Repointing Fieldstone Foundations
It should be expected that the mortar used in fieldstone foundation walls will deteriorate over time. Repointing is something that is usually required every 80-100 years if done properly. To illustrate this let use a typical New England fieldstone foundation wall correctly built in 1850. It would probably need repointing sometime around 1930 to 1950. Now the question is was that repointing done correctly when it was repaired back then?
One problem we run into as a MA Foundation Repair company is when an inexperienced DIY or contractor repointed the walls with a Portland cement type mortar sometimes in the past. This mix is very strong, that is the problem it is too strong. Fieldstone foundations move because of seasonal changes. This type of mortar is actually too rigid. It is best to use a hand mixed lime mortar that adheres to the fieldstone better.
When Should Repointing Be Done
This is when the mortar joints are crumbling and loose. Sometimes even smalls stones will fall out. It is important that a fieldstone wall be repointed before the wall is allowed to weaken and collapse. If left unattended ground water passing through the foundation mortar joints and cause structural damage.
Lime mortar repointing should not be done during periods of heavy rain or in the case of the exterior, freezing weather.
Move all items away from walls at least 3 feet. Cover items with plastic.
Safety precautions should be taken to ensure a safe work area. It is important for workers to wear protective safety glasses and gloves.
Cleaning Out Joints:
In order to do a good sound repointing job it is vital that the loose mortar be removed and if at some time in the past Portland cement mortar was used a hammer and chisel may be necessary to clean this off the wall to get down to the original lime mortar.
A foundation repair from Ram Jack New England of MA would be happy to answer any question you have about chimney repair or retaining wall repair.
DO NOT use power tools like hammer drills to remove the mortar joints as you might disturb the pinnings (small stones used to lock in the larger stones).
Using various sizes of thin metal tools a worker can rake out the decayed mortar. Many times it is advantageous to wash out these joints with a fine stream of water.
It is important to make sure all roots and vegetation are removed from between the joints.
It is a good practice to take pictures of all sections of the walls prior to repointing.
This is the process of pushing new mortar back into the joints. Using various metal tools this mortar will be tamped into the joints. This first coat or tamping coat should be batched by mixing the following – 7 parts sand, 1 part lime, 1 part cement.
Final Pointing Mortar:
This mortar should be batched by mixing the following 6 parts sand, 1 part lime, and ¾ part cement. This mortar should be used to fill the finish pointing. Carry should be made to clean the stone of any excessive mortar.
The work area should be broom cleaned, then remove the plastic from items and make sure to leave the work area clean. A Clean job is a great job.