Do I need an SDS drill or will a combi hammer drill do?

The mechanical difference between a rattling combi drill and a piston-driven SDS, and the jobs where only one of them will finish.

Difficulty: beginner
Time: 15 min
Read: 3 min

Short answer

A combi drill hammers by rattling two ridged discs together, which is enough for brick and lightweight block. An SDS drives the bit with a piston and hits far harder, which is what dense concrete, engineering brick and anything over about 12mm needs. For occasional plugs in brick, a combi is fine; for concrete, hire or buy SDS.

The two look similar and the marketing blurs them, but they are different machines. The distinction is worth understanding because the wrong one on a concrete wall wastes an afternoon and a fistful of bits.

Step by step

Understand the combi drill's hammer.Two ratchet-toothed discs behind the chuck click over each other as the chuck turns, vibrating the whole drill forwards. Blow energy is small and depends on you pushing.
Understand the SDS mechanism.A piston driven by the motor fires a striker at the back of the bit. Blow energy is measured in joules and is delivered whether you push or not.
Match the machine to the material.Soft brick, lightweight block, plaster: combi is fine. Dense concrete, engineering brick, hard aggregate, floor slabs, lintels: SDS.
Match the machine to the hole size.Up to about 8mm in brick, a combi copes. Beyond 10 to 12mm, or any depth over about 60mm, an SDS is the difference between minutes and never.
Note that the chucks are not interchangeable.SDS bits have a splined shank that slides and locks into the tool; they will not fit a keyless chuck, and ordinary round-shank bits will not fit an SDS holder without an adaptor.
Understand SDS-plus versus SDS-max.SDS-plus is the domestic and light trade size and covers everything a house needs. SDS-max is a heavier format for breaking and large-diameter work.
Check whether you need rotation-off.Most SDS drills have a chisel mode: hammer with no rotation, for removing tile, chasing and breaking out. A combi cannot do this at all.
Decide between buying and hiring.An SDS gets used rarely in most houses. Hiring for a weekend costs a fraction of buying, though basic corded SDS drills are now cheap enough that regular masonry work justifies one.

Tips

An SDS drill still needs its own SDS bits — budget for those, not just the tool.
Do not use an SDS in hammer mode on tile, plasterboard or anything brittle. It is far too aggressive.
A combi drill remains the better tool for screwdriving and for wood and metal; an SDS is a masonry specialist, not a replacement.

Common mistakes

Buying a more powerful combi drill instead of an SDS — The limitation is the hammer mechanism, not the motor. A more powerful combi delivers the same feeble blow energy, so it fails on concrete in exactly the same way.
Leaning on a combi drill to force it through concrete — Extra pressure prevents the ratchet discs from separating and reduces the hammering rather than increasing it, while overheating the bit.

If it doesn't work

Combi drill making no progress in a wall

Cause: Concrete or dense engineering brick — Fix: Switch to SDS. No amount of pressure or bit changing will make a combi drill perform in concrete.

SDS bit will not go into the tool

Cause: Wrong shank format, or grit in the holder — Fix: Check it is SDS-plus rather than SDS-max, clean the shank, grease it lightly and push in with a twist until it clicks.

SDS drill spinning but not hammering

Cause: Mode selector between positions, or bit not fully seated — Fix: Select hammer-and-rotate firmly, then pull the bit to confirm it is locked in but still free to slide a few millimetres.

Questions people ask

Can I use an SDS drill as a normal drill?

Many have a rotation-only mode and accept a keyed chuck adaptor, but they are heavy, coarse in speed control and poor at screwdriving. Keep a combi or driver for that work.

Do SDS bits wear out faster?

No, generally slower, because they are being driven properly rather than rubbed. Blunt tips still happen, and the same rules about clearing dust apply.

Is a cordless SDS worth it?

For occasional holes away from a socket, yes. For sustained work a corded SDS costs less and never stops. Battery drain in masonry work is heavy.