Block Basements: Part Two

Why They Fail?

The number one issue with a block basement is moisture; blocks are not waterproof…period. Water will penetrate any mortar crack, open seam or just about any opening for that matter and we all know that the blocks themselves are porous. I have seen cases where a contractor drilled a hole in the second course of blocks and the water actually flowed out. The CMHC guide to damp basements has a good list of reasons why basements leak.  Ineffective or non-existent eavestrough and downspouts are top of the list. Lack of slope away from the building, poorly draining backfill around the basement, ineffective window well drains, defective or missing below grade drainage systems are the common culprits for a wet basement. There are three other issues which are serious and very expensive to fix, a basement built in an area of high water table, in a valley or on the side of a long sloping grade.

A block basement with moisture issues is usually fairly easy to detect, provided its not covered in drywall or paneling. This is where Thermal Imaging Cameras are beneficial. This technology, which was developed by the military, can actually detect moisture in most finished basements.  If the blocks are exposed and you have a white chalking or powder along the first or second row of blocks you can bet you have a drainage issue. This chalking is called efflorescence and is not uncommon in a block foundation, especially an older one. Efflorescence by itself is not that harmful, what it means is there is a salt content in the blocks and the external ground water is seeping thru and depositing this white crystal like powder on your block wall. There are as many reactions from a wet basement as there are terms. Mold, Mildew and that “Damp Smell” are all signs of a water issue. Mold or rot at the end of your floor joists or along the sill plate are also a sign. The CMHC publication “A Guide to Fixing your Damp Basement” is a good starter manual. Contact CMHC at 1-800-668-2642 and ask for catalogue number NH15-72/2007E. Moisture in a block wall can usually be fixed by excavation and installing any number of waterproofing membranes around the foundation. We will discuss that next week.

The other issue is foundation failure and this is usually caused by seasonal expansion and contraction of the soil around the basement which puts pressure on the actual block foundation. During the freeze-thaw cycle this pressure builds up against the wall and often causes horizontal cracks along the mortar lines. This can and does lead to a bow in some block walls. I generally see this at or near the frost line or grade, anywhere from the fourth to sixth row of blocks in a standard basement which is usually 11 rows high. The next defect that I see too often is the lack of anchors in the sill plate and the soil pressure is actually pushing the wall vertically inward.  While not as common, a shear crack at the base or second row of blocks is usually caused by poorly drained soil that has moved these blocks inwards. These types of block failure are more common than many people think and usually mean major foundation restoration or replacement if not caught in time.

Another common problem is step cracks that follow the mortar line. These can be the result of settling or a poorly installed footing base. This is reasonably common in block basements and if there is no continued movement there are some reasonable cost repairs available for this type of crack. A wall with a bulge or a full bow can often be stabilized by means of buttress work or infilling the blocks with rebar and concrete slurry. I have seen a Kevlar wrap used on a bowed wall.  I have also seen steel columns installed that were anchored to the pad and the floor joists. The benchmark most contractors use is ½’’ to 3/4’’ of deflection for a bowed wall to be considered for repair. While the rebar and slurry does work it has some drawbacks. Getting an even pour inside a block wall is difficult and air pockets are not uncommon. Feeding a 6-7 foot long piece of rebar inside a block wall is a daunting task; it takes patience and a skilled hand. Any block wall with more than a ¾-1” of displacement should be excavated and brought back to plumb or replaced.

Ground water drainage and eavestrough discharge are two major issues and I can’t stress enough how important these are. Every time we get called to inspect a basement problem it’s often a home that is 25 plus years old, the downspout discharge is limited to the elbow on the end and the wall has a major crack. I shake my head thinking that spending twenty dollars for eavestrough extensions would likely have saved what quite conceivably will be a five figure repair. If you have a block basement and see activity in your wall, don’t delay getting a professional foundation company to look at it. Caught in the early stages you can save thousands of dollars.

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