Filling stepped plaster cracks

danrv

Hi
I’m just redecorating my kitchen/diner and have used Polyfilla on some large cracks in the plaster on one wall (lower pic).
It’s ready for painting but since the prep, a couple of small hairline cracks have appeared where I’ve filled.
One of the cracks can be seen on the other side of the wall (concrete) in the cupboard.
Just wondering if a better, flexible filler could be used or would I need to re plaster the walls? 
The cause of the cracks is the main problem though I guess.

Ebe_Scrooge

As far as it goes then yes, you just need to go over it again with Polyfilla (or whatever generic alternative takes your fancy).  If you're filling large cracks, then you can sometimes get hairline crack appearing as it dries out, meaning you need to do it in two goes.  Flexible filler can help, but it's not absolutely necessary.  But once it's done then you shouldn't get any more cracks, unless there's movement of some sort.

Yes, you really ought to identify the reason for the cracks.  On a new house it's not at all uncommon for cracks to appear as everything settles a bit.  So if those cracks have been there from day one, it's probably nothing to worry about.  If the house is not a new-build, though, and they've only recently appeared, it would be worthwhile investigating the cause.  Might be a simple and innocuous explanation, but better to get it checked out.

FreeBear

Cracks need to be opened out so that filler can be pushed right in to the depths. Just plastering over the top is an open invite for the cracks to reappear. Sometimes, some mesh jointing tape can be used to give a bit of strength to the surface, but....

That is a fairly significant crack on the other side, and there is a gap opening up between the wall & ceiling. Both are pointing towards movement. Whether this is historical or ongoing needs to be investigated & monitored.

plumb1_2

Pointless filling in the cracks, the wall needs to be tied together otherwise it will keep opening up.

danrv

Ebe_Scrooge said:

So if those cracks have been there from day one, it's probably nothing to worry about.  If the house is not a new-build, though, and they've only recently appeared, it would be worthwhile investigating the cause.  Might be a simple and innocuous explanation, but better to get it checked out.

They’ve been there for the three years I’ve been here.
It’s an ex rental property and has been hastily redecorated on the cheap at some point.
This may be original 70’s plaster or could have been redone. If so, not very well as it’s blown in several places.
This crack is along the bottom of the downstairs loo wall. A 30cm ruler goes right through so I’ll need to refill the mortar and plaster there.

danrv

Ebe_Scrooge said:

If you're filling large cracks, then you can sometimes get hairline crack appearing as it dries out, meaning you need to do it in two goes.  

Yes, could be that. 

stuart45

If those walls are partition walls built off the slab, it could be the slab is dropping. This often happened in the older unreinforced concrete slabs that had a really deep fill. Block and beam is now used in these cases.

danrv

stuart45 said:

If those walls are partition walls built off the slab, it could be the slab is dropping. This often happened in the older unreinforced concrete slabs that had a really deep fill. Block and beam is now used in these cases.

Yes, partition walls.
The cracks are quite near a central electric warm air heater that weighs almost half a ton.
It’s not used anymore and does need to come out. Not sure if the weight and forty five years of heating the surrounding walls has anything to do with it.

danrv

Ebe_Scrooge said:

Flexible filler can help, but it's not absolutely necessary.  But once it's done then you shouldn't get any more cracks, unless there's movement of some sort.

The hairline cracks do seem to reappear after a few weeks. I think I’ll just have to try something like Toupret Fibacryl as I need to paint.

Bendy_House

If you are going to try a refill - and why not - then perhaps worth brushing/squeezing/forcing in some dilute builder's PVA in there first.

Vacuum out the cracks, dilute the PVA ~15%, and bush it liberally along and sideways and over the cracks, stibble it in where you can. Pile it up, and keep it going as long as it continues to be drawn in.

When you are confident that no more is going in, then wipe the outer wall surface with a damp cloth to remove any PVA from the surfaces to be painted. 

Allow to dry, and then fill. It might help.

Megaross

Personally I'd knock the plaster back and investigate the cause of the cracks, get a structural engineer is probably your best bet. Those look like structural deterioration - diagonal zig zag, wider at the top. I'd air on the side of caution, particularly given there are major cracks elsewhere in the property.

Even if they turn out to be innocuous I'd probably install helical stitching bars for good measure before replastering. Easy and cheap to do yourself.


But if you're taking the "We'll pretend that we didn't see that" approach. Rake out the cracks and easifill. Odds are they will crack again though.