Anchor Bolts: Which Type, Which Wall, and Why It Matters

A steel beam bearing plate bolted to blockwork with four M12 wedge anchors. The builder tightened them up, left site, and within a week the plate had pulled 15mm away from the wall. The blockwork behind had cracked in a starburst pattern around every hole. The substrate was hollow lightweight block. Wedge anchors generate enormous expansion forces, and there was nothing solid for them to grip. Four new holes, chemical anchors this time, and a conversation with building control about remedial work. That job cost three times what it should have.

The anchor bolt you choose matters far less than the wall you're putting it into. Get that decision wrong and the fixing either cracks the substrate, spins in a void, or pulls straight out under load. Get it right and a single M10 anchor will hold over 300kg in tension from solid concrete. The substrate dictates everything.

What an expansion anchor bolt is

An expansion anchor is a steel bolt designed to grip inside a hole drilled in masonry or concrete. You drill a hole, push or hammer the anchor in, then tighten the nut. As the nut tightens, an internal mechanism forces a sleeve, wedge or shield outward against the walls of the hole. That outward pressure creates friction against the substrate, locking the bolt in place.

The grip is purely mechanical. Nothing is glued. The bolt stays put because the expanded section is jammed against solid material. This is why substrate density is everything: the expansion mechanism needs something hard and continuous to push against. A void, a soft block, or a crack in the concrete means the expansion has nowhere to go.

Expansion anchors carry load in two directions. Tension (pulling straight out of the wall) and shear (pulling downward or sideways across the face). Shear capacity is always higher than tension capacity for the same bolt, because the full cross-section of the steel resists shear while only the expansion grip resists pull-out.

Types of expansion anchor

Three main types are sold in the UK, and they're designed for different substrates and load requirements. Picking the wrong type for your wall is where most failures start.

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Wedge anchors

The strongest type. A tapered cone at the base of the bolt is pulled up into an expansion clip when you tighten the nut. This creates intense, concentrated force at the base of the hole. Wedge anchors deliver the highest pull-out resistance of any mechanical anchor in solid concrete.

But that concentrated force is also their weakness. They impose massive stress on the substrate around the expansion zone. In solid C20/25 concrete, that stress is absorbed without issue. In anything softer, lighter, or hollow, the concrete cracks or the block splits. Wedge anchors are concrete-only fixings. Not brick. Not block. Concrete.

Drill diameter matches the bolt size exactly. M10 wedge anchor = 10mm drill bit. Simple.

Sleeve anchors (throughbolts)

More versatile than wedge anchors. The expansion force is distributed along the full length of the sleeve rather than concentrated at one point. This makes them more forgiving in variable substrates and suitable for both cracked and non-cracked concrete.

Sleeve anchors are the practical choice for most extension work. They work in dense concrete, engineering brick, and solid common brick. Load capacity is lower than wedge anchors in ideal concrete, but the reduced substrate stress means fewer cracking problems.

The drill diameter is larger than the bolt size. An M8 sleeve anchor needs a 10mm hole. An M10 needs a 12mm. Check the manufacturer's specification for every size because getting this wrong means the sleeve can't expand properly.

Shield anchors (Rawlbolts)

The traditional UK anchor bolt, and still the most commonly stocked at Screwfix and Toolstation. A split metal shield folds outward as you tighten the bolt, gripping the inside of the hole. The expansion is gentler and more distributed than a wedge anchor.

Shield anchors work in solid concrete, solid brick, and dense block. Rawlplug's technical data rates certain shield anchors for use in hollow lightweight block, but the community experience is overwhelmingly negative. In practice, the shield has too little material to grip in a void, and the expansion force can crack lightweight block. For DIY installation, treat shield anchors as solid-substrate fixings only.

The drill diameter is significantly larger than the bolt size. An M8 shield anchor needs a 14mm hole. An M10 needs 16mm. An M12 needs 20mm. These are big holes, so get it right first time.

The substrate suitability matrix

This is the single most important table on this page. Every fixing failure traced back to "wrong anchor for the wall" could have been avoided by checking this first.

How to install expansion anchors

The process is straightforward, but every step has a specification that must be followed exactly. "Near enough" drilling or "hand-tight" torque causes failures.

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Drill the right hole

The drill diameter must match the anchor specification exactly. Not approximately. Exactly.

For wedge anchors, the drill matches the bolt: M10 bolt, 10mm SDS bit. For sleeve anchors, the drill is larger: M10 bolt, 12mm SDS bit. For shield anchors, the drill is larger still: M10 bolt, 16mm SDS bit.

Use an SDS drill in hammer mode for concrete and masonry. A standard drill will struggle and overheat in anything harder than soft brick. Mark the drill depth on the bit with tape. The hole must be deep enough for full embedment plus 5-10mm clearance for dust at the bottom.

Clean the hole

Drill dust sitting at the bottom of the hole reduces embedment depth and prevents the anchor seating properly. Blow the hole out with a hand pump or compressed air. Brush the sides with a wire brush. Blow again. This sounds excessive. It isn't. BS 8539 specifies a clean-brush-clean sequence for structural applications, and it exists because dust in holes is one of the top three causes of anchor failure.

Set the embedment depth

Every anchor size has a minimum embedment depth. Go shallower and the expansion mechanism doesn't have enough substrate to grip. The minimum member thickness (how thick the concrete or masonry must be) is also specified, because an anchor that's close to the back face of a block can blow out the rear.

Size	Min Embedment (Shield)	Min Hole Depth (Shield)	Min Embedment (Sleeve)	Min Member Thickness
M8	40mm	55mm	35mm	70-100mm
M10	50mm	65mm	40mm	80-120mm
M12	60mm	85mm	50mm	100-140mm
M16	95mm	125mm	N/A	170mm+

Edge distances and spacing

Put an anchor too close to the edge of a block or concrete pad and the expansion force blows out the corner. The minimum edge distance for an M10 shield anchor is 75mm. For M12, it's 90mm. Minimum spacing between anchors is similar: M10 at 50mm, M12 at 60mm.

These aren't guidelines. They're failure thresholds. Go below them and the concrete or masonry physically cannot resist the expansion force.

Torque

Every anchor has a specified torque setting. This is the point where the expansion mechanism is fully engaged but the substrate isn't overstressed.

Size	Torque in Concrete	Torque in Brickwork
M6	6.5 Nm	5 Nm
M8	15 Nm	7.5 Nm
M10	27 Nm	13 Nm
M12	50 Nm	23 Nm
M16	120 Nm	N/A

Brickwork torque is roughly half the concrete torque. This catches people out. They tighten an M10 anchor in a brick wall to the same 27Nm they'd use in concrete, and the brick cracks. Use a torque wrench for structural connections. Hand-tight then "a bit more" is not a specification.

How many you need

Anchor bolts are specified per connection, not per project. A steel beam bearing plate typically needs four M12 anchors per pad. A heavy wall bracket for kitchen cabinets might need two M10 anchors. The structural engineer or the fitting manufacturer specifies the number, size and pattern.

Buy a few spares. If a hole goes wrong (drill wanders, hits a void, substrate cracks), you need to drill a new hole in a different position. You can't reuse the damaged hole. Buying a five-pack when you need four gives you one spare at minimal extra cost.

For non-structural work (handrails, shelving brackets, general brackets), two M8 or M10 anchors per bracket is standard. Space them vertically, not horizontally, because the top anchor takes the majority of the shear load.

What they cost

Expansion anchors are cheap. Even premium branded ones rarely exceed TBC3 each for common sizes. The anchor itself is never the expensive part of a fixing job. The SDS drill, the correct masonry bits, and the time spent drilling are where the cost sits.

Budget brands (Easyfix at Screwfix, F&F at Toolstation) are perfectly adequate for non-structural fixings. For structural work (steel bearing plates, heavy brackets that building control will inspect), use a branded product with an ETA number: Rawlplug, Fischer, or Hilti. The price difference is pence per bolt. The accountability difference is substantial.

Zinc-plated steel is standard and fine for internal use. For external fixings or anywhere exposed to moisture, specify galvanised or stainless steel. Stainless adds roughly 50-100% to the per-bolt cost, but corrosion on a structural anchor is not something you want to discover ten years later.

Where to buy

Screwfix and Toolstation stock the full range of shield anchors in M6 to M12, plus sleeve anchors from Fischer and Rawlplug. Both offer next-day delivery or click-and-collect. Toolstation often has better prices on Fischer bulk packs.

For wedge anchors and ETA-rated products, specialist fixings suppliers like Midfix and Fixmart carry wider ranges. Worth the extra few days' delivery time for structural applications where you need a specific ETA number.

When chemical anchors are the better choice

Chemical anchors (injection resin with threaded rod) are the upgrade path when expansion anchors can't be used safely. They work by bonding resin to the inside of the drilled hole, creating a solid connection that doesn't rely on expansion force. No outward pressure on the substrate.

Use chemical anchors when:

• The substrate is hollow block, aerated block, or lightweight aggregate block
• You're fixing close to an edge (less than 50mm for M8, less than 75mm for M10)
• The concrete is cracked or of uncertain quality
• The application involves vibration (machinery mounts, for example)
• Building control requires traceable, testable fixings for structural connections
• The consequences of failure are severe (overhead fixings, structural bearing plates)

Chemical anchors cost more per fixing (typically TBC8-15 for a cartridge that does 10-15 fixings, plus TBC1-2 per length of threaded rod) and take 20-45 minutes to cure before loading. But they work in every substrate where expansion anchors fail, and they don't impose expansion stress on the surrounding material.

For hollow block specifically, injection resin with a mesh sleeve (Hilti HIT HY 20 system or equivalent) is the professional solution. The mesh contains the resin inside the void, creating a solid anchor point in material that has no solid core.

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Where you'll need anchor bolts

Steel beams and lintels

When a steel beam bears onto blockwork or a concrete pad, the bearing plate needs bolting down. This is the most common structural use of expansion anchors on a domestic extension. Typically four M12 anchors per bearing plate, torqued to specification, with building control expecting to see the installation before the steel is loaded.

If the beam bears onto a concrete pad (which it should, on any properly designed scheme), wedge anchors are ideal. Maximum grip in the ideal substrate. If it bears onto blockwork (common on older properties where a padstone is used), the substrate determines the anchor type. Dense concrete block: shield anchor with reduced torque. Lightweight or hollow block: chemical anchor, no question.

Kitchen installation

Heavy wall units, tall larder units, and worktop brackets all need fixing to the wall behind. In a new extension, that wall is usually blockwork with plaster or plasterboard over it. Standard wall plugs with screws are adequate for most kitchen units, but the anchor points for heavy items (full-height pull-out larders, wall-mounted ovens, reinforced worktop brackets for breakfast bars) need expansion anchors or chemical anchors depending on the substrate.

An M8 or M10 shield anchor through the cabinet back, through any plasterboard, and into solid blockwork gives a reliable connection. Account for the thickness of the plasterboard when calculating embedment depth. A 12.5mm plasterboard layer means 12.5mm of your anchor's length contributes nothing to the grip.

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Common mistakes

Using expansion anchors in hollow block. The single most common anchor failure. It deserves repeating: no wedge anchor, no sleeve anchor, and (despite what some manufacturers claim) no shield anchor should be trusted in hollow blockwork for DIY installation. Use chemical anchors.

Wrong drill diameter. A hole that's 1mm too large means the sleeve or shield can't generate proper expansion pressure. A hole that's 1mm too small means the anchor won't seat to its full depth. Check the specification on the box, not a general table. Different brands use different drill sizes for the same bolt diameter, particularly for sleeve anchors.

Overtightening in brickwork. Brickwork torque is half the concrete torque. An M10 anchor in brick gets 13Nm, not 27Nm. Overtightening cracks the brick, and once the brick is cracked, the anchor has zero pull-out resistance. Use a torque wrench.

Skipping hole cleaning. Drill dust compacts at the bottom of the hole, preventing the anchor from reaching full embedment. The anchor sits proud, the expansion zone is in the wrong position, and the pull-out capacity drops. Blow the hole out, brush, blow again.

Ignoring edge distances. A bracket bolted 30mm from the edge of a concrete pad with M10 anchors will blow out the corner of the concrete when the expansion force kicks in. The minimum edge distance for M10 is 50-75mm depending on the anchor type. Measure it.

Not accounting for plasterboard thickness. A 75mm anchor through 12.5mm of plasterboard leaves only 62.5mm in the substrate. If the minimum embedment is 50mm, you're cutting it fine. Always calculate embedment based on what's behind the plasterboard, not the total anchor length.

Using a standard drill instead of an SDS. Standard drills overheat in concrete, wander off-line, and produce oversized holes with rough walls. An SDS drill in hammer mode cuts clean, accurate holes in concrete and hard masonry. It's the right tool. There isn't a cheaper shortcut that works reliably.

Standards and building control

BS 8539:2012+A1:2021 is the British Standard for post-installed anchors in concrete and masonry. It covers selection, installation and testing. For structural applications on a domestic extension (primarily steel beam bearing plates), building control will expect compliance with BS 8539.

The standard requires:

• ETA-approved anchors for structural connections
• Trained installers (not just general site experience)
• Correct drilling, cleaning, and torque procedures
• Documentation of anchor make, type, ETA number and size

If an ETA-approved anchor is used and installed correctly by a competent person, pull-out testing is not required. If any of those conditions aren't met, testing under BS 8539 may be required, and that adds cost and delay.

For non-structural fixings (kitchen units, shelving, general brackets), BS 8539 doesn't apply. Use manufacturer guidance, check the substrate, and apply the correct torque. That's sufficient.