How do I drill right through an external wall for a cable or a vent?

This is the job for a tumble dryer vent, an extractor duct, a boiler condensate, an outside tap, an aerial or a network cable, and it is done badly far more often than it is done well. Almost all the failures are the same three: the hole falls the wrong way and carries water in, it emerges in the wrong place outside, or it lets the cavity get wet.

Safety

What you'll need

• Long masonry bit or core, the right length for the wall
• SDS drill for anything beyond a small hole
• Cable and pipe detector
• Pipe sleeve or plastic conduit (optional)
• External sealant and a wall grille or cover plate
• Goggles and FFP3 mask

Step by step

1. Establish what the wall is and how thick.Measure at a window or door reveal. A modern cavity wall is typically two leaves with a gap; a solid Victorian wall is a single thickness of brick, often more than one brick deep. The bit length you need follows from this measurement, not from a guess.
2. Check both faces for services.Detector inside across the whole area, and outside look for pipes, meter boxes, cables, gas runs and anything mounted on the wall that might be fixed through it. Cables run in predictable zones near switches and sockets, and a long bit passes straight through them.
3. Pick the position from both sides.Measure from a fixed reference that exists inside and outside — a corner, a window reveal — and mark both. Coming out through a downpipe, a soil stack, a meter box or somebody's rendered detail is a very common and very visible mistake.
4. Set the fall towards the outside.Angle the drill so the outside end of the hole is lower than the inside end. Not much, a few degrees is enough. Every hole through an external wall is a potential water path, and the fall is what decides which way water travels.
5. Pilot with a long thin bit first.A 6 or 8mm long bit through the whole wall lets you check outside where it has emerged before committing. If it is wrong, one small hole is easy to fill.
6. Open up from the inside.Drilling the full size from inside means the breakout — the ragged spalled edge where the bit exits — happens on the outer face where a grille or cover plate hides it, and the neat side is the one you look at every day.
7. Deal with the cavity.If the cavity is filled or partly filled with insulation, sleeve the hole with plastic conduit or a purpose-made sleeve so loose fibre or bead cannot pack round the cable or duct and carry water across.
8. Expect to meet a wall tie.A sudden hard stop with a metallic feel partway through is a tie. Do not push through it — move the hole 50mm along and start again. Cutting a tie weakens the wall and destroys the bit.
9. Seal the outside first, and shape the seal.Fill round the duct or cable on the outer face and tool the sealant so it slopes away and downwards, with no ledge that holds water. Fit the external grille or cover last.
10. Make good inside afterwards.Fill and decorate the inner face once everything is through and tested. Doing it before the cable is pulled means doing it twice.

Tips

• For a 100mm or larger vent hole, a diamond or TCT core in an SDS drill is the right tool and a hire shop will have both. Do not attempt a large hole with a small bit and a chisel.
• If the outside face is render, drill the render carefully at low speed with hammer off before switching hammer on for the masonry behind — render spalls badly under percussion.
• Where the hole is for an electrical cable, how and where the cable may pass through and be terminated is regulated. The hole is yours; the electrical connection at either end may not be.

Common mistakes

• Drilling level or uphill towards the outside — The hole becomes a channel that delivers rainwater into the wall and eventually into the room. This is a very common cause of a mysterious damp patch near an extractor or a dryer vent.
• Drilling from outside inwards — The breakout lands on the inside face, blowing plaster off around the hole in the room you have to look at.
• Not sleeving a hole through a filled cavity — Insulation packed against a cable or duct bridges the cavity and gives water a path across it.

Questions people ask