The short answer
Bird spikes do not damage a roof when they are fitted properly, but a poor fixing can. Spikes screwed through tiles, lead or pointing leave holes and splits that let water in. Done well, they sit on a clipped or glued base that leaves the covering untouched. Wires and netting carry the same risk at their anchor points, so every fixing is the part to check.
The spike is rarely the problem, the fixing is
A strip of plastic or steel spikes weighs very little, and it rests on the surface instead of cutting into it. What matters is the way it stays put. There are three common methods: adhesive, clips that grip an edge, and screws or nails driven through the surface.
Screws are the one to question. A screw through a roof tile makes a hole in a material that was made to shed water. A screw into lead or into mortar does the same. Each hole is small, but a roof is exposed to rain, frost and wind-driven water on every slope, and a small hole at a ridge or flashing is exactly where water finds its way in. If the person fitting the spikes is working from a ladder and wants to do it quickly, screws are what they reach for.
Adhesive can be gentler, but it depends on a clean, dry surface. Moss, droppings and algae stop it from holding, so spikes glued to a dirty ridge come loose in the first hard winter and end up in the gutter. A good installer cleans the surface first, uses a base that suits the material, and chooses the fixing to match the roof.
Tiles and slate
On a tiled roof, spikes normally go along the ridge, which is the line of tiles capping the top, and along the hips, the angled edges formed where two slopes join. Clay and concrete tiles are brittle at their edges, so a clamp that grips too hard can chip them, and a screw can split one outright. A cracked tile may not look like much from the ground, but it lets water in under the tile and wets the batten, the timber strip the tile hangs from.
Slate is more delicate again. It is thin, layered stone, and drilling through it risks flaking the surface. For slate we would expect a clipped or glued system that avoids the slate altogether, and ideally one that sits on the ridge tile rather than on the slope.
Spikes also change how water runs. A long continuous strip along the eaves can trap leaves and moss, keep water sitting against the edge of the roof and block the line where rain should drain into the gutter. That is worth thinking about before spikes are placed low on a roof.
Lead, stonework and parapets
Older Bath houses have more at risk than a plain tiled roof. A parapet is a short wall built up at the roof edge. Behind it you will often find a parapet gutter, a trough finished in sheet lead that leads rainwater off to a downpipe. Lead flashing, a strip of sheet lead dressed across the meeting of roof and wall, runs along the wall and around chimneys. Coping stones cap the top of the wall.
All of these are bad places for a screw. Lead is soft and thin, and a hole in it is a leak waiting to happen. Fixing through lead also stops it from moving as it warms and cools, which can lead to cracking beside the fixing. On stone copings and old pointing, drilling can crack the stone or break the bed of lime mortar beneath it. Where the building is listed, the council planning team at B&NES needs to know before anything is attached to the fabric.
The better approach on these roofs is a clamp or a base that grips a coping without penetrating it, or a glued unit set on a clean surface. Where the roof detail is delicate, wire or a sprung system running along the ledge can do the same job with no holes in the stone.
Spikes, wires and netting compared
No deterrent suits every roof, and none works on every bird. Pigeons and gulls both react differently to each type.
- Spikes. Good on flat ledges, copings and ridges where a bird wants to perch. Gulls may still nest between or behind them if the strips leave gaps.
- Tensioned wire. Light and unobtrusive on a period building, held by small posts at each end. The posts are the only fixing, so they are the points that need to be sealed.
- Netting. Covers a whole recess, valley or open area. It needs fixings around its perimeter, so a large net means many anchor points and is only as good as each one.
- Mesh or proofing at the eaves. Closes the gaps under tiles where birds get in, which does more good than a perch deterrent on some roofs.
Our page on bird-proofing a roof goes through where birds actually get in, and if gulls are the issue then seagulls nesting on a roof covers the timing and the legal limits.
Having spikes fitted without leaks
If you have a quote for spikes from any firm doing roof repairs in Bath, ask how each fixing works and what it fixes to. A fitter who is clear about this is usually a good sign. One who says only that it is quick and cheap is not telling you enough. Ask whether anything goes through tile, lead or stone, and what is done to seal it.
If spikes are already up and you are seeing damp, look at the nearest fixings first. A stain on a ceiling below a ridge or a chimney may trace back to a screw hole. Our page on leaks that appear in windy weather covers holes that only let water in when the rain is driven sideways. A leak above a flat is a different case again, covered in roof repairs in flats.
Where spikes have damaged the covering, the repair is to take out the offending fixings, replace any split tile, patch the lead with a proper lead repair and reset the deterrent on a better base. If spikes are going on a ridge or chimney, ask how they will be fixed, because a strip stuck down with adhesive leaves the tiles whole while screws driven through a tile or the lead open a path for water. Droppings are worth clearing as well, since they hold moisture against the roof and can block gutters and outlets.
