Hollow sounding patch of plaster.

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Hi all,

We are having some sockets added to our house, and as such, we needed to chase the brick walls in preparation - I have had the week off and have done this in preparation for the sparks next week.

All of them have gone well, except for one. At the top end of the chase, there is a slight crack (hairline crack), about 4 inches in length and at the end of the crack, the wall sounds hollow when you tap it over an area about 3 inches square.

Annoyingly, this chase was for a flex outlet for a wall mounted electric fire, and of course, the hollow sounding area is exactly where we need to drill. My worry is, that when we do, the plaster is going to fall off.

Ideally, I don't want to remove the plaster, but I obviously don't want the fire to fall off - if I successfully drill a hole and fix the fire to the wall, is the plaster likely to stay in place? Additionally, should the plaster fall off after the fire is mounted, will the fixing remain securely in place? The plaster is about 12mm before the bricks, and the fixings are 60mm in length. There will be 3 other fixings in a 'non' hollow part of the wall.

Any ideas as to how you guys would proceed?
 
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Tap around the hollow area moving outwards till you get a solid sound then hack off the hollow plaster till it is solid, then patch in accordingly. No use hanging things on a hollow area...As a matter of interest what is the plaster, sand and cement or a light-weight backing plaster?
 
Hi Roy - Thanks for the reply, I had a feeling you would say something like that.

I'm unsure of what is underneath - it could be sand and cement, but it doesn't really look like mortar as I know it (of course, it could be a different ration or something), could it just be a 'bonding'.

I was going to use 'Richard C's' method (//www.diynot.com/forums/viewtopic.php?p=1278796) of using a bonding to fill up the chases and then using a very fine filler over top (at least on one chase, and if I can't get a decent finish, I'll get a plasterer in.) Would this work over a larger area?

Additionally, if we were to not hang the fire in the location and move it somewhere else, would the hollow plaster just stay there, or would it just fall off one day.

Do you know what could of caused it to come off, would the plaster have not set properly when it was put on (long before we bought the place), or might I have hit the chisel too hard and it caused it to come off?

Sorry to bug you with these questions, but it's really frustrating that I did 12 chases, 11 are perfect and the one that really matters does this - but that does sum up my luck really!!
 
It sounds like it could be a sand and lime mix, depends on how old the house is.As for a part of it being "boxy" (hollow) some times when areas get rendered the plasterer or the labour usually dowses the wall to kill the suction so that the plaster doesn't dry too quickly. And it sounds like this area may have dried too quickly and let go of its hold on the substrate. I hacked off a bedroom wall a couple of years ago because it was boxy and the whloe wall then room came off in sheets when we put the scrappers under it. Pictures in my album (dabbing walls).If there hasn't been any damp or problems with your walls you will be ok with bonding then either skim it or easi-fill it...As for hollow plaster coming off on its own , I think it would be unlikely for you to get up one morning and find it on your carpet. ;) There are loads of people out there with boxy plaster that dont even know about it. But as for what you do with it when you find it is Dont Panic!! Get a plasterer in to have a look at it and advise you .I am not going to go into all the scenarios of what to do with out having a look at it. But at the end of the day it can be repaired ;)
 
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All of them have gone well, except for one. At the top end of the chase, there is a slight crack (hairline crack), about 4 inches in length and at the end of the crack, the wall sounds hollow when you tap it over an area about 3 inches square.

Annoyingly, this chase was for a flex outlet for a wall mounted electric fire, and of course, the hollow sounding area is exactly where we need to drill. My worry is, that when we do, the plaster is going to fall off.

Ideally, I don't want to remove the plaster, but I obviously don't want the fire to fall off - if I successfully drill a hole and fix the fire to the wall, is the plaster likely to stay in place? Additionally, should the plaster fall off after the fire is mounted, will the fixing remain securely in place? The plaster is about 12mm before the bricks, and the fixings are 60mm in length. There will be 3 other fixings in a 'non' hollow part of the wall.

Any ideas as to how you guys would proceed?

Firstly, my preference would be cut out the offending area and patch (as the guys suggest for you). I would also add - use a stanley knife to cut out the extent to give a neat edge to work with and minimise damage to the surrounding area.

However, plan B, you may be ok to proceed without patching this bit with some care in doing your prep for fixing the fire.
Some things to consider....if you drill through the plaster without hammer action on until you reach brick/block and then only turn it on when you get there, it may help protect the weakened plaster.
You could also consider a longer fixing for this one of the four. The strength from the fixing will be mostly between the rawl plug and brick/block (not the plaster). If three are in secure beckground and a slightly longer fixing on the fourth, this may be fine.
A longer fixing may also depend if it is brick or block behind. I know blocks in my house had big cavities and if you drilled into one of those, you had less behind.
A final point, if you did drill as-is, once the fixing is tightened, it will help hold the loose area in place anyway. 3" square is not a massive area.

Some options for you!
 
Thank you both for the replies.

Thats a very good tip on using a Stanley knife - I actually used it when chasing and it gave very good results.

The house was built in approximately 1850, and the wall in question is solid brick. I think you could be Right Roy - in that it is a sand and lime mix. Should I use this instead of bonding when filling the chases, or would bonding be sufficient? As I mentioned, my plan was to follow Richard C's advise: Apply PVA, apply bonding whilst PVA still Tacky, allow Bonding to harden overnight, level off with fine filler or easi-fill.

As for the hollow patch, I would like to take it out as you suggest DIYnewbee99 and re patch it, but do you think the plaster has come away from the sand/lime mix, or this has come away from the wall?
 
Out of interest - and I know I have swayed from the original topic - but would hardwall be better than bonding seen as how it is brick?

Would I still need to PVA the chase, or just wet it?

Sorry to be a pain!!
 

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