I need info/recommendations on sealing limestone foundation. I want 100+ year solution.
I have older home with limestone + mortar basement walls about 150 years old. Good slope & drainage around house and downspouts get water away. Walls are solid, about 2′ thick @ base, 18″ @ sill. Thin plaster finish inside. In 17 years I have not seen water leaking thru walls & onto the floor. But walls DO let in some water (vapor?) — plaster “bubbles” off near bottom and in corners. Not OK for finished basement. We want to redo inside surfaces with metal lath & plaster mortar (which ain’t cheap), but I am concerned that slow ingress of water will loosen it over time. I think the only way to REALLY stop this water is from the outside in.
On the outside limestone is OK but mortar joints weather rather fast. Rather than tuckpointing again, I would prefer to finish off the whole surface with a more durable material.
I’m considering pouring a concrete wall outside the existing one. Concrete about 6″ thick below grade, tapering to 1-2″ at water table. Trowel visible portion with attractive pattern. Sealer on concrete below grade.
Your thoughts please. Warnings/cautions? Efficient methods to minimize the cost/extent of excavation? Proper concrete mix (or whatever is used)? Cleaning and adhesion to the existing wall? Recommended thickness below and above soil line? Are there firms out there that do this?
Thanks!
Replies
the outside surface of a stone foundation is usually very, very rough. They only dug just enough of a hole and build the basement walls right up against the dirt, stacking the stones to be smooth on the inside only. By "rough" I mean that you'll find stones that stick out several inches from each other, and crevaces that go many inches into the wall 'surface' (picture a stone porcupine). Your idea of a concrete cap is just about the only way to try and seal such a surface because you could never get it clean enough to get anything to stick no matter how hard you tried.
Stone foundation walls were not designed to stand on their own, they use the earth to help hold them up, and any mortar you see is mainly decorative (the mortar they put between the stones will be almost sand by now). This kind of wall will stand for a very long time if properly built, left undisturbed and kept dry.
But, if you excavate all around your old foundation to cover it with a modern, smooth concrete surface that can then be sealed with modern water controlling stuff you'll have to be VERY careful about it to support the wall to prevent collapse. It must be done in many small steps so that the minimum amount of wall is put at risk at any time, this means that your concrete 'cap' will be made of many small segments and will be VERY expensive/time consuming.
Hire the best, most experienced (and most expensive) expert in this very rare and highly risky buisness of undermining stone foundations to do the work for you.
The usual solutions are:
1) leave the wall alone (see above) and focus on surface water prevention instead(french drains carefully installed can add protection beyond gutters with 10foot run out and good grade drainage). Add a dehumidifier to deal with the remaining humidity that will occur in any basement due to its coolness when confronted with warm, moist summer air.
2) demo the whole thing and install a new foundation, with the house up on jacks (not a preferrable solution, but if you try to excavate around your existing walls this will always be a risk that you might end up having to do this anyway if they do collapse!
Woody,
"Your thoughts please. Warnings/cautions? Efficient methods to minimize the cost/extent of excavation?"
Just my thoughts, though many will disagree.
It is a basement. Not meant to be finished. The reason that it lasted 150 years without failure is that the previous owners realized this. You should, too. In your case, messing with the foundation is going to be a long and expensive mistake.
If you want to make a cellar into a habitable space, I would recommend the space be conditioned by means of forced air/dehumidification. The stone foundation and mortar wash that was there before, will pass moisture. Most modern constructions would include something like a big sheet of plastic as a vapor barrier. This would be a bad idea. The stone gets wet. Then the stone gets dry. It has worked for 150 years. You cannot keep out the moisture. You CAN trap it in a place that you don't want it (in the stone/mortar) and, without starting with a new hole in the ground, is all that you will probably accomplish. Then, the stone gets wet, the stone stays wet, the stone falls apart.
Get a good dehumidifying system (not the little portable things with a bucket attached), like a DesertAire, replaster/mortar wash the wall surface and enjoy your basement. A decent dehumidying system will probably cost less than anything else (worth doing) and it will work. Not for 100 years, though. The only thing that will work for the next 100, is what has worked for the last 100.