Richard Greenland

Last week I looked at common types of flat roofs and the problems they can cause. I didn’t mention good flat roof coverings of anodised metal and rubberised plastics, which are durable and often come with long guarantees, but are rare on domestic buildings.

Nor did I mention the very worst, bituminous felt/ chipboard. Condensation makes the chipboard sag, causing puddling on the roof above, making the felt stretch, causing leaking, making the chipboard sag more… Reckon on imminent replacement!

The pale stone chippings on bituminous roofs provide essential shade from the sun’s desiccating rays and will need replacement if missing.

Chimneys are attacked from inside and out by acids from burning coal, ferns, colonising plants and the weather. Loose pointing and plant life can easily be observed from outside. With deterioration they become porous and a conduit for moisture into the building. Problems with chimneys are indicated by a lot of sandy material coming down the chimney, a blocked ‘damper plate’ (openable metal flap above the fireplace) if there is one, or damp patches on chimney breasts or on ceilings above. This last could also indicate deterioration of the lead joining chimney to the roof or even birds-nest debris in the chimney.

The chimneys in my last project were so bad I could see daylight from front to back through the mortar gaps! I erected scaffolding, demolished them and roofed over the hole. This solved both problems of porous masonry and damaged lead-work. Taking them right down to ground level also made a lot of extra room in the house. If chimneys must remain, provided the lead-work is OK, the cheapest and most thorough waterproofing method is three coats of waterproof render. Repointing looks nicer but costs more and is less thorough. Unused chimney pots need capping to prevent rain coming in. You can tell which are in use by lighting a fire in the grate. Use a slate cut to size, preventing it being blown off, and weighted with a brick and a dob of mortar. Unused fireplaces can be blocked off but an air-vent should be fitted to reduce condensation. If fitting a wood-burning stove it’ll need lining.

three parapets through roofs on a modern building

Chimneys in newer buildings from the 1990s onwards should have lead trays or plastic DPM (damp proof membrane) continuing right through them, preventing water in the masonry above soaking into the walls below. Their chimney breasts should never become damp, but there’s no guarantee. The same goes for parapet walls, 1990s onwards should have no problems with porous masonry, although the lead joining it to the roof can still fail. On older buildings, if interior walls below the parapet are damp, removing the parapet and roofing over, as I did with my chimneys, is the best solution. Alternatively the parapet can be removed and rebuilt with a lead tray dressed through it and under the roof tiles. Failing this, renewing mortar in the joints between the capping slabs sometimes provides a partial solution.

Sometimes parapets and chimneys are blamed for penetrating damp of other causes. I recently looked over a block of flats for a friend, and although the parapets could have been the problem, to my mind the outside render (sand/ cement coating) was more suspect. It was very soft, perhaps only a 1:12 cement/sand mix, which would be very porous.

I’ll discuss assessing gutters and downpipes next week.