Plastered ceiling cracked again!

[alec green]

Hi

I had my living room ceiling plastered earlier on this year, but was left we a ridge across the middle of the ceiling which cracked. I called back the plasterer back and to my disappointment he opened up the crack and filled it with filler, but left the ridge and overall made the ceiling look worse. Thinking I had made a bad choice of plasterer, I called another guy who taped up the crack with scrim tape and skimmed the ceiling again. I was delighted with his work, but 3 months later there is a hair line crack appeared in the same place. Can anyone suggest what I can do about it, or what is wrong. It really annoying me now.

Thanks

[NT]

structures move, hairline cracks happen. Fix it by just smearing a little filler along it with a finger, pressing to remove all excess. Very simple. And yes, you'll have to repeat this another year.

NT

[Cash]

Or widen the crack slightly and use painters caulking (similar to silicone mastic but paintable). I used that on a similar crack many years ago and the problem never returned.

Cash

[Another John]

I'm interested: how did you widen the crack? Did you run the corner of a chisel (for example) along it, i.e. maybe a 2400mm run along a plaster board join? And was it on a ceiling?

Cheers John

[Cash]

John,

Usually with an old, wide, flat-bladed screwdriver - on any length of crack in walls and ceilings - it would be sacrilege for me to use one of my [wood] chisels on that stuff.

Cash

[Gib Bogle]

We have the same issue in our living room, which was made much longer by the removal of a non-load-bearing wall. Some months after the plastering two hairline cracks appeared, on each edge of the plasterboard strip that was used to fill the gap made by removing the wall. We've been told that the plasterers were at fault for not using paper tape on the edges, and another plasterer is booked to fix it up.

I'm having trouble understanding how tape can prevent the cracking. Clearly there is some expansion movement in the ceiling structure, and although the paper may stretch to accommodate the movement, surely any plaster on top will still crack. Advice from plastering experts would be appreciated.

[Dave Liquorice]

The idea is that the tape doesn't stretch and stops (reduces...) the movement that is cracking the plaster.

IANAPE

[Gib Bogle]

The idea that tape can prevent movement that is being created by expansion-contraction (temperate/moisture driven) in a large area of roof and ceiling doesn't make sense to me.

[Gib Bogle]

Since I've now heard about this tape (I didn't know it was a fibreglass mesh) several times now, I believe it must work, but I'm still having trouble understanding exactly how it works. Somehow the stretching is spread over a width of plaster (about 2") - this means the plaster has a degree of flexibility. What amount of movement does this scrim tape handle? Clearly there is a limit.

[Brian Gaff]

Its obviously moving I'd suggest. Is this some old Victorian house? You really need something that has some flexibility I think.

Brian

[Dave Liquorice]

It's not thermal or moisture driven movement. Think about it, the whole ceiling is affected by those so the whole thing moves together, there is no differential movement. It is differential movement that causes cracking. One board moveing relative to another, a joist flexing a bit more than it's neighbours etc.

I don't know how much movement scrim tape can take before it gives up but the modern glass fibre stuff is very strong, mind you so was the old fibre stuff as well.

[Tim Watts]

it will bind the edges stopping the crack appearing again, it's a gamble wheather the stresses will just cause a crack somewhere else.

As others have said, the other option is rake out and apple some flexible filler/caulk. I have a kitchen ceiling to paint next weekend, and have a few

2-3' hairline cracks. Sometimes you just have to live with it - especially in large rooms - houses are just a bundle of imperfections. If anyone is that worried, lining paper....

[Tim Watts]

Doesn't need to handle much. A hairline crack may be 1/2-1/4mm wide which is nothing is spread over 20-30cm width of tape (well within the elasticity of paint). Without tape, the plaster has to take the stain and plaster is, well, rather less flexible.

[Gib Bogle]

Sorry if I'm being obtuse, but ... as I understand it, the tape is plastered over (so I don't understand the mention of paint). Since the movement is still there, the plaster must stretch. Is it that instead of a single fracture, the plaster, being bound to the tape, experiences multiple fractures, each too small to be noticed?

[NT]

Raking the joint out only makes it worse. Its not as if it'll stop further cracking, and the finger wipe method has always worked well for me, it doesnt normally drop out.

NT

[Tim Watts]

Plaster is not *totally* inelastic, just very nearly. If the strain (ratio of movement divided by distance over which movement must be accomodated) is small enough, even plaster can cope. The art here is to take a tiny movement and spread it out over a bigger distance so that the plaster can take it without cracking.

Overall I think plaster is a crappy material anyway. We use it because it is cheap, can be applied wet and polished to a good finish (by a genius). But in terms of physical properties after that, it is s**te. However, we don't have a lot of other options that work the same way.

Cheers

Tim

[george [dicegeorge]]

perhaps widen the gap and put flexible frame filler in it then paint it?

its used between wooden window frames and brick.

[g]

[Phil L]

It isn't tape, it's mesh.

It used to be made from heavy brown sacking like material, then in the 60's and 70's it was cotton. Now it is fibre of some kind but i don't think it's fibreglass...in any case it does the same thing as reinfocing mesh in cocnrete - it holds it together, especially in areas of known weakness - where boards meet and where a ceiling meets a wall

[Phil L]

This ridge you speak of is either a joist or a steel beam which is lower than the rest, and more to the point, it appears to be moving - fine if it's steel because it's probably just expanding and conctracting which could be causing the cracks. If it's timber then there could be something more sinister afoot, questions:

1) how long have you lived there and how long has it been cracked? 2) have you had a wall removed downstairs where this crack is?

3) is there evidence that a wall has been removed? - you would see some uneveness on the walls at each side possibly or maybe marks in the floor - paint from long ago removed skirtings etc.

4) what is directly above this crack?

5) are there any protrudences (piers) on the walls in the room where this crack is that girders/beams could be sitting on?

[Weatherlawyer]

Originally plasterboard was fixed with a slight gap between the boards, about the thickness of a plasterboard nail. And the abutting edges were dished.

The plasterer forced plaster in the gap and laid a thin strip of plaster along the dished edge; running a ribbon of hemp along the line whilst doing this.

When he skimmed the boards the hemp provided a reinforced joint, similar in principle to reinforcing concrete. Having the gap tended to negate any flexing between joints. But usually this gap was omitted. You could get over that problem with brown paper tape under the hemp run if you were diligent.

These days nobody leaves that gap and more often than not the paper tape finish used in walls not being plastered allows for that. I don't know the ins and outs of that aspect.

Glass fibre tape is narrower than the old hemp bandages but of course much stronger. And it is sticky backed, so a lot easier for novices to apply. Perhaps the thinness of the modern tech allows for plain flat sheets to abut with no trouble. But you still need the reinforcing to take place.

IOW the plaster must be pressed through the cells in the tape.

HTH.