By Charles Weiner
Last year I published an article on how to repair our stone foundations in the Glebe area. (Glebe Report August 2018, page 27). I was taken aback by the wonderful response that included over 40 email with questions and concerns as to this maintenance regime. In particular you queried about the possibility of lowering your stone foundation to allow for more headroom in the basement. On several occasions, I came to visit your homes so that different aspects (drainage depth, footings, etc.) could be explored and the possibility of doing this ascertained.
Lowering a basement
Because stone foundations seldom if ever have a concrete footing, underpinning (which involves excavating under the walls in three-foot sections and pouring a concrete footing) is financially impractical and virtually impossible without compromising the structural integrity of the walls.
The second method used to lower the depth of a foundation is called the bench footing. This method is financially viable and does maintain the structural integrity of the basement if properly crafted. I recommend that this procedure be carried out under the guidance of a professional structural engineer. Surprisingly, having their input on an important project such as this is not as expensive as one would think. On average, following last year’s prices, this may cost anywhere from $500 to $1,000 depending on the extent of the report. A visit to inspect should cost approximately $400. Prices will vary depending on the chosen firm.
With or without an engineer, I do not recommend a bench-footing if your basement is to be finished. The reason is that a high percentage of benches allow water from the exterior to leak in, sometimes in uncontrollable amounts. This is why I recommend either saving the building of a full-height basement for a new adjoining addition, or constructing a basement with either a poured or concrete block foundation on a pre-existing footing so that the underpinning procedure could be employed.
There are primarily three types of original foundations in the Glebe area: stone (which I already covered), rubble, and cinder-block.
Repairing rubble foundations
The rubble foundation is a poured concrete foundation filled with small stones, usually river stones, and sand. The stone size may vary from pea size to fist or larger. During the early part of the century or before, Portland cement (the bonding agent) was at a premium and used sparingly. For this reason rubble walls in the area are on average 15” in width and in most cases do not have a footing. The width acts as the stabilizing factor.
Over time, on average 70 years, rubble walls will begin to degenerate from the outside in. In many cases the walls become soft enough that a screwdriver can be pushed through. Even like this, if there are no visible signs of structural cracking, the walls can still be protected from further degeneration. Installing a membrane against the wall is not enough to protect the wall from further damage.
The first step in restoration is to excavate from grade to footing. I recommend selecting small test areas first so that the extent of the damage can be properly ascertained. These test areas also allow a structural engineer to inspect the problem. In most cases the degeneration occurs within the top two to three feet or in the frost zone and at the base of the wall where water accumulates. Higher areas of the wall, especially close to ground grade, may also show signs of salt deterioration. In severe cases, especially when salt has been used over many years, the degeneration may be more widespread.
Remember that salt is cancer for cement and it is essential that all compromised parts of the foundation that retain salt residue be removed. This is carried out by scraping the lose material off the wall with a small scaling jack-hammer.
When the repair has been assessed by the contractor or engineer, reconstruction can begin. Once the softer material has been removed, the contractor will test the integrity of the remaining wall to see whether steel reinforcing rods can be installed. The rods are held in place by epoxy. Many times the interior of the wall is too soft for the epoxy to grip, so steel cannot be used. Again, by using a small scaling hammer, the base of each damaged area is squared and concrete is installed upwards until the cavity is filled. Regular concrete is not used for rebuilding. The preferred product is a polymer-based reconstructive cement. The first step is to dilute the product to slurry and coat the repair areas so that the polymers seep into the structure. After 24 hours, the cavities can be filled with the undiluted product.
The most important part in protecting the structure is to address the bottom. This is vital and I cannot stress this enough. Unless the base of the wall is sealed from the elements, the work can be for nothing. A rubble wall acts like a sponge and even if you protect the exterior of the wall completely, water will wick up from the base and allow degeneration to continue. For this reason I underpin the walls with a minimum 8” (truck ready) concrete. This will act as a moisture barrier and prevent degeneration of the underside. Once this is complete, the wall can be covered with a concrete base waterproofing agent like Thermoshield®. This will prevent all moisture from entering the wall from the exposed outer surface. The Thermoshield® coat should be applied over the footing, the joint between the new footing and wall and the entire exposed foundation to the top.
The below-grade foundation area can then have a final barrier placed over it. If a nipple barrier is used, a 4” horizontal strip 2” deep must be cut from the wall. This is because any fastenings used, including expansion bolts, will not grab onto the interior rubble surface and in time, the top of the barrier will open allowing water to penetrate. For this reason, a rebuilding polymer cement is added to the crack and once it is filled and dried, fasteners can be drilled into it to hold the barrier tight to the wall.
In cases where the wall has degenerated to where a structural engineer does not feel confident that restoration is an alternative, they may ask that a new footing be poured outside parallel to the wall and a protective concrete wall installed against the old wall. When structural damage is too severe, a home might have to be shored, the old foundation wall removed and a new poured foundation wall installed. That is why prevention is the best solution; doing these essential repairs early will save you great expenditures in the future.
The most important element in preventing water penetration into an interior basement area is of course drainage. In many older dwellings this is not an available alternative, unless storm-water from the drainage tile is connected to city storm system. Another and more cost-effective alternative is to install an interior sump pump that will discharge water above grade so that it runs along the ground away from the house. This may be a fraction of the cost of connecting to the city storm system. Of course, gravity is the preferred method of draining water away from the foundation, except with the heavy downpours of recent years that have caused storm sewers in some areas of the city to back up. Unless your house is equipped with a back-up storm valve, sewage from the overflow where systems are jointed can end up in your basement.
Repairing cinder-block foundations
Unlike stone and rubble walls, the cinder-block foundation can, depending on environmental stresses like rain and frost, have a much shorter life expectancy than the other types of foundations. An unprotected block foundation may be compromised in its structural integrity after 55 years on average and will have to be replaced. This is, fortunately, easier to accomplish than for stone or rubble. The deciding factor is if both the exterior walls of the block and the interior web that supports it are so degenerated that they have lost structural integrity.
One is, unfortunately, not aware of this damage from the inside of the home until the exterior brick above the foundation begins to show signs of cracking in joints and through the brick itself. It is harder to recognize the signs of blocks decomposing when brick has been replaced by siding or other exterior envelopes. It is a good idea to have a foundation expert dig down to the footing and check the integrity of the blocks, especially if your home is older than 55 years. If the block is still in good enough condition to salvage, waterproofing procedures can be implemented and the wall can maintain its strength for another 100 years or more. A bit of foresight can save you a bundle of money in the end.
Waterproofing a block foundation is best achieved by removing all residue tar from the top and front face of the footing as well as the bottom 12” of the foundation wall. Once clean of all debris, two coats of a cement-based waterproofing and sealing product like Thermoshield® can be applied. After 48 hours the entire foundation wall and footing can be covered with two coats of a tar-based roll-on coating.
As with a rubble foundation or stone wall, I do not recommend installing a nipple barrier over a below-grade block foundation. Nipple barriers are made for newer poured concrete walls where drilling into the wall to apply screws or fasteners will not puncture any hollow cavities as it would with a block wall. Remember, the centre of a block is hollow, and a hole, no matter how tiny, can potentially become a leak point. For this reason I install below-grade Styrofoam® or polystyrene with a ship-lock edge. These products are pressed against the wall by the soil backfill and will stay in place. Another reason why below-grade Styrofoam® or polystyrene is a good choice is because it will protect your block foundation against frost that can push blocks inward (see photograph).
Charles Weiner is a structural expert and foundation repair specialist living in the Ottawa area. He would be pleased to answer any of your questions by phone 613-915-8377 or email firstname.lastname@example.org.