Foundation Types and Where Termites Get In

Why this matters

Subterranean termites live in the soil and need a protected path from soil to wood. The foundation decides where those paths can hide. A tech who walks a slab home looking for tubes on a crawlspace pier, or treats a supported slab as if it were monolithic, misses the entry the colony is actually using and leaves a gap the next inspection finds as fresh damage. Identifying the construction type in the first five minutes of the visit tells you where to look, what you will never be able to see, and what you must write on the report as inaccessible. It also decides which treatment approaches are even on the table, which is why the proposal starts here.

Read the construction before you read the wood

From the outside, three clues usually give the type away. The height of the finished floor above grade: a slab home sits a step or two up, a crawlspace home several steps, with foundation vents or an access door in the wall. The presence of foundation vents or a crawl door. And the garage: an attached garage is almost always a separate slab, poured at a different time, with its own joint against the house.

Inside, a slab home has no floor hatch and the floor feels solid underfoot with no bounce. Look in the water heater closet or at a bath with an access panel for the plumbing penetrations, and in the garage for where the house slab meets the garage slab. Many homes are a mix: a basement under the original house, a slab addition, a crawlspace under a sunroom. Treat each part as its own foundation and record the joints between them, because the junction between two pours is one of the most common hidden entries on any house.

The five foundation types and their hidden entries

MONOLITHIC SLAB (slab and footing in one pour)
 siding|
 ------+=====slab======pipe=====tub drain====
 grade |thickened edge  |       |bath trap,
       \_______________/        |open soil box
 entry: pipe sleeves, bath trap, cracks,
        slab edge hidden behind siding or foam

FLOATING SLAB (slab poured inside the wall)
 wall |expansion joint
      ||=====slab=====pipe=====
      ||  fill         |
 _____|_______ footing
 entry: joint around full perimeter, pipes

SUPPORTED SLAB (slab edge bears on wall top)
 =====slab======+=========
         joint->|  block |
                | hollow |
                | cores  |
                +--------+ footing
 entry: slab-to-wall joint, block cores, pipes

CRAWLSPACE
 ===sill plate=====joist========sill plate===
 |block|    pier     pier         |block|
 |cores|    |  |     |  |         |cores|
 ---soil----debris-----wood stake---soil---
 entry: tubes on walls and piers, block cores,
        wood touching soil, debris, sill plate

BASEMENT
 ===sill===
 |block wall,     |
 |hollow cores    |
 |===floor slab===|cold joint at wall base
 entry: cold joint, wall cracks, pipes,
        finished walls hide the face

Monolithic slab. The footing and slab are one pour with a thickened edge. The slab itself is a good barrier; termites come through the breaks in it. Plumbing penetrations where pipes rise through the slab, the bath trap (a box of open soil left under a tub drain so the plumber could connect it, often reachable only through an access panel), shrinkage cracks, and the outside edge of the slab where siding, stucco, brick veneer or rigid foam insulation extends to or below grade and hides tubes from view. The slab edge is the place to look first from outside.

Floating slab. The slab is poured inside a separate foundation wall and is not tied to it, leaving an expansion joint around the entire perimeter. That joint is a continuous hidden path along every exterior wall, under the baseboard and flooring, and it cannot be inspected from inside without lifting floor covering.

Supported slab. The slab is poured on top of a block or concrete foundation wall and bears on it. The joint where the slab meets the top of the wall is hidden, and if the wall is hollow block, the cores are open vertical shafts from footing to slab. Termites can travel up inside the block and reach the sill or framing without ever showing on the face of the wall.

Crawlspace. The most inspectable foundation, and the one where most termite evidence is found by eye. Tubes run up foundation walls, piers and plumbing lines. Hollow block walls and block piers have the same core problem as a supported slab. Wood in contact with soil (form boards left in place, stakes, stair stringers set on dirt, debris piles) is a bridge that needs no tube at all. The sill plate on top of the foundation wall is where most crawlspace damage is found.

Basement. A deep foundation, often hollow block in older homes, with a floor slab poured inside the walls and a cold joint at the base of the wall where the two pours meet. Cracks in the wall, pipe and utility penetrations, and the cold joint are the entries. Finished basements bury the inside face of the wall behind framing and drywall, which is why so many basement infestations are first found as swarmers in spring.

Why the construction drives access

The table below is about what you can see, which decides what the WDI report can honestly claim.

Every item in the right column goes on the report as an area not inspected, with the reason. See related: The WDI Report Form, Section by Section.

Why it drives the treatment approach

This card does not give treatment steps; it explains why the foundation narrows the choice. A liquid barrier needs continuous treated soil along every entry, which is straightforward against an exposed crawlspace or basement wall and hard under a slab, where the soil next to a hidden joint is reached only by drilling through the slab or by rodding from outside. Hollow block walls add the cores as a separate void to address. Where the slab, finished floors or a well nearby make a continuous barrier impractical, baiting becomes more attractive because it does not depend on reaching every joint. See related: Termite Bait vs Liquid Barrier vs Fumigation Decision Matrix and Liquid Termiticide Trench and Treat Standard for the methods themselves, and the product label for every site and method it permits.

Rigid foam below grade deserves a separate note. Foam board insulation on the outside of a foundation wall or slab edge gives termites a hidden tunnel from soil to sill, and the model residential code restricts foam below grade in the heaviest termite areas unless an approved termite protection method or another of the section's exceptions applies (IRC Section R318, as adopted by your jurisdiction; check the current edition and your local termite infestation map). Whether or not the house was built to that rule, foam to grade is an inaccessible area on your report.

Worked example: one house, three foundations

A two-story home: the original 1960s house sits over a hollow-block basement, partly finished; a 1990s family-room addition on the back is a monolithic slab; the attached garage is a separate slab with a joint where it meets the house.

Step one, read it. From the yard: the original house has small basement windows in the block, the addition sits two steps above grade with no vents, and the garage floor is at grade. Three foundations, two junctions (addition to basement wall, garage to house).

Step two, assign the hidden entries. Basement: block cores in all four walls, the cold joint at the base of each wall, pipe penetrations at the laundry and the water service, and the finished family-room wall in the basement that hides two of the walls entirely. Addition: plumbing for a wet bar through the slab, the exterior slab edge behind vinyl siding that runs below the mulch line, and the joint where the addition's slab meets the original block wall. Garage: the joint along the house wall.

Step three, inspect in that light. The tech finds nothing on the exposed basement walls, then finds a working tube on the outside of the addition, coming up from the mulch where the siding meets the slab edge, eight feet from the corner where the addition meets the original house. The tube is in the exact place the construction predicted: the edge of a monolithic slab where the cladding hides it.

Step four, record what could not be seen. The report lists as not inspected: the two finished basement walls (drywall), the slab edge of the addition along the remaining siding (siding to grade), the underside of the addition slab, and the block cores (concealed). The tube location goes on the graph with distance from the corner. See related: How to Draw the WDI Inspection Graph.

Why the order mattered. A tech who treated the basement as the whole job because that is "where termites are" would have spent the visit on wall faces that showed nothing and might not have pulled mulch back from the addition's siding at all. The construction pointed to the addition's slab edge and the junction with the original house before a single tube was found.

What would change the read. If the addition had been built over a crawlspace instead of a slab, the same junction would be inspectable from underneath and the slab-edge concern would disappear, replaced by the sill plate and piers of the new crawl. If the basement were poured concrete rather than block, the cores would come off the list, and the cold joint and cracks would remain.

Crawlspace and basement entry hazards

Before you enter a crawlspace, check for standing water near wiring, exposed or damaged conductors, and signs of a sewer leak. Do not enter where water and live wiring meet; have the electrical isolated by the person who controls it and confirm dead before going in, then have it restored by the same person once you are out and confirm the affected equipment (a sump pump, a crawlspace light) runs again. Rodent droppings are a hantavirus exposure when disturbed and dried; do not sweep or vacuum them, and wear a respirator with a particulate filter under your company's respiratory protection program (29 CFR 1910.134) where you will disturb them. Carry a light and tell someone when you go in and come out.

References

• See related: Termite Bait vs Liquid Barrier vs Fumigation Decision Matrix
• See related: Liquid Termiticide Trench and Treat Standard
• See related: The WDI Report Form, Section by Section
• International Residential Code Section R318 (termite protection, foam plastic below grade), as adopted by your jurisdiction
• OSHA 29 CFR 1910.134, Respiratory Protection