Second the no-rigid-filler advice. If you want to close them visually, the slivers method is the only one I've seen still look right after five years. It's slow work — figure a full weekend for a decent sized room — but it's permanent.
Teodora Vlasic
@immutable_infra
I build CI pipelines and container images for a living. Reproducible builds, small base images, and no SSH into production.
Also resist the advice to pull it all down and board it. It's a legitimate option and sometimes the right one, but a sound lath ceiling with two detached patches is a repair job, not a replacement job, and the acoustic and thermal behaviour of the original is better than most people expect.
Parallel cracks a metre long in a lath ceiling are usually following the joists, and what's cracking is the plaster keys — the blobs that squeeze through the gaps between laths and hold the whole thing up. Filler on the surface does nothing about that, which is exactly why it comes back in the same lines every time.
Go into the loft or floor above if you can and look at the top of the ceiling. If you see a lot of broken key material lying loose on the laths, you've got a section that's slowly detaching. The fix from above is to reattach the plaster with plaster washers and screws into the joists, or to run a bonding coat over the top of the laths.
If you can't get access above, the from-below fix is plaster washers on drywall screws driven up into the joists, spaced along the crack line, then scrim tape and a skim coat over the top. Ugly to do, permanent when done.
Not if you countersink them slightly and skim over. They're about 40mm across and dished specifically so they sit below the surface. Two coats of finish plaster and you'd never find them again.
Worth adding: firms that sell injected damp courses diagnose rising damp at a remarkable rate, because that's what they sell. An independent damp surveyor who doesn't do the remedial work costs a few hundred and is the right money to spend if you're unsure.
Check the parting bead too. On a window that's been painted a dozen times the bead often gets loose or has been replaced with something too thin, which lets the upper sash move front to back. You can pull it out and shim it or replace it for a few pounds a metre.
That's roughly the partial quote's scope. Good to hear it isn't obviously the wrong choice.