RA, Co Limerick
First, I presume the leak has been repaired and you have had no further leaks. However, if the leak had been ongoing for a while, it is possible that it was repaired but the insulation remained wet, sitting on the ceiling and soaking into the plasterboard.
The majority of leaks show through at the weakest point, in your case where the plaster slabs are joined to a ceiling joist. Combined with the natural movement of the timbers (even very slight movement) the leak may have created cracks.
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The fact that the ceiling is stained suggests a considerable amount of water had to dry out. Such stains can be very hard to cover, especially with everyday emulsion. If your DIY skills are up to scratch, you could use screws to fix a piece of timber to both sides of the ceiling joist where the crack is recurring. Take care not to overtighten the screws as this may split the board. Rake out the cracks and refill both these and the screw heads to achieve a smooth finish. Allow them to dry, and then apply a product called Seal-Lock to the stained areas before applying two coats of ceiling emulsion.
You should also check that the insulation hasn’t gone flat, as water can damage insulation and in some cases cause mould growth. If you are in doubt, install new insulation.
Take care when working in the roofspace. Use a light, crawl boards, a dust mask, gloves, goggles and standard protection equipment.
Peter Crawley is managing director of Crawley and Sons, roofing contractors, Dublin