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Question DetailsAsked on 7/8/2017

Looking to purchase a house and I had a question about block foundations.

I looked at a house built in the 40's today and saw something I've never seen before. The block foundation had strips of wood about 3/8 thick separating some of the levels of block. First question would be why and the second would be is there any concern? There didn't appear to be any rot going on but they didn't look like they were treated with anything either. I live in Minnesota if that helps.

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2 Answers


Have not seen that for a LONG time but then I no longer live in an area with a lot of old homes either. Before real building codes came into practice, say before about 1960 or so, lots of basement builders would lay a strip of lath (3/8 or 1/2" - same thickness as the mortar joint) about every 2nd course of cinder block, or about every 5th or 6th course of brick, to serve as a firring strip for nailing into to put up panelling or to (more commonly back then) to nail reinforcing wire mesh to preparatory for plastering. The wood strip, in those cases, would typically be about 2 inches wide, so not full depth of the wall - that dimension would be easy to confirm by drilling into one.

Obviously a rot issue if the wall is damp - but if not moldy/rotted or punky if you push a nail or other sharp item into the ones nearest the bottom (the ones most likley to be wet) probably not an issue if OK after 70 years or so. Plus back then quality masons doing this would commonly use cypress, sassafras, locust or heart cedar or redwood for maximum rot resistance - or in that area probably more commonly locally available white oak or American Larch (locally called Tamarack, but not same tree as the generlly much larger western Tamarack Larch of the western US).

This wood strip was originally done in double-thickness brick walls for nailing into, because driving fasterners into brick broke the brick up plus did not hold worth beans plus back kthen their avilable nails would not penetrate brick well - this before the days of ramset nails and expansion anchors. The practice carried over to cinder block walls - tapering off and going away in the 60's when ramset nails and hardened concrete nails came into use.

The piece of wood obviously eliminates a few inches of mortar at those joints, so it somewhat weakens the wall's resistance to inward bowing but not dramatically (since the tensile strength of mortar joints is awfully low anyway) - but certainly if the wall is not significantly bowed inward after 70 years and the strips are in decent condition I would say I would not be at all concerned by these strips. They also are not hard to cut out with a circular saw (with a sacrificial carbide blade) and replace or repoint the joint if only a few are deteriorated.

Answered 3 years ago by LCD


I rashly assumed you meant these strips were part of the joint - laid flat on the inner edge of the blocks and forming the inside part of the joint. If you instead meant 5/16" lath (typically about 5/16" x 1-1/2 to 2 inch wide) mounted edge up and flush to the FACE of the wall (nailed through it into the joints), then those would just be spacer strips for plastering - to space the mesh out from the wall so the plaster would properly penetrate through and bond to the mesh. If that is the case and they and the wall are not all gunked up with plaster from a previously plastered wall, would probably be originally installed for that purpose and the wall never got plastered - for whatever reason (like they realized a plaster basement foundation wall is an invitation for trapping moisture and mold). Or maybe they had softening and mold problems in a plaster wall and tore it off, leaving the lath strips.

In that case, many times the strips lying on top of the courses which were discussed in the first response were left sticking out 1/4-3/8 or so for the same reason, so not flush to the face. In fact, in the "good old days" - up to about 1960, you could buy "double-sawn spacer strips" for this purpose where you bought plastering supplies - about 1/4-3/8" on the part intended for sticking into the wall joint, and about 3/4" thick on the part that stuck out to better take nails or mesh U-staples without splitting it, so it looked like it had been rabetted.

[An aside- if "plastering" a basement wall like that, usually it is best "plastered" with a Portland cement coat rather than lime based, so is technically more a waterproof stucco (which may be sandless or have only very fine sand in it) than a "plaster"].

Answered 3 years ago by LCD

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