Removing hard cement concrete render

[Geoff Pritchard]

I want to remove hard cement concrete render from the walls of a cellar in an 18th Century town house because this has acted as tanking and is driving moisture up to the room above causing salt tidemarks as the moisture dries out depending on room temperature. I wondered if anyone has any tips on removal of cement render without damaging the lime mortared brickwork behind. If I hacked off some of the cement without taking it all off would it allow the wall to dry out sufficiently through evaporation into the cellar or would I need to take it all off ? I am worried about damaging the foundations behind. Any suggestions gratefully received. Thanks. Geoff

 

[MdB]

Hi Geoff, welcome to the forum.

I'm not sure there are any clever techniques. It mainly depends on how well it was put on in the first place. I've just been removing cement render from an old stone wall and it was very poorly fixed so came off easily with little damage. Using an SDS drill on hammer setting removed the cement in smallish pieces but also damaged the stone in places when I misjudged the depth of cement. Using a hammer and chisel was a bit more hard work and the cement came off in large sections but did less damage to the stone. The odd bits that were properly stuck though were a complete pig to get off.

One trick I did find useful was to take the cement off in the same layers it was put on with. My wall had two layers of cement and in some places I could remove the top layer with the SDS making it easier to get the second layer off with the chisel afterwards.

 

[Moo]

Our cellar was (and mostly still is) tanked with cement.
Before we moved in I removed it from one area of external wall (the one facing me now as I sit at the computer) and it made a real pig's ear of the bricks, so I left it at that. But a couple of years ago I noticed that some of the render further along the wall was beginning to sound a bit hollow, so I did a bit of picking and it came off in quite gratifyingly large chunks, revealing what I guess (and hope) is a mixture of limewash and very thin lime render.
Every so often I do a bit more tapping, hoping to find more hollow areas, but so far no luck.
For the time being I'm leaving it at that, because I just can't face the prospect of carting everything movable upstairs into the hall while I spend weeks making an unholy mess down below.

Not very helpful, I'm afraid, but I thought you might take some comfort from the fact that you're not alone.

 

[kevmiles1]

This isnt going to sound very technical but having removed lots or cement render from many listed buildings (some cob, some stone, some brick) the best way is to find a blown area and hit it face on with a club hammer and it will just fall off or crack and then that gives you the level and the depth to get a bolster behind it and chip away, the odd bits will just come off clean and easy and in large lumps as they wont have bonded, the ones that are stuck fast will take a little bit more time and patience, but if done carefully (and not using any power tools) will come away without doing any major damage even to very old brick or very soft cob.

 

[JoceAndChris]

Geoff- only a very amateur contribution to make to this but in my (limited) experience removal of some render is better than removal of none. Where I've removed it on the exterior a lot of drying out on the inside has taken place.

If it won't come off with a modest amount of force you have to wonder if it should come off at all - like you I worry about the foundations.

 

[AMc]

My experience of removing cement is mostly pulling up floor screed from pamments but the same rules apply I think.

Careful application of an angle grinder can score lines in smooth unbroken patches to allow a bolster/cold chisel to get in underneath - however the angle grinder is not easy to be careful with and you will slip and score what's underneath - it also makes an UNHOLY amount of dust and you may wish you hadn't bothered.

Finding lose/hollow bits to knock off and work out from works well.

I armed myself with the longest, thinnest pry bar I could (this one in fact http://cpc.farnell.com/jsp/level5/module.jsp?moduleId=cpc/454213.xml)
A metal chisel the size of my thumb
A narrow cold chisel
A wide cold chisel
A small lump hammer
Goggles, mask and gloves.

Then I spent a lot of time wacking the hell out of the slabs. Trying to get behind as much as possible and levering off in preference to smashing into the surface underneath. It's quite satisfying but incredibly hard work - wear the safety gear as cement in the eyes is horrid and it hurts much less when you smash into your gloved hand not your bare thumb.

In your cellar if you don't have to remove everything then leave the best adhered stuff alone - most uncovered will work to deal with the moisture - but also think about how you're going to vent that moisture out of the cellar or you're swapping one problem for another.

Happy smashing!

 

[Penners]

The odd bits that were properly stuck though were a complete pig to get off.

And that's the problem. Concrete rendering is notoriously unpredictable. You can work away at an area that comes cleanly off the bricks, and you get all cocky about it. Suddenly, and for no apparent reason (other than downright cussedness) you strike an area that's apparently welded to the brickwork. Removing this can cause considerable damage to the faces of the bricks.

You have little choice but to try it and see. For my amateur opinion is worth, I think if I were in your place I would start by removing all the area of concrete that will come away satisfactorily. Then I would have a go at one or two of the difficult patches.

If this starts to cause real damage to the bricks, then you may have to consider leaving those patches of concrete and lime-rendering the rest of the wall flush with them. Provided you end up with a good percentage of the wall lime-rendered, this should still have a beneficial effect on the damp problem. You may find that you'll get some hairline cracking around the joins between the cement and lime rendering, but this isn't so difficult to live with and deal with.

Needless to say, it would be beneficial to leave the exposed brickwork for a while to dry out, before you re-render it.

 

[robgil]

i used one of these to great effect.

i was lucky though because most of the cement render was put onto wire gauze nailed to the wall.
still , the bits that were not on gauze still came off quite easily with the breaker. you just need a very light hand at it , dont go into the wall with the gun , rather along its face and take one layer of render off at a time , being exceedingly careful on the last layer. the most important thing is knowing when to put the breaker down and pick up the bolster.
i think the breaker uses tiny little punches in succession and the bolster has one huge punch from a lump hammer at a time , so the breaker can be more effective at breaking the cement and not damaging the brick if used properly , but try it on a tiny bit first out of the way , just in case it all goes pear shaped.
also , a small , sharp bit chisel or point for the drill would be better than a big bolster bit.

 

[Jill]

Geoff, I've spent many an hour chipping render off interior stone and brick walls, just using a chisel and hammer. Like other have said it all depends on how well it is stuck on, try doing it when you've had a bit of a bad day or someone has annoyed you! Takes along time but very satisfying when its all gone.

 

[Inspector John]

i used one of these to great effect.

i was lucky though because most of the cement render was put onto wire gauze nailed to the wall.
still , the bits that were not on gauze still came off quite easily with the breaker. you just need a very light hand at it , dont go into the wall with the gun , rather along its face and take one layer of render off at a time , being exceedingly careful on the last layer. the most important thing is knowing when to put the breaker down and pick up the bolster.
i think the breaker uses tiny little punches in succession and the bolster has one huge punch from a lump hammer at a time , so the breaker can be more effective at breaking the cement and not damaging the brick if used properly , but try it on a tiny bit first out of the way , just in case it all goes pear shaped.
also , a small , sharp bit chisel or point for the drill would be better than a big bolster bit.

I have the same Makita Had it since 92 and it's still going well
Trouble with taking cement of is the damage it leaves with bolster or breaker If the render is sound and is bonded well I'd leave it alone
If it's cracked well you either take it off and maybe have damaged masonry or the frost will do it for you
Just one of those things....Your call

 

[Geoff Pritchard]

Thanks to all for your helpful comments re removal of render from cellar. I will proceed carefully and let you know in due course. On another point, what is the score with frost damage to limewash? The builders who re-rendered an exposed gable wall with lime mortar/horsehair mix last year put on a load of limewash last year when it was very cold and some came off with frost damage shortly after and now a load more has come off. All is on places where there is a ledge or bulge in the wall and snow can lie. If this is going to continue to be a problem do I need to use something other than lime mortar eg lead flashing (which won't look very nice though) or should it be OK if re-limewashed properly in the spring. Thanks, Geoff (Norwich)