How do I repair a fixing hole in masonry that has blown out?

A blown hole is one that has become larger and softer than the bit that made it, usually from hammer action in weak material, from a plug spinning, or from a fixing that has been loaded and worked loose over time.

What you'll need

• chemical anchor resin or rapid repair mortar
• dust brush and blower
• threaded studbar or long screws (optional)
• masking tape (optional)

Step by step

1. Decide whether the position is genuinely fixed.For a shelf or a bracket, moving 75 to 100mm is quicker, cheaper and stronger than any repair. Only commit to a repair when the position cannot move.
2. Clean out the hole completely.Remove old plug fragments, dust and loose material. A drill bit run gently by hand, then a brush, then a vacuum or a blower. Every repair method depends on this.
3. Assess how far the damage goes.Probe with a screwdriver. If the material is soft for 20mm and sound behind, a deeper fixing solves it. If it is soft all the way, it needs filling or resin.
4. Option one: chemical anchor.Inject resin into the cleaned hole, push in a threaded stud or a long screw, and leave it undisturbed to cure. Resin fills whatever shape the hole is, which is why it is the natural answer to a bad hole.
5. Option two: fill and redrill.Pack the hole with a rapid-set repair mortar or a resin filler, leave it proud, and let it cure fully — hours, not minutes. Then drill a fresh hole through the cured plug and fix normally.
6. Do not use ordinary filler or plaster.Interior filler has no compressive strength worth speaking of. A fixing in a filler plug will pull out at once.
7. Use a sleeve in hollow or very friable material.A perforated mesh sleeve stops resin disappearing into a void and forms a mechanical key of its own.
8. Let it cure before loading.Read the cure time and double it in cold weather. Loading a partly cured anchor destroys it permanently and the failure is not obvious until it drops.
9. Reconsider the original cause.If the hole blew because the fixing was undersized for the load, repeating it at the same size in a repaired hole will fail again. Increase the fixing, or spread the load over more of them.

Tips

• Moving the hole is not defeat. It is almost always the strongest and quickest result.
• Masking tape over the hole with a slit in it stops resin running back out on a vertical face.
• Cure times on resin roughly double for every ten degrees the temperature drops. In an unheated garage in winter, leave it overnight.

Common mistakes

• Packing the hole with matchsticks or plastic and re-driving the screw — It restores friction, not strength. The material around the hole is already crushed, so the fixing is holding on debris and it works loose again quickly.
• Redrilling a filled hole before the filler has fully cured — A partly cured plug tears out with the bit and you are back where you started, having wasted the material and the wait.

If it doesn't work

Questions people ask