Hole Saws: Sizes, Technique, and What to Buy for Extension Work

Your plumber is running waste pipes through the first floor and needs a 44mm hole through six joists. He drills them in the wrong zone, too close to the bearing wall, and building control flags every one at inspection. Now someone has to sister (double up) those joists before the floor can be boarded. That's a day's extra labour and £200 in timber because nobody checked the rules before picking up the drill. Hole saws are dead simple to use. Knowing where and what size to drill is where the real knowledge sits.

What it is and when you need one

A hole saw is a cup-shaped blade with teeth around its rim that fits onto a drill. It cuts large-diameter circular holes in timber, plasterboard, thin sheet metal, and plastic. The blade attaches to an arbor (a shaft with a pilot drill bit in the centre) that chucks into a standard combi drill or SDS drill. The pilot bit bites first, centres the cut, and the toothed cup follows to cut the circle.

You need hole saws at three distinct points during extension work. During first-fix electrics, you're cutting holes in plasterboard ceilings for recessed downlights. During first-fix plumbing, you're drilling through timber joists and floor decking for pipe runs. During kitchen installation, you're cutting access holes in the backs and sides of base units for waste pipes, water supply, and cables. A set covering 19mm to 76mm handles all three jobs.

They're a beginner-friendly tool. No special skill required. But there are rules about where you can drill through structural timber, and ignoring them creates problems that are expensive to fix.

Types of hole saw

Not all hole saws are the same metal. The material the teeth are made from determines what you can cut, how long the saw lasts, and how much you'll pay.

For extension work, bi-metal is the right default. It cuts everything you'll encounter during first fix and kitchen installation: softwood joists, plasterboard ceilings, plywood decking, and the thin MDF or chipboard backs of kitchen units. Carbon steel sets work for a single project but teeth go blunt mid-job on anything dense. TCT is better quality but costs more, and the extra durability is wasted if you're only making 20-30 cuts across the whole build.

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One thing to understand: bi-metal doesn't mean two separate metals welded crudely together. It's a strip of high-speed steel (the cutting edge) bonded to a flexible spring-steel body. The HSS stays sharp while the spring steel flexes without cracking. Versions with 8% cobalt added to the HSS (labelled M42 or "cobalt bi-metal") stay sharper for longer and are worth the small premium. Erbauer and Bosch both use this formulation in their mid-range sets.

The arbor: the bit everyone forgets

The arbor (also called a mandrel) is the shaft that connects the hole saw to your drill. It has a pilot drill bit running through its centre and a threaded end that screws into the back of the hole saw. Without an arbor, the saw is just a metal cup.

Budget sets usually include one arbor. Better sets include two: a small one for saws up to about 30mm diameter, and a larger one for everything above. The thread size differs between the two, so you can't put a large saw on a small arbor.

Arbor quality matters more than you'd expect. Cheap arbors wobble, and wobble means the pilot drill doesn't centre properly, which means the hole saw wanders across the surface before it bites. If the pilot bit snaps (a common failure on budget arbors), the saw has nothing to anchor it and will skate violently across the workpiece. Bosch's Power Change system and similar quick-release arbors from Makita and Milwaukee let you swap saws without unscrewing anything, which saves time when you're switching between sizes repeatedly during first fix.

Which size for which job

This is the practical reference that no other guide publishes cleanly. Pipe sizes in the UK are standardised, so the hole saw size you need is predictable.

For kitchen installation, you'll cut holes in the backs and sides of base units for waste pipes (38mm or 44mm) and supply pipes (19mm or 29mm). These go through thin chipboard or MDF, so even a budget carbon steel saw handles them easily.

Joist drilling rules: the bit that actually matters

Building control restricts where and how large a hole you can drill through a timber joist. These rules exist because joists are structural. Drill in the wrong place, or drill too large a hole, and you weaken the floor. Building control inspects joist penetrations at the first-fix stage, and they will require remediation if the holes are wrong.

The rules for solid timber joists (the most common type in UK domestic construction):

Permitted zone: Holes must be drilled between 0.25 and 0.4 of the span from the nearest support. On a 3,000mm span, that means your holes must be between 750mm and 1,200mm from each end where the joist sits on a wall or beam. Drill outside this zone and the hole is in a high-stress area.

Position on the joist: Holes must be drilled on the neutral axis, which is the centreline of the joist depth. Not near the top, not near the bottom. The centre.

Spacing between holes: Centre-to-centre distance between adjacent holes must be at least three times the diameter of the larger hole. Two 40mm holes need at least 120mm between their centres.

Distance from notches: No hole within 100mm of a notch in the same joist.

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For engineered joists (I-joists), the rules are different. Don't cut the flanges (the top and bottom horizontal sections). Pre-formed web holes already exist in most I-joists. Any additional holes larger than 38mm in the web must be centred with spacing of at least twice the larger hole diameter. Always check the manufacturer's literature for your specific joist.

Metal web joists have no flexibility at all. You cannot drill or cut the flanges or the steel webs. Services run in the gaps between the webs, and conduit may need installing before the joists are fixed in place.

Always drill rather than notch where you have a choice. A correctly positioned hole on the neutral axis causes far less structural weakening than a notch of equivalent size. For waste pipe runs that need to pass through multiple joists, plan the route before drilling. Waste pipes need a minimum fall of 18mm per metre, and achieving this fall while keeping all holes within the permitted zone across four or five joists requires thought. Mark every hole position, check every measurement, then drill.

How to use a hole saw properly

Setup

Thread the hole saw onto the arbor by hand until it's finger-tight, then use the flats on the arbor with a spanner or the supplied hex key to snug it up. Don't over-tighten. The saw needs to come off again later, and heat from cutting can seize an over-tightened joint.

Chuck the arbor shank into your drill. A standard combi drill handles saws up to about 50-60mm diameter comfortably. Larger saws generate more resistance and may stall an 18V drill. For anything above 60mm, a more powerful drill (high-torque combi or SDS in drill-only mode) makes the job easier.

Cutting technique

Mark the hole centre on the workpiece. For plasterboard ceilings, mark from the room side. For joists, mark the neutral axis with a pencil line first, then mark each hole centre along it.

Set your drill to a low-to-medium speed. This matters. Large hole saws on high RPM generate heat, burn timber, dull teeth, and are harder to control. The larger the saw diameter, the slower the speed should be. As a rough guide: saws under 30mm can run at 600-800 RPM; saws from 30mm to 60mm at 300-500 RPM; saws above 60mm at 200-400 RPM. Most combi drills don't display RPM, so use the lowest gear setting and control speed with trigger pressure.

Place the pilot bit on the mark. Start the drill at low speed to let the pilot bit bite and centre the cut. Once the saw teeth contact the surface, apply steady, moderate pressure. Let the teeth do the cutting. Pushing too hard causes the saw to bind, the drill to stall, or the whole assembly to wrench sideways in your hands.

Preventing tearout

When cutting through timber or plasterboard from one side, the teeth will tear the material on the exit side, leaving a ragged edge. For visible work (downlight holes in a finished ceiling), this matters.

The fix is simple: drill from one side until the pilot bit just pokes through the other side. Stop. Go to the other side and use the pilot hole to re-centre the saw. Finish the cut from the opposite direction. Both faces get a clean entry cut.

Removing the plug

After cutting, a disc of material (the plug) will be trapped inside the cup of the saw. This is the part that drives everyone mad.

Most hole saws have slots or holes in the side of the cup. Push a flat-blade screwdriver through these slots to lever the plug out. If there are no slots, drive a wood screw into the plug face and use it as a pull handle. Whichever method you use, remove the plug before making the next cut. A jammed plug prevents the teeth from cutting and causes overheating.

What to buy

Budget: £7 – £25

A carbon steel or basic bi-metal set from Titan, Toolpak, or Silverline. The Titan 9-saw set at Screwfix runs £6.59and includes sizes from 19mm to 86mm. The Toolpak 10-piece at Toolstation is £7.98(ex VAT). The Silverline 16-piece on Amazon covers 19mm to 127mm for around £15 – £20.

These are genuinely adequate for a single extension project where you're cutting softwood joists and plasterboard. The teeth will dull after 20-30 cuts in timber. Trades treat budget saws as consumables, buying a cheap one for a specific job and binning it when it's done. For a homeowner who needs 15-25 holes across the whole build, that's enough.

The catch: pilot bits and arbors at this price point are the weak link. Pilot bits snap, arbors wobble. Budget for a replacement arbor (£5 – £10) as a spare.

If you only need downlight holes, the Toolpak Downlight Kit at Toolstation (£14.48, 4.3 stars from 409 reviews) includes the six sizes that cover virtually every UK downlight fitting: 51mm, 60mm, 64mm, 72mm, 76mm, and 86mm. It's the most-reviewed hole saw product on the site for a reason. The arbor and pilot drill are included.

Mid-range: £29 – £55

Bi-metal with cobalt (M42 grade) or multi-material sets from Erbauer, Forgefix, or Makita. The Erbauer 6-saw electricians set at Screwfix (£28.99) uses 8% cobalt bi-metal and covers 22mm to 64mm. The Erbauer 11-saw multi-material set (£52.99, 4.6 stars from 52 reviews) spans 19mm to 76mm with HSS tips and includes two arbors.

At Toolstation, the Forgefix M3 Bi-Metal Electricians Kit (9-piece, £35.98) and Plumbers Kit (9-piece, £39.98) are trade-focused sets with the sizes that electricians and plumbers actually use. The Makita 6-saw plumbers set at Screwfix (£49.89) covers 19mm to 57mm.

This tier lasts noticeably longer. The cobalt formulation keeps its edge through 50+ cuts in softwood, and the arbors are solid. If you're managing a build and expect to use hole saws across first-fix plumbing, first-fix electrics, and kitchen installation, mid-range sets pay for themselves in not having to replace dull saws halfway through.

Professional: £65 – £180

Starrett, DeWalt, Bosch Progressor, and Milwaukee Hole Dozer sets. The Starrett 6-saw Fast Cut electricians set runs £64.99at Screwfix. The Bosch PRO 11-saw multi-material (8% cobalt) costs £132.99. The Milwaukee Hole Dozer 13-saw set sits at £179.97.

These are built for tradespeople making hundreds of cuts a week. Electricians on forums report Starrett saws lasting decades with proper care. Milwaukee's Hole Dozer uses a thick-wall construction designed to survive repeated plug removal. For a homeowner on a single project, this is over-buying. The money is better spent elsewhere on your build.

One exception: if you're doing significant work in hardwood or dense manufactured boards, a professional TCT set (Milwaukee TCT 4-piece at Toolstation, £119.98) genuinely cuts faster and lasts longer than any bi-metal alternative. TCT teeth don't clog the way bi-metal teeth do in dense timber.

Alternatives

For small holes up to about 25mm, a spade bit (flat wood bit) does the same job faster and cheaper. A set of spade bits costs under £10and they're quicker through softwood joists than a hole saw of the same diameter. But they leave a rougher hole and can't cut plasterboard cleanly.

For very large holes (above 76mm) or high-volume joist drilling, self-feed bits are the trade tool. They pull themselves through timber using a screw tip and cut fast. But they need a powerful drill (high-torque combi or SDS in rotation-only mode) and they're aggressive. Not a beginner tool.

For a single specific hole size, buying one individual hole saw (£3 – £15 depending on size and material) plus an arbor is cheaper than buying a whole set. This makes sense if you only need, say, a 44mm saw for kitchen waste pipe holes and nothing else.

Where you'll need this

• First fix electrics - cutting downlight holes in plasterboard ceilings and cable access holes through joists
• First fix plumbing - drilling through timber joists and floor decking for hot/cold supply and waste pipe runs
• Kitchen installation - cutting access holes in unit backs and sides for waste pipes, supply pipes, and electrical cables

These tasks appear across any extension or renovation project, not just kitchens. Anywhere you have pipes or cables passing through timber structure or plasterboard, a hole saw is the tool for the job.

Common mistakes

Not checking joist type before drilling. Solid timber, I-joists, and metal web joists have completely different rules. Drilling through an I-joist flange or a metal web joist is a structural failure that requires replacement, not repair.

Drilling in the wrong zone. The 0.25 times depth rule is well known. The zone rule (0.25 to 0.4 of span from the nearest support) is not. Holes near the bearing points of a joist, where shear stress is highest, cause disproportionate weakening.

Running too fast. High RPM on a large hole saw generates friction heat instead of cutting action. The teeth glaze over and stop cutting. Slow down, clear chips regularly, and let the teeth work.

Forgetting to clear the plug. A plug jammed in the cup from the last cut means the next cut produces nothing but heat and frustration. Check and clear the cup every single time.

Buying the wrong arbor shank. Hex shank arbors fit standard drill chucks. SDS shank arbors fit SDS drills only. If you're using a combi drill, you need hex shank. It's printed on the packaging but easy to miss.

Cutting downlight holes before buying the lights. Cutout sizes vary by manufacturer and model. A 70mm hole is useless if your chosen fitting needs 73mm. Buy the lights first, read the spec sheet, then cut.

Safety

Wear safety glasses. Cutting timber throws fine sawdust and splinters upward, and cutting plasterboard produces irritating gypsum dust. If you're cutting overhead (downlight holes in a ceiling), dust falls directly onto your face. Safety glasses are non-negotiable for overhead work.

Clamp thin materials. Cutting a hole in a loose piece of chipboard or MDF lets the saw grab the material and spin it. Hold it down or clamp it to the bench. Kitchen unit panels being cut for pipe access should be laid flat and clamped, not held in your hand.

When drilling through joists from above, check what's below. First-fix work happens before boarding, so there's usually nothing but air beneath, but if there's existing wiring, pipework, or insulation from a previous installation, cutting into it with a hole saw is a bad day.