How do I remove crumbling mortar from fieldstone foundation?

Question

I have a fieldstone foundation. As with most such foundations, the mortar is crumbling, and the sandy mortar is falling out onto the basement floor. I have just read that I need to remove crumbling mortar and point (to fill the voids with new lime mortar).

How aggressive should I be about removing existing mortar, and what is the best way or removing it? Should I use a steel brush, scrape with a chisel, or actually chisel away mortar? Should I be concerned if the mortar is crumbled and loose far into the foundation?

The basement is about 130 years old and has had no water leaks so far.

Answer 1

I've no experience of lime mortar, but I would work under the same assumptions as I use for plaster and normal mortar.

That is, when removing the loose material go a little way into the sound areas, but don't be too aggressive.

So in this case I'd start with a steel brush to get the really loose material out and then use a chisel (or an old long screwdriver) to scrape away a little bit further and test out the remaining mortar to see how loose and crumbly it is.

If you find that you are removing a lot of mortar then I'd start to get concerned, but if you only do a small area at a time you shouldn't affect the stability of the wall.

It might be useful to get a couple of people in to give you quotes for the work. They should explain what they would do and give you an indication of just how bad the mortar is. Ask them if there could be structural problems when repointing - it's a perfectly valid question.

Just don't tell them you intend to the work yourself :)

Answer 2

Supposing it is the same wall that is pictured in the other question, you should go as far as necessary to find solid stable old mortar. Scraping the crumbly material away with a chisel or an old screwdriver as ChrisF suggests should be fine. Usually you could scratch your way throughout the wall, so stop before you loosen some of the big stones to avoid rebuilding the whole wall. The old stone walls (foundations) are usually very thick (60cm - 1m) so probably don't need to be afraid about it's stability if the wall is in a good condition. If some of the smaller stones falls out it's no big deal you can hammer it back to the place (so that it is surrounded by new mortar).