Don’t use wood to damp-proof foundation

Hey there, time traveller!
This article was published 14/06/2009 (6281 days ago), so information in it may no longer be current.

QUESTION: I wanted to know your opinion on waterproofing a 1960 basement foundation. We are planning on doing only two sides of our foundation and the other two in about five to eight years. We can’t afford to do it all at once.

The house is a three-level split, with only about four feet below grade. We had some leaking around the joists as we graded to high outside. This problem has been fixed by an epoxy injection on the inside and by lowering the grade outside. We would like to waterproof the foundation for peace of mind.

Is a Delta brand membrane a good option for our climate? The other quotes we have received include a paper application with the sprayed on rubber type material and another one that applies a wood on the outside and additional material for a seal. Any information you can shed on this type of work would be greatly appreciated.

— Karen Joyal, email

ANSWER: Repair to foundations and reinstallation of damp-proofing is quite common on homes after they hit the 40-year milestone. Yours may be a little more complex due to the multi-level design of the home and the low-lying floor system, but the methods of repair may be quite similar to a single-level home.

Most concrete foundations, especially those over 20 years old, use a simple method to prevent saturation of the semi-porous concrete from exterior moisture. Most have a bitumen-based damp-proofing material sprayed, rolled or trowelled over the fresh concrete before backfilling. This asphalt-based material may be installed by itself, or with an embedded membrane or mesh to prevent deterioration. It may also be covered with rigid foam insulation or protective sheathing to prevent damage from backfilling, but is often uncovered. Along with weeping tile, installed near the footing below the foundation wall, this thin membrane is the main protection the concrete foundation has from moisture intrusion.

This method has been successfully used for decades and works well, but may have a limited life expectancy, certainly less than that of the concrete foundation. Frequent movement of the soil outside the foundation, caused by seasonal temperature changes and freeze-thaw cycles, can wear away this thin foundation coating. After several decades, water may begin to permeate the concrete in areas where the coating is worn or small cracks or holes have opened. This is particularly problematic in older homes where weeping tiles have become blocked. The only real solution is to excavate around the perimeter of the foundation and redo the membrane and weeping tile systems. There are several newer products available, but the bitumen-based membrane system is still used.

The “Delta” membrane is well suited to our climate, but may be the most difficult option to properly install without complete excavation around the home. This very durable material prevents moisture absorption by the concrete foundation. The dimples provide added strength to the membrane while providing a small drainage plane between the bumps. This allows any moisture which seeps behind the membrane a route of escape. There are several manufactures of similar products.

A third option is a self-adhesive, rubberized membrane often called “blueskin” because of the blue colour of one of the major manufacturers. This material is very pliable and highly resistant to moisture. It is often used along with rigid foam insulation or other sheathing over top to prevent damage from backfilling.

While it may seem either of the two types of membranes may be superior to the bitumen based coatings, they are not nearly as widely used. On new homes, the reason is normally an economic one. Some foundation contractors have told me that they have difficulty adhering the membranes to older, uneven foundations. They often prefer the trowel or roll on products due to their ease of application for complete coverage of the foundation. Most will also add an embedded layer of mesh or other material to give added durability and strength to the foundation coating.

Using a paper or wood-based product below grade is a bad idea. It may be subject to rot or moisture damage. Pressure treated plywood is sometimes used to protect a foundation coating or membrane from soil damage, but that is not typical.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors – Manitoba (www.cahpi.mb.ca). Questions can be emailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358 or check out his website at www.trainedeye.ca.

trainedeye@iname.com