at the core, and the code (R317.1.1, as adopted) requires field cuts, holes and notches to be treated with an approved field preservative. And the copper-based preservatives in modern treated lumber corrode ordinary steel fasteners, which is why the code (R317.3, as adopted) calls for hot-dipped galvanized, stainless or otherwise approved fasteners and connectors in treated wood.

When you cut or drill treated wood, the sawdust carries preservative, so cut outdoors where you can, collect the dust rather than blowing it around, wear eye protection and a dust mask or respirator suited to wood dust, wash before eating, and never burn the scraps.

What other trades disturb at grade

The pest company row deserves care. Many termite treatment contracts and warranties carry conditions about soil disturbance, construction and wood-to-ground contact, and a trench through the treated zone can affect coverage depending on that contract's terms. You do not know what the owner's contract says, and you should not guess. Saying "your pest company may want to know we dug along this wall" is accurate and enough.

Worked example: one patio, one planting bed, one house

A house sits in a county whose design criteria table lists termite as moderate to heavy. At the back wall the bottom of the wood siding is 6 inches above grade and the sill is 9 inches above grade. At the side wall the siding is 5 inches above grade and the sill is 8 inches above grade.

The patio at the back. A concrete crew plans a 4 inch patio slab with its top surface 4 inches above the existing grade, against the back wall. The slab is a horizontal surface, so the rule is 2 inches from the slab's top to the siding. 6 inches minus 4 inches leaves 2 inches, which meets the 2 inch minimum with nothing to spare. The crew holds the slab top exactly there and slopes the slab away from the house, and records the measurement. Had they poured to the bottom of the siding to "make it look finished," the clearance would have been zero and the bottom course would sit in water every rain.

The planting bed at the side. A landscaper plans a raised bed that adds 3 inches of soil and mulch along the side wall. The bed is ground, so the rules are 6 inches siding to ground and 8 inches sill to ground. Siding: 5 inches minus 3 inches leaves 2 inches, which fails the 6 inch minimum by 4 inches. Sill: 8 inches minus 3 inches leaves 5 inches, which fails the 8 inch minimum by 3 inches. Note that the side wall already failed the siding clearance before anyone arrived, at 5 inches against 6; the bed turns a 1 inch shortfall into a 4 inch one and adds a new failure at the sill.

Same house, two outcomes. The back wall passes with a patio on it; the side wall fails with nothing more than dirt added. The difference is the datum: a slab is measured at 2 inches, the ground at 6 and 8, and a crew that applies the slab rule to a planting bed approves something the code does not.

What the landscaper does. Rather than building the bed against the wall, the landscaper sets it 18 inches out with a gravel strip between bed and house, keeping the soil at existing grade at the wall. He tells the owner the side wall is already an inch short on siding clearance, which is worth having the pest company or a carpenter look at, and that their pest provider may want to know about the work along the foundation.

What would change it. In a county where the termite entry is none to slight, the decay clearances still apply in full, because rot does not read the map. If the siding were fiber cement or another non-wood product, the siding clearances come from that product's installation instructions rather than the wood decay rule, but the wood sill and framing behind it are still wood. And if the bed held no soil, only loose stone with drainage beneath, some inspectors would treat it differently; ask the building department rather than assuming.

How to verify you got it right

Measure from the named datum and write it down: "back wall siding 6 in above existing grade; patio top set 4 in above grade; 2 in to siding." A photo with a tape measure in frame against the wall, taken before backfill or before the bed is planted, is the record that settles the question later.

References

• International Residential Code, Sections R301.2 design criteria, R317 protection of wood against decay and R318 protection against subterranean termites, 2021 edition, as adopted by your jurisdiction
• The treated-wood end tag and the preservative manufacturer's handling instructions
• See related: The Local Design Criteria Table
• See related: Lot Drainage and Where the Water From Your Work Goes