View Full Version : Limestone foundation blocks sinking.
4th Nov 2010, 02:36 PM
I have a house build in the mid 1800's which has a limestone foundation made of blocks about 2 feet wide by 1 foot tall. In the corner there some sinking due to the previous owners' neglect to fig the downspout on the gutter. lots of water eroded the stone and mortar.
The mortar is mostly gone around a few of the blocks but because there was a little sagging I decided not to tuck-point but to see what below the ground looked like. After digging 2 feet below I have found there are lots of small plates of limestone (or some other rock) which is all just stacked under the huge blocks but are not really being held together except by dirt.
I know there is brick (or maybe more limestone) further into the inside of the wall as there is no visible damage on the inside of the basement, but I am afraid that if I don't re-enforce those stacked rocks then the blocks will just break loose from the new mortar (not yet placed) and sag all over again.
I'm curious of the best way to fix the loose below-ground stone before doing any work on the above ground foundation. I can post photos tomorrow if needed.
Thank you for any ideas,
4th Nov 2010, 03:20 PM
Here is a photo to give a better idea of what I'm talking about.
4th Nov 2010, 09:59 PM
Step one is to work out how much the wall has dropped by, if it is significant it needs jacking and underpinning - not a DIY job unless you know what you are doing. If it hasn't sunk too much then just fill in the hole you dug with concrete and fix the mortar between the limestone blocks. Don't forget to divert that downpipe, water and footings don't mix. (lime mortar not cement) There is nothing wrong with the rocks below ground, don't dig them out or the house will fall on you!!
6th Nov 2010, 12:57 PM
Thank you for the reply. I think it's only sunk about 3/4 of an inch, but it was enough that I wanted to know why it sank which is what started me digging. I had considered pouring cement into a wooden frame around the loose rocks but wasn't sure if that was smart or not. We are spending a lot of time and money fixing the previous homeowner's "handy-work," and didn't want to do anything I'd regret.
I spoke with a cement company who does some foundation work. They suggested putting mortar in before the cement. This conversation was over the phone so he may have thought they were more brick-like, as opposed to the rocks just kind of stacked and in random sizes. Is there a reason to use mortar first other than to try to really fill the spaces? I'm not sure if morter (using a bag) would be easier to fill all the voids, or if cement poured in and vibrated would be better. If I were to use mortar would I need limestone mortar even if it were to be encased with cement?
Thanks for reminding me about the downspout. It was just temporarily moved so I could get at the ground. Otherwise it's usually being diverted into the driveway where it runs away from my house.
I'll check with our local brick company tomorrow about getting lime mortar.
Thank you again for your helpful answer, I'm getting excited to make progress on this!
22nd Mar 2011, 02:13 PM
I have a simmilar problem to yours, that the water damage has both caused subsidance, and also damage to the mortar between the stones.
I would suggest that cleaning out the mortar, and repointing with lime putty and sand in 1:3 mix into the stone joins first would help stabilise the wall for want of a better term, then any further fixing under the wall/ closing of the gaps etc could be done.
If you have any large movement cracks, dont fill these up, only where the mortar has eroded. If you fill the cracks from movement, and you want to lift the wall in the future, your wall has nowhere to go.
Begin at the bottom of the wall, and work your way up.
Pre wet the stone often for a few days before repointing, and if the cracks are very deep, only point to 25mm deep at a time to allow the mortar to set and shrink, then add further mortar untill your at the surface and finish as required for your house.
Use lime putty, as you dont have any dpc, so the lime putty mix allows the walls to breathe and moisture to escape, and its softer than your stone, so it wont damage your stone unlike portland cement that will not allow moisture to move thru the wall, and as the house moves, the cement wont, and the weak point is your lovely limestone blocks which will be damaged.
Im sick and tired already of removing portland cement from my old stonework to see the damage its done.
Ive already gone thru bags of lime putty now and the place is starting to look good.
Good prep, wet it out, and let it dry slowly, and it will look great.
Ps, have a close look at the old mortar, and with a sample try and match up your sand.
Looks like our place sourced its sand from the local creek, so ive used a mix of concrete sand and RF plaster sand.
22nd Mar 2011, 02:45 PM
In the 1800's you are really pre concrete, that loose rock was most likely placed in to simply level the foundation blocks, although it was normal practice by the early 1900's to use bricks and lime mortar ending up with thin pieces of stone to finish off to level. The only reason your footing has dropped is water, and once the area dries the loose material will sit firm. If you need to pack beneath the stone footing and above the loose stuff you can mix a dryish brew of 1/8th rock, sand and cement (and a little water) to pack the gap. You just get an amount on a shovel, rest the shovel tip on the rubble and pack it underneath using a thin piece of wood or brickies trowel to firmly pack the gap. If the gap is more than an inch then you will need something a bit more substantial. The pointing has already been covered.
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