Repairing Water Damaged Plasterboard

Unfortunately for us, we ended up with a soggy ceiling at the start of 2022. Assumed sealant failure in the flat upstairs allowed some water to escape, with no obviously logical route, it trickled its way down into a ~60cm long blob on our bathroom ceiling.

The developers builders swapped the neighbours sealant out and patched our ceiling. Weeks later our plasterboard had started to deteriorate. The blob had become boil. Popped, dried and undesirable.

Steps to fix:

These are the high level steps I planned out from a bit of YouTube research and a sense check post on r/DIYUK.

  1. Dry out everything thoroughly
  2. Remove damaged material and surrounding material
  3. Cut a trench for new tape installation over any joins that were damaged
  4. Install plasterboard tape
  5. Cover damaged area with plasterboard bonding agent
  6. Apply plasterboard filler in staged layers, never more than needed
  7. Sand back excess edges of filler to blend level with existing
  8. Apply stain remover paint or spray
  9. Paint two coats of primer
  10. Paint two coats of moisture resistant bathroom paint


I made a couple of mistakes along the way which took more time and effort to fix. I’d also learnt a few tips from the research.

Not removing the right amount of damage. It’s a Goldilocks problem. Too little and you’re repairing on compromised material. Too much and you’re wasting time and energy. I used a hand sander power tool to sand back the area so I was aware of what was damaged on the surface. This helped determine which bits were dead and could be picked out using a knife. Feeling the board gives you a sense of what’s crumbling and what is cosmetic water marking.

Setting your tape replacement too high or uneven. The damage I was working with was directly on a tape line, this ripped off with ease once I started removing the mess. A join and the material added to boards needs this tape to help form structure, reduce chances of visible lines between boards and hides fixing screws. When you remove the damage you’ll want to cut an area the width of the tape. Don’t make the tape so long that it starts to come above it’s repair trench.

Skipping adding a plasterboard treatment material. Luckily I learnt this step from a helpful person on Reddit that responded to a post I made about my repair plan. Plasterboard that has previously been finished and now needs a repair can be tricky to work with. The new filler can struggle to bond to the surface. Before filling you’ll want to add a material that helps improve the chances of the filler sticking. You can use Polyvinyl Acetate (PVA) – not the school glue – but it requires mixing it perfectly. Alternatively you can use a product like the Bostik plasterers stabilising primer, thats what I used. Only downside is it is sold in large quantities which you might need all of it for a small job.

Adding too much filler that will need lots of sanding back. I made exactly this mistake. Slapping it on without a care in the world and not checking the levelling to the existing ceiling, not working in stages between drying or using a range of tools will, ultimately, you’ll end you up just like me. Next time I repair a larger area my bet is that I need a larger skimming tool and a wetter mix of filler to keep that horizontal line smoothly consistent.

Rushing the sanding. I ended up using a torch clipped to my cap to inspect the ridges and lumps in the filler. Spend time on this bit as you’ll paint over it and still see the difference in levels.

Rushing the painting. Primer helps mask the repair and is essential for getting a surface ready to make paint. I’d never really thought about what primer is until I looked into it. It makes paint work. You’d think paint is paint, it paints stuff and changes it’s colour. Well, it turns out it’s mostly rubbish at being paint without prime underneath it. I’m no expert, so have look at it yourself and what you’re about to slather in liquid pigment.

Tools & Materials

  • Power sander
  • 80 and 180 grit sandpaper
  • Scraper
  • Stanley knife
  • Plastic dust sheets
  • Small paintbrushes
  • Paint roller
  • Dust mask with filter units
  • High powered torch
  • Bostick cementone plasterers stabilising primer
  • Pollyfilla
  • Ploycell stain stop spray
  • Primer and undercoat paint (multipurpose)
  • Bathroom paint, resistant to moisture and steam

There’s a few tools I wish I’d had for the job but I made do with the basics.

  • Vacuum adapter and vacuum for the hand sander. This could have reduced the mess and improved visibility when working on levelling the plaster.
  • Plastering taping knife. This is a wider metal edge tool for getting a consistent line across the application of plaster
  • Angled brush for cutting in paint edges. Our house is all white paint so it’s not a big deal if the edges are not accurate. I learnt as I was about to start painting that an angled brush is the way to make edging easy.

Wrapping up

I’d assumed this job was one for a pro. A bit of research built up my confidence and being methodical with each step helps get a good result. If you’re reading this and wondering if you should do the same type of repair – do it. Contractors might be reluctant to do this job on its own as it requires a lot of short steps and drying time between each. Further more you start to look at your own home with a new level of confidence that “it’s just material and I can repair, replace and upgrade that when I need to”. It’s liberating.

Final note: methodology is a variable based on research, experience and skill. I’m a beginner and learnt through research. Then doing. Once. If you’ve done this more than once you’ll probably do it differently. Email me and let me know what you would have done, I’ll add it here.


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