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Leaking Basement Walls

Dear Tim,

What is the best method to ensure new basement walls don’t leak?

Tom H., Pocatello, ID

Properly constructed concrete is, by itself, waterproof. Consider the thousands of plain reinforced concrete water storage tanks out there that have performed well over the last century.

Why, then, do so many basement walls leak? The rub comes in the “properly constructed” part. Failure of any of the following can cause groundwater intrusion problems:

• Bad concrete mix. Too much water or too little cement in the mix are common culprits resulting in low strength concrete prone to cracking.
• Improper consolidation. Wet concrete must be vibrated (consolidated) or voids will occur. Banging the outside of the forms doesn’t count. Mechanical vibrators must be used.
• Hot or cold weather conditions. When curing, concrete is sensitive to extreme temperatures. Too hot or too cold will cause weakness.
• Cold joints. Interrupting a pour mid-wall (cold joint) leaves a seam prone to leakage.
• Free-falling wet concrete too high. Pouring a wall from the top of tall forms, particularly if a lot of rebar exists, causes large aggregate to segregate, causing rock pockets. Four-foot maximum fall height is a good rule of thumb.

• Bad structural design. Insufficient rebar quantity or improper rebar placement can result in wall cracking.

In my opinion, the best method of ensuring a waterproof wall is to use a subsurface drain system on the outside of the wall. This intercepts groundwater before it hits the wall. Either drain rock backfill or a proprietary fabric-polyethylene sheet system works well. In each case, a perforated drain pipe must be installed at the bottom of the footing to collect and transport groundwater away.

Membrane systems adhering to the outside of the wall can also work well, as long as they’re applied correctly and no damage occurs to them during construction — a tall order, generally.

Waterproofing admixtures are great; however, because concrete is by itself waterproof, they tend to be redundant. Also, improper design or construction (see above bullet points) can trump any benefits this system might provide.

I was once involved as an expert witness in a leaking basement wall case. The house was situated on a clayey hillside — an indicator that migrating groundwater would be likely. The walls were built in the winter; rain had transformed the excavation into a muck pit. It was obvious the contractor was in a hurry. He did many things wrong, but here are the ones that caused leakage:

• He did not mechanically vibrate the concrete, leaving voids and rock pocket conduits for groundwater to get through.
• His perforated pipe drain system was installed in the slop of his excavation, causing it to foul and plug.
• Rather than backfilling with washed drain rock, he used a sand-gravel mix, which in the mucky conditions was not porous enough to ensure groundwater would go down rather than sideways.
• His stick-on membrane was punctured and was not properly overlapped.

In short, this contractor was begging for the lawsuit and judicial spanking he ultimately got. Even the best expert witness could not bail him out!