Masonry vs Concrete Screws: What Size Hole?

Masonry screws and concrete screws are the same fixing under two names: a hardened, plug-free screw that cuts its own thread straight into brick, block or concrete. You will also see them called concrete frame screws or concrete bolts. The common trade size is 7.5mm diameter, and it needs a 6mm pilot hole. Pick the length so the screw goes at least 40 to 50mm into solid masonry past whatever you are fixing. No wall plug required.

Is there a difference between a masonry screw and a concrete screw?

No. They are the same product, and the different names are just what different trades and suppliers call it. A masonry screw, a concrete screw, a concrete frame screw and a concrete bolt all describe the same thing: a high-strength screw with a hardened, aggressive thread that taps into masonry without a plug. That is exactly why searches split across all those terms, everyone is looking for the same fixing.

The one that does stand slightly apart is a "concrete bolt", which people sometimes use to mean a heavier hex-head version for structural work. The screw-head versions (T30 Torx, countersunk or pan head) are what you reach for on the vast majority of jobs, fixing timber, frames, battens, brackets and unistrut back to masonry.

 

What size hole do you drill for a masonry screw?

For the standard 7.5mm masonry screw, drill a 6mm hole. The 1.5mm difference is what the hardened thread bites into. Drill it with a masonry drill bit, or an SDS bit in denser concrete, and follow three rules:

• Drill deeper than the screw. Go at least 10mm deeper than the screw will sit, so dust collects below the thread instead of jacking the screw back out.
• Clear the dust. Blow or vacuum the hole out. A hole packed with dust is the single most common reason a masonry screw spins and fails to grip.
• Drive at low RPM. These are hardened screws. Run the driver slow, with the T30 bit fully seated, so you do not shear the head or strip the thread you have just cut.

 

What size masonry screw do you need?

Diameter is easy: 7.5mm covers almost every job. Length is where you choose. The rule is the screw length equals the thickness of what you are fixing (the fixture) plus the depth it needs in the masonry, and you want at least 40 to 50mm biting into solid masonry. Our 7.5mm masonry screws run from 42mm to 152mm, in boxes of 100:

For frame fixing, measure the frame plus the gap you are packing out and add the embedment. Keep packers behind each fixing so you are tightening onto a solid point, not pulling the frame out of line.

Brick, block or concrete: does it matter?

The same 7.5mm screw and 6mm hole works across all three, but the substrate changes how it behaves:

• Dense concrete and engineering brick give the best hold. Use an SDS bit to drill, and do not overdrive.
• Common brick and medium block are the everyday case and hold well.
• Aircrete and soft block hold, but ease off the torque, the thread can strip a soft block if you keep driving after it is home. Fix into the solid part, not the perforations or a mortar joint if you can help it.

One real advantage of a masonry screw: because there is no plug expanding in the hole, it fixes closer to an edge without blowing the masonry. That makes it the better choice near the end of a lintel, a reveal or a slab edge.

When should you still use a wall plug instead?

Masonry screws have replaced plugs for most day-to-day fixing, but a plug (or resin) still wins in a few cases:

• Very soft or friable substrate that will not hold a cut thread, an old lime-mortar wall or crumbling block.
• An oversized or worn hole, if you have drilled too big or a screw has been in and out, a plug fills the hole where the screw no longer grips.
• Heavy, sustained structural loads where the design specifies a through-bolt or resin anchor. Follow the spec, not habit.

For everything else, timber, frames, brackets, battens, the plug-free screw is quicker and there is nothing to push in first.

How to fix a masonry screw

Step One: drill the hole

Mark your point, and drill a 6mm hole at least 10mm deeper than the screw. Hold the drill square so the screw sits straight. In concrete, use an SDS bit; in brick or block, a standard masonry bit is fine.

Step Two: clear the dust

Blow or vacuum the hole out. This step gets skipped and it is the one that costs you grip, the screw needs to bite masonry, not compacted dust.

Step Three: drive the screw

Fit the T30 bit fully into the head, offer the fixture up, and drive the screw at low speed until the head pulls home. Stop there. Overdriving a hardened screw in soft masonry strips the thread and you lose the fixing.

Common mistakes to avoid

• Wrong pilot size. A 7.5mm screw wants a 6mm hole. Drill 6.5 or 7mm and it spins; drill 5mm and you shear the head.
• Hole not deep enough. If the screw bottoms out on dust or solid masonry it jacks back out. Drill deeper than the screw.
• Not clearing the dust. The most common cause of a masonry screw that will not grip. Clear every hole.
• Driving too fast. High RPM shears heads and strips threads. Slow and steady.
• Too little embedment. Under about 40mm in solid masonry the fixing is weak. Choose the length for the embedment, not just to reach.

The short version

Masonry screws and concrete screws are the same plug-free fixing. Standard is 7.5mm diameter into a 6mm hole, drilled 10mm deeper than the screw and blown clean, driven slow on a T30 bit, with 40 to 50mm biting into solid masonry. Pick the length for the fixture plus the embedment. Browse the full range of 7.5mm masonry screws in boxes of 100, and the masonry drill bits to match.

By Steve, Builders Hardware Direct. Steve runs the BHD trade counter and has spent years supplying fixings and drill bits to builders, roofers and window fitters across South Wales. Not sure on a size? Call the counter on 01269 267030 or order online for free UK delivery.