Trained Eye

 
 
 

Mortar Repairs in Stone Foundation

Question:

I read with interest a column you wrote on moisture in basements, which prompts me to ask about the following problem.

We recently purchased an older home, approximately 100 years old, with thick stone foundation walls that have been neglected.  While there is no sign of shifting or buckling, the mortar joints have deteriorated to the point where it is completely missing in some areas. In several places on the outside of the house you can poke a stick 12 inches deep in between stones. 

Is this a tucking job that a handyman/homeowner can tackle or should it best be left to a professional?  If a handyman/homeowner can do the job, how should I proceed?  What kind of mortar should I use and how would I force it far enough inside the joints to deal with voids that are 12 inches deep?

Thanks for your consideration of this question.

Answer:

You have raised an excellent issue with relation to older stone foundations. Unlike concrete foundations, which require little maintenance, stone foundations require regular care and re-pointing to prevent movement and deterioration. You may indeed be able to tackle this project yourself and I will provide some guidance in that regard.

Stone or rubble foundations are comprised of many smaller units, often limestone blocks, which are held together with mortar. This method of foundation construction has been found to be very durable, although seepage is a very common problem. Another issue is the deterioration of the mortar over a period of many years. This can occur on the exterior, above or below grade, or on the interior of the foundation walls. This deterioration can lead to large gaps, as you are seeing, between the individual stones. In some cases this can lead to significant structural issues or constant seepage and moisture intrusion. In other situations, similar to yours, there may be little structural movement, but this will most certainly occur if left unchecked.

From your description, it appears that most of your worn out mortar joints are on the exterior above grade. These are the easiest to repair and may be within your means to do yourself. The first item that must be addressed is the loose mortar and sand that remains in the old joints. This must all be scraped, brushed, washed or pulled out before proceeding. This is critical to ensure adequate adhesion of the new mortar to the older stone. While a purely a labour intensive job, it is one that can be accomplished yourself with much patience. Once this is complete, re-pointing can begin.

At this point you can decide whether to buy redi-mixed mortar, or make your own. The redi-mixed mortar is the simplest to use and is available in small bags at most home centres. Depending on the quantity required, this could be the most costly option. If you only have a small area to repair, this should be the best way to proceed. The second option is to purchase a sufficient quantity of sand from a landscaping or building supply retailer, and the appropriate masonry cement. The type of cement used may vary depending on the composition of your foundation and other factors, but normal Portland Cement is often suitable. If you choose the second option, I would suggest further research at Building Centres or on the Internet to determine the best ratio of sand to cement for your particular application before mixing and use. There are also liquid bonding agents available that may be added to the mortar mix to help with adhesion to the old foundation walls.

Once these decisions have been made, the mortar joints cleaned and the mortar mixed with clean water to the proper consistency, application should be very straightforward. Filling the old joints with a pointed trowel, or other tools necessary, is all that is required. The joints can be further worked when almost set, to improve the aesthetics, but this is not essential. Replacing the old, worn mortar is the important consideration here, not appearance. A simpler alternative may be to apply a complete parging coat of mortar over the entire exposed exterior foundation after the joints are filled. While this may be an easier cosmetic option for the novice, the real benefit of this method is for future maintenance. It will be much easier to see new cracking and mortar wear if the entire stone exterior is parged. This will make further inspections and patching a simple matter, in subsequent years.

The final thing to be considered is the condition of the mortar joints below grade. If seepage is a regular occurrence, it is likely that the mortar is worn where it is not visible, as well. The interior of the foundation should be uncovered and exposed and inspected for the same deterioration seen on the outside. If this is the case, the solution is the same as the exterior. If the interior mortar is in good condition, you repair the section above grade and you still experience significant leakage, then professional help may be required. Excavation and repairs on the exterior should be done by qualified foundation contractors, but above grade maintenance may be within the skill level of the average handyman.

 

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