Chapter 1: Why are tiles slipping on our Georgian terrace near College Street?
What we find
On the older clay roofs in the city, each plain tile hangs from the batten (the timber lath running across the rafters) by two small nibs and is held by a nail or a wooden peg. Over a couple of centuries the iron nails rust and thin, and the pegs soften. The tile then drops a little in its course, which roofers call nail sickness. Frost makes it worse, because water that gets behind a tile freezes and pushes it further out of line.
You will usually spot a single tile sitting low, a gap showing a dark strip of underlay, or broken pieces on a path or in a rear light well. In the loft, look for pale stains on the underside of the roof boards and for daylight at the line of the slipped tile. A wavy roof is not the same thing as a failing roof, and the undulation alone is no cause for alarm.
What we fix
We lift the affected area and check what is left of the fixings, then re-hang the sound tiles on new stainless or copper nails and replace the broken ones with reclaimed plain tiles of matching colour and camber (the slight curve across each tile). Where the battens have rotted, they are renewed in the same section. Breathable underlay goes under the work if the old felt has perished. The rest of the roof is left alone, so the character the appraisal values stays in place.
If slipping is widespread across a whole slope, patching stops making sense and a strip and re-lay of that slope is the sensible repair, reusing as many tiles as survive. Our nail sickness guide explains the fault in detail, and the pitched roofing page shows how a larger job runs.
