How To Look At A House
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How do termites get into a concrete block house?
Friday, September 21, 2018
Termites can infest a block house from the top or bottom, depending on the species. The ones that get in from the bottom, subterranean termites, live in the soil and enter the house to consume wood, then return to their nest in the ground. Older concrete block houses are vulnerable to subterraneans because of a type of construction called “block stemwall” that was used until about the mid-1980s.
The block walls were laid on top of a concrete foundation footing in the ground. Then a floor slab was poured inside the walls, with a thin strip of fiber-board placed on the wall around the perimeter of the slab area to allow room for the thermal expansion and contraction of the floor slab without damaging the walls. It’s called an “expansion joint,” and moisture in the ground causes it to deteriorate over time, leaving about a half-inch gap between the floor slab and the inside surface of the block wall. Subterraneans that find the gap between the floor slab and wall encounter the bottom of a tasty wood baseboard, and also the thin vertical strips that run up the wall from the floor to the ceiling behind the drywall, called furring strips. After they munch on the baseboard a while, the furring strips provide a highway directly up to the wood roof trusses at the top of the wall. And, best of all for the little critters, they can do all their damage undetected.
The photo above shows a fiber expansion between a floor slab and concrete block stemwall. Newer homes combine the floor slab and concrete footing into one piece, called a “thickened edge slab” or “monolithic foundation.” The concrete block is laid along the perimeter of the slab, over the thickened edge, and there is no concealed gap in the ground.
Termites can still find their way into the home thru penetrations in the floor slab created for plumbing pipes, electrical conduits, air conditioning refrigerant lines, and small cracks that develop in the slab as it hardens or settles. But access to the home’s wood components is not as easy as in a stemwall home. Drywood termites have a different strategy: during the spring swarming season, a winged queen termite and her flying entourage enter the house from above, typically through a torn soffit vent or gap in the roof around a chimney or plumbing vent.
They are slower to establish a colony than subterraneans and less voracious eaters, but can be harder to detect because they don’t leave the telltale surface mud-tubes that are characteristic of subterraneans. Whether it’s subterraneans or drywoods, concrete block houses may be less vulnerable to termite attack than a frame house, but not immune.
Subterranean termite mud tube running
across drywall on interior of concrete
block wall, from wood window trim to
wood cove molding.
Also, see our blog posts How do termites get into a brick house? and Are homes in Florida required to have termite protection? and Does the presence of carpenter ants in a house indicate that there are probably also termites?
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