How do I fix into aerated concrete blocks?

Aerated blocks, sold under names like Thermalite and Celcon, are foamed concrete: strong enough in compression to carry a wall, but soft and crumbly against a point load. Fixings that rely on expansion work against that weakness rather than with it.

What you'll need

• aerated block fixings or resin anchors
• drill with hammer switched off
• dust brush or vacuum
• long screws (optional)

Step by step

1. Confirm what the wall is.Aerated block drills very fast, gives pale grey almost sandy dust, and the bit often sinks under its own weight. It is common in internal partitions and modern inner leaves.
2. Turn hammer action off.Percussion smashes the cell structure and produces an oversized, ragged hole. Rotation alone cuts it easily.
3. Drill the hole undersized where the fixing allows.Screw-in anchors for aerated block generally need a hole smaller than the anchor's outside diameter, exactly the reverse of a masonry plug. Follow the fixing's own instruction.
4. Clear the hole thoroughly.Aerated block produces a lot of loose dust which stops any fixing gripping. Brush and vacuum it out.
5. Use a fixing designed for the material.Options are a long coarse-spiral nylon plug, a screw-in nylon or metal aerated concrete anchor, or a nylon-sleeve frame fixing. All spread load over a long length rather than expanding at a point.
6. Use resin for anything heavy.For a radiator, a boiler, a wall-hung basin or a TV bracket, a chemical anchor with a sieve sleeve is the reliable answer. The resin bonds along the whole hole rather than pressing outwards.
7. Go long.Depth is the substitute for grip. A 50mm fixing in aerated block will hold a fraction of what a 100mm one will.
8. Spread the load over more fixings.Four moderate fixings in aerated block beat two heavy ones, because the failure mode is local crushing rather than the fixing itself pulling.
9. Test before you trust it.Pull firmly on the fixing before hanging anything. Aerated block gives no warning between holding and letting go.

Tips

• Hammer off, always. It is the single most common mistake in this material.
• If a fixing spins, it has crushed its hole. Move at least 75mm rather than trying a bigger plug in the same place.
• Where the wall is plastered, remember the plaster gives a false sense of firmness in the first 12mm — the fixing must work in the block behind it.

Common mistakes

• Using ordinary red or brown expansion plugs — They work by pressing sideways against the hole wall. Aerated block cannot resist that pressure, so the plug crushes its own hole and the fixing has nothing to grip.
• Leaving hammer action on — The percussion shatters the aerated structure and makes the hole substantially larger and looser than the bit diameter.

If it doesn't work

Questions people ask