Footing capillary break vs. bonding of masonry to footing
hallie17
| Posted in GBA Pro Help on
I am an architect in Ohio where we often use CMU for foundation wall, especially on addtions. I’ve been showing “damp proofing as capillary break over footing” on my wall sections based on GBA recommended details. I recently got a question from a mason about this. He had never heard of this detail, and was concerned that it would create a bond break, preventing the mortar from properly connecting the block to the footing. Is his concern a valid one?
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What product is being used?
Do you also specify rebar and fill the voids in the CMUs with cement?
Hallie,
Three comments:
1. In parts of the country where the use of CMUs is rare (like northern Vermont), many people might ask, "Why aren't foundation contractors in Ohio using poured concrete for their foundations?" Local variations in construction practices (like the one you describe) are mysterious.
2. In most cases, a CMU foundation wall will require the wall to be connected to the footing with rebar, and will require a certain percentage of the voids to be filled with rebar and grout, as Steve noted. When in doubt, talk to an engineer.
3. For more information, see this article: Capillary Breaks Above Footings.
I'm not an engineer, but the one thing I want to point out is that the capillary break will prevent the wall from sucking water up from below the footing, but if the water table rises above the footing it becomes problematic. A detail like https://www.mtidry.com/hyperspecs/interior-below-grade-cmu-moisture-control-system creates a path for that water to drain back to the drain tile and sump system. If concerned about soil gases in a detail like this the courses of Control Cavity can be sealed with a polyurethane sealant, or a poly barrier can be installed up the wall.
Tyler,
On excavations where high water tables might present a problem we deal with it by tying the perimeter drains to a stub out of perforated pipe run through the foundation wall to a small rock pit on the interior.
Look at the Mfg info. But it seem a cement based waterproofing such as Throseal or a crystalline waterproof product such as Xypex would be a better than a latex based material like Drylok.
Hallie, I'm glad you asked the question as I was concerned about the same issue. There is no requirement in the code for reinforcing and grout to tie the footing to the foundation wall. The bond of mortar and block to the footing seems to be important in walls without reinforcing. So adding dowels and some grout seems to be necessary to achieve that detail. A basement concrete slab would help to resist the force. I think this issue needs some empirical study.