Termite Infestation Probability Map: What Your Zone Means

The termite infestation probability map in the International Residential Code (IRC) sorts the continental United States into four zones: very heavy, moderate to heavy, slight to moderate, and none to slight. The zone shapes the termite rules that apply when a house is built. It says nothing about whether termites are in a particular house today; only an inspection answers that.

What the four zones cover

The very heavy zone covers the Deep South and parts of California and Texas; the lighter zones cover the northern tier. The code uses the map in its section on protection against subterranean termites, the kind that nest in soil and tunnel into wood. The Building America Solution Center at Pacific Northwest National Laboratory (PNNL), which publishes an adaptation of the 2021 IRC map for the Department of Energy, places Alabama, Florida, Georgia, Louisiana, Mississippi and South Carolina, plus parts of California and Texas, in the very heavy zone.

On the same map, the moderate-to-heavy band spans most of the Southwest, the southern Plains, the lower Midwest and the Mid-Atlantic. The two lighter zones run across the northern tier, from the Pacific Northwest through the northern Plains and the upper Great Lakes to northern New England.

The code prints a caution beside the figure: “Lines defining areas are approximate only. Local conditions may be more or less severe than indicated by the region classification.”

The figure has moved between editions. Codes based on the 2018 IRC show it as Figure R301.2(7). The 2021 IRC moved it into the termite section as Figure R318.4. The 2024 IRC renumbered that section to R305 and the map to Figure R305.4.

Who decides whether an area counts as termite country

The local building department does. The IRC asks each jurisdiction to fill in Table R301.2, its list of local design conditions, including whether local experience shows damage from subterranean termites. In a December 2021 bulletin, New York’s Division of Building Standards and Codes told local officials to treat the map as a guide and, where the map and local history disagree, to fill in the table from well-documented local conditions. An entry of “yes” switches on the code’s termite provisions.

So two towns in the same map zone can apply different rules. The building department can say what its adopted table shows.

What the code requires in new construction

Where the table marks termite risk, the IRC requires at least one of six protection methods, alone or combined. They fall into three groups:

Group	Methods the code accepts
Chemical	termiticide applied to soil or to wood in the field, at the concentration, rate and method on the label; bait stations installed and maintained per the label
Materials	pressure-treated wood with a quality mark; wood species that resist termites naturally; cold-formed steel framing
Physical	approved barriers made for termite prevention, such as metal or plastic sheeting or collars

Shields placed on top of an exterior foundation wall count only in combination with another method, under Section R305.3 of the 2024 edition.

The very heavy zone adds a rule for foundation insulation. Extruded and expanded polystyrene, polyisocyanurate and other foam plastics may not be installed on the exterior face of, or under, foundation walls or slabs below grade. Foam installed above grade must stay at least 6 inches from exposed earth. The exceptions cover buildings framed entirely in noncombustible or pressure-treated material, the interior side of basement walls, and assemblies that add an approved method of protecting the foam and structure.

Clearances that apply in every zone

Separate from the termite map, the code’s decay rules set minimum distances between untreated wood and soil. Section R317.1 of the 2021 IRC requires naturally durable or preservative-treated wood in these situations:

Wood member	When treated or durable wood is required
Floor joists in a crawl space	closer than 18 inches to exposed ground
Girders in a crawl space	closer than 12 inches to exposed ground
Framing resting on exterior foundation walls	less than 8 inches from exposed ground
Siding, sheathing and exterior wall framing	less than 6 inches from the ground, or less than 2 inches above patios, steps and similar slabs

The same numbers work as a checklist for an older house. Soil, mulch or a new patio built up against siding closes the gap the code was written to keep.

What FHA adds for new homes

For new houses financed with FHA-insured mortgages, HUD requires termite treatment documentation in most of the country. Its Termite Treatment Exception Areas list, last revised November 3, 2021, waives the requirement for all of Alaska, Idaho, North Dakota, Oregon and Washington; for Maine outside York and Cumberland counties; and for named counties in Colorado, Michigan, Minnesota, Montana, New Hampshire, New York, South Dakota, Utah, Vermont and Wyoming. Everywhere else, it applies.

The builder records the protection on form HUD-NPMA-99-A. Where a pest control company applied the treatment, the builder guarantees on that form that if subterranean termites infest the structure within one year of closing, a licensed or state-authorized company will treat it without cost to the buyer, and that the builder will repair termite damage during that year. The form states that pressure-treated sills alone are not acceptable protection. The guarantee excludes additions and alterations made by the buyer, including landscaping and mulch changes that disturb the treated area.

Termites the map does not separate

Formosan and drywood termites add risks that the map does not show. It does not distinguish termite species, and these two threats have their own geography.

The Formosan subterranean termite is an invasive species. University of Florida IFAS Extension reports it in Hawaii and across the southeastern states, north along the Atlantic coast to Virginia, inland to Tennessee, and in southern California, based on distribution records through July 2024. Its colonies take 5 to 8 years to mature. UF/IFAS states that controlling it requires professional pest control service and that over-the-counter products are unlikely to be effective.

Drywood termites nest in the wood they feed on rather than in the ground. The University of California Statewide IPM Program finds the western drywood termite along the Pacific coast, into the Central Valley and in the deserts of Southern California, and treats its fecal pellets, which have six hexagonal sides, as diagnostic. The IRC’s termite section is titled protection against subterranean termites, and its soil treatments and barriers are not aimed at drywood colonies.

What the zone means for a house that already stands

For an existing house, the zone is a reason to watch for the conditions that invite termites, since the code’s termite provisions only tell builders what to install. HUD’s advisory on the builder’s guarantee form lists those conditions: foam insulation at the foundation, wood touching soil, faulty grading, firewood against the house, poor ventilation, moisture, wood debris in a crawl space, wood mulch, tree branches touching the structure, landscape timbers and wood rot.

Mississippi State University Extension recommends preventive treatment and yearly inspections as the defense against hidden infestations, and advises that buildings in areas with Formosan termites be inspected every year, whether or not they have ever been infested.

When to call a professional, and what to ask

Call a licensed pest control company if winged termites emerge indoors, mud tubes appear on the foundation, or probing exposed wood finds hollow spots, a check EPA suggests. Call before adding foam insulation to a foundation in a very heavy area, and before an addition or new slab disturbs soil that was treated when the house was built. Questions worth asking:

• Which termite species are active in this county, and does the Formosan or drywood termite change the plan?
• Which of the code’s protection methods do you propose for this foundation, and why that one?
• Is there foam insulation below grade here, and can the foundation still be inspected?
• Which conditions around the house would you correct first?
• Does your state license cover termite work, and will I receive a written record of the treatment?

For new construction financed through FHA or VA, the form’s own instructions route a copy of HUD-NPMA-99-A to the buyer through the lender at closing; where a pest control company treated the site, the company’s HUD-NPMA-99-B service record is attached to it. The form describes itself as a record of termite treatment for builders, pest control companies, lenders, homebuyers and HUD, which makes it worth keeping with the house papers.