| Mar 16, 2006

Feature Article - February 23, 2006

Back toHome

Feature Article - March 16, 2006

Everything you never wanted to know about cement block foundations - and were afraid to ask

by JohnMcEwen, CMHCauthor

For those looking to confirm what I have to say please consult the Ontario Building Code Sections 9.12 through 9.15., or visit my web site at www.e-o-f.com. And I stress that any repair or material alteration of a foundation wall requires a building permit issued by your municipality. Always check with your municipal building department.

In the short space we have here I will explain how block foundations are built, why they suffer, how to fix them, and what to watch out for in hiring a contractor.


The problem: In a best case scenario: a hole is dug for the foundation. The footings are cast, these are simply a wide strip of concrete designed to spread the weight of the house out over a larger surface area. The inside perimeter should be filled to the top of the footings with or 1 inch clean stone (no dust, just rocks). Eventually a floor stab will be cast on top of the stone, but let’s build the walls first. Mortar is laid on top of the footing and the first courses of blocks are laid. A post-1975 eight inch wide block weighs about 33 pounds, has two big holes in it: 30% concrete, 70% air. 3/8 of an inch of mortar holds the whole thing together.

Once the walls are up they are "parged" with a thin layer of cement. The parged wall is then damp-proofed with a single layer of tar. Tar by itself has nothing to do with waterproofing, which we shall discuss shortly. A weeping tile or 4 in. perforated Big 0 is laid beside the footing, no higher no lower. Clay tiles are simply 4 in. diameter foot long tubes, laid end-to-end with a slight gap between each tube where water “weeps" into them. During the early 70s they were quickly replaced by me modem plastic version. The tiles should lead directly to either a sump pit or a storm sewer. Six inches of clean stone is placed over the tiles, and then whatever came out of that hole in the first place is used to fill the balance of the trench. Usually clay, sometimes sand.

And that's it. Water is expected to drain through 4 to 9 feet of clay, hit the clean stone, and weep into the tiles, which carry the water directly to either a sump pit or a storm sewer. Time and pressure enter the picture. From above the weight of the house is transferred through the foundation walls to the footings. This force tends to break the walls at the mortar joints. Corners and window well areas tend to break first, and the broken mortar line tends to "step" down towards the bottom of the wall. Pressure is also applied to the face of the wall. Bone dry clay will weigh in at 2700 pounds per cubic yard; it will reach saturation at about 3800 pounds. The deeper you go, the greater the pressure. This force tends to create horizontal splits between the first and second course of blocks.

At some point it becomes easier for the water to pass through these fault lines and enter the cavity of the blocks, then it is to get down to the tiles (again covered by only six inches of clean stone). Water fills the cavity of the blocks until pressure forces it through the same crack on your side of the wall. A puddle begins to form where the wall meets the floor slab.

The solution: To fix the problem you do exactly what the Building Code dictates. Excavate and remove everything to the bottom of the footings. Create a clean, flat, dry surface and apply a waterproof membrane to the exterior of the wall. Replace the weeping tiles, and make absolutely sure that they are physically connected to either a sump pit, a storm sewer. Rigid board insulation goes in front of the membrane (more for the protection of the membrane then anything else). Use clean stone for the backfill material, at least half of the wall, preferably the entire trench. Water hits the clean stone and falls directly to the tiles. The tiles conduct the water to either a gravity feed system (a storm sewer) or a sump pit with a pump. And that's it. Letter perfect to the Ontario Building Code; and it really, really works.

This from Canada 's National Building Code: "A-5.8.2 Moisture Protection. Moisture protection for building elements in contact with the ground is generally categorized as either waterproofing or damp-proofing. Waterproofing provides a continuous protection against water ingress and is intended to resist hydrostatic load. Damp-proofing, on the other hand, does not provide a seal against water ingress and cannot withstand hydrostatic pressure."

To fix it that's what you have to do. No other options exist under the Code. And, with block walls, because of the cavity, everything must be done from the exterior. To alleviate the symptoms, get as much surface water away from the foundation as possible.

Clean eavestroughing and downspouts, and improve grading if possible. This won't fix the cracks in the wall, but if the water isn't there in the first place it's less likely to cause you a problem. Secondly it is extraordinarily common for the weeping tiles never to have been connected to anything in the first place. If you have a sump pit look to see if the tiles are directly coming into it. If you are in the country, and you have no sump pit, where are your tiles going? If you're in the city and you have no sump pit, the only other option is the storm sewer. Call your municipal engineering department to see if your weeping tiles are directly hooked into their storm sewer system.

For many years I have come across companies that will offer "water control" or “pressure relief” systems, usually in conjunction with '"bentonite injection". It always seems that the more extraordinary the claim, the less extraordinary the proof, of which I have found nothing. Typically this will include digging a trench inside the basement, and drilling a series of 1 inch holes off the top of the footings into the cavity of each block. The floor slab is recast with the exception of a "rigid sealer". The rigid sealer actually creates an open air gap between the wall and the floor and allows water dripping down the wall to follow a direct path to the underside of the floor slab. The same gap also allows methane and radon to escape back up into the living space, which violates numerous soil gas emission rules. Also water is an excellent solvent, and your mortar lines are soluble in water. As water passes through the mortar it picks it up in solution, which later shows up as a white powdery material (Calcium Carbonate precipitate). This weakens the mortar joints, and may eventually cause structural problems. Draining the cavity of the blocks will have no bearing on the amount of pressure being applied to the wall from the exterior.

On bentonite injection: Most diagrams I view are of bentonite clay replacing existing clay (and rock, and construction debris) by about eight inches to create a ''waterproof barrier". Why wouldn't this displacement push a block wall into the basement? Also, prior to 1975, it was common for weeping tiles to be directly coupled into your sanitary line. Again, from the diagrams I normally see, the bentonite would necessarily fill your sewer lateral. And if bentonite created a 'waterproof barrier", why the additional trench in the basement? And so on.

Under the Building Code Act it is your responsibility to apply for permits, and it is your Building Department's responsibility to physically inspect the project prior to backfilling. Certainly, this is the best way to ensure that you or the company you hire to perform the task gets the job done properly.

Other Stories this Week View RSS feed

Support local
independant journalism by becoming a patron of the Frontenac News.