How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

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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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To learn more about termites, see these other blog posts:

Should I be worried about termites if my neighbor's house is being tented?

Is the WDO (termite) inspector allowed to poke holes in my wood siding and trim? 

Do carpenter ants cause structural damage to houses in Florida?

How long before closing can you have a WDO (termite) inspection done?

How long does Bora-Care® last? 

 Why is it a mistake to store lumber in the crawl space under a house?

How do termites infest a house and remain hidden while doing major damage?

 If termite damage appears to be old, does that mean that termites may no longer be present?

How do I know if my WDO/termite report is "clear"? 

When do termites swarm in Florida?

Does a recent termite company inspection sticker mean there are no termites? 

Can a mobile/manufactured home get termites?

Do I have to tent the house if I have termites? 

What is the difference between a subterranean termite and a drywood termite?

What are the green plastic discs in the ground around the house? 

What is a termite shield? 

Do termites eat concrete?

What is a clean WDO?  

What do termites eat?

How do I treat wood rot  that's listed in my termite-WDO report? 

Do I really need a termite-WDO inspection? 

What's causing those holes in the fascia?

Does wood chip mulch in the yard attract termites?

I think I have termites. What does a termite look like?

I'm buying a concrete block house. Do I still need a termite inspection? 

• I saw a little termite damage on the baseboard. Should I be concerned?

   Visit our TERMITES, WOOD ROT AND PESTS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

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