“Slow”, thorough engineering investigation solves flooding problems

Going slow – like many months for a simple investigation, longer still for complex ones, ensures the cause of the problem is determined.  And the investigator doesn’t succumb to the tyranny of the obvious – as I almost did. (Ref. 1)  You’ve got to have time to think and reflect.  Going slow also helps the owner adjust to seeing his property taken apart during the work.

I investigated the cause of two wet basements in the past 1.5 years.  More than just wet, a flood in one case, 3 inches deep, and very wet in the other.

There was also water in depressions on the properties that sloped down to nearby lakes.  That meant poor surface drainage and probably high water tables – evidence of a possible cause of the wet basements..

The homeowners helped in both cases.  One used a novel method for determining the correct cause of her wet basement.  I’ll use her simple technique in future.  The other was in the right place at the right time to see the actual cause of their wet basement, and in a very striking way.

Both Houses

Both basements were finished including the floors.  But you could see water flowing from under the finished floors and across the exposed concrete floor in adjacent furnace rooms.  The water came from the direction of the basement walls on the up-slope side of the properties.

We cut small holes in the gyproc at the bottom of the walls and gradually added other holes and enlarged them – in a sense, we chased the wet basement problem.  This exposed the wood sill at the bottom of the walls and the area where the concrete floor abuts the concrete wall.  We also took up part of the finished floor in one house.

The owners helped and we went slowly so they could get their heads around the dismantling and the mess.  These were well-appointed, $350,000 plus homes, one about 30 years old and the other 40.

The exposed wood sills were water stained at both properties.  The stain gradually faded along the length of the wood sills from a dark area in the middle.  The stain indicated the wall was leaking, and the dark area suggested the location of the leak.

Just to be sure, we cut small holes in another wall in each house well away from where the water was seen in the houses.  We saw clean, unstained wood sill indicating no leaks.  There was a leak along one of these adjacent walls 20 years earlier that was fixed by constructing a new, perimeter footing drain.  Fine soil clogs these drains often enough after a few decades.

I concluded a clogged footing drain was the cause of the flooding at both houses, a good initial hypothesis as to cause.  But, I was in for a surprise.

(You can imagine there was quite a mess in both houses now with dismantled wall debris everywhere.  But we were going slowly – weeks now, and soon months)

Where was the leak?  How was water getting from a clogged footing drain into the basement – if that was the source?  The concrete wall was stained a little at the location of a hairline crack in one house.  But this crack was so fine I quickly dismissed it as the source of the leak, and it was above the suspect footing drain.  Surely such a tiny crack was not the cause.  Surely.

Where was the leak then?  I thought about the construction joint where the concrete floor abuts the concrete basement wall in both houses.  It measured 1 to 1.5 mm wide and ran the length of the walls.  The construction joint was also down near the suspect footing drains on the outside of the basement wall.

I concluded that the footing drain at both homes was clogged after 30 and 40 years, water was backing up in the drains and getting into the basement through the construction joints.  We would dig up the footing drains at both houses and fix them.

It took me a while to conclude that construction joints could admit so much water.  The penny dropped, so I thought, when I realized that not much water would flow through a 1.5 mm hole but a lot would flow through 100s of 1.5 mm holes joined together.  Like a line of holes in a sieve or the holes in a garden hose used for irrigation.

House #1

But, again, just to be sure, we uncovered a greater height of wall in House #1 – more time more debris, and saw that an area of the wall was honeycombed a few feet above the wood sill.  There were small holes in the wall between the pieces of gravel in the concrete.  The inside wall was porous.  This happens when the concrete is not well mixed during construction.  It doesn’t usually cause a problem because it’s localized and above the footing drain.

Fortunately, we had a very heavy rain a few hours after work.  My client called to say water was flowing from the honeycombed area like water from a tap.  It stopped shortly after the rain stopped.  He videotaped and I saw that it was so.  My client was in the right place at the right time.

We uncovered more wall later and found that a large area was honeycombed.  We also uncovered the outside of the wall and saw that the honeycombing – the porous area, continued through the wall.  We also saw that the water table was at the level of the honeycombing.

There was a source of water and a means for the water to get through the wall, through the porous honeycombing.  The honeycombing and the high water table were the cause of the wet basement in this house, not the footing drain.

We fixed the leak by patching the outside of the wall well above the footing drain that we had considered digging up, and at much lower cost.

(The patching details are not so important to my message here about the advantages of a “slow”, thorough engineering investigation)

House #2

Also, again to be sure, my other client, House #2, decided to investigate the innocent-looking, fine crack in their wall when I was away.  She simply took a garden hose and let it run for some time at different locations against the wall starting at the fine crack.

She saw that water flowed through the crack and stopped when she removed the hose.  She also saw that less water flowed when the hose was at increasing distances from the fine crack.  The fine crack and a water filled depression in the sloping ground were the cause of the wet basement, not the footing drain. 

We fixed it too by patching the outside of the wall at the location of the crack, also at a much lower cost than digging up the footing drain.  We did expose the top of the footing drain over a short distance during the patching.  It appeared to be well constructed.

(We are going to monitor the effectiveness of this repair over the next couple of years)


So, four or five months later in both cases after a “slow”, thorough investigation – and a lot of gradually accepted mess, we determined the correct cause of the wet basements.  And we fixed them for a lower cost than might have been the case if I had remained in the grip of the obvious.


  1. “Getting seduced by the tyranny of the obvious”  Posted December 9, 2013 at www.ericjorden.com/blog



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