Why Power Tools Keep Cutting Out on Roofing Jobs – and the Cheap Fix Most Crews Miss

Every roofer has had it happen: the grinder or the drill cuts out mid-cut, comes back when you wiggle the lead or give the tool a knock, then dies again an hour later.

Most crews assume the tool’s on its way out and either nurse it through the job or budget for a replacement.

Nine times out of ten it’s a quick fix once you know which fault you’re looking at – and on site tools, it’s usually the cheapest part in the whole machine.

Here are the classic problems and their quick fixes:

Worn carbon brushes

Carbon brushes are the small blocks of carbon inside a corded tool’s motor that press against the spinning commutator and carry the electrical current across that moving joint.

They wear down every time the tool runs, and roofing work is especially hard on them – slate, tile and fibre-cement dust gets pulled straight through the motor housing and grinds away at both the brushes and the commutator they run against. Angle grinders and wall chasers, run hard and close to the dust, take the worst of it.

The signs:

• Sparking at the motor vents that’s heavier than usual, or brighter and more constant. A little faint sparking is normal on a brushed tool – what you’re watching for is a jump in how much you’re seeing.
• Power that cuts out, then comes back when you knock or tap the tool casing.
• A hot, acrid smell coming from the tool body, especially after it’s been running under load.
• Power fading noticeably when you push the tool into a cut, then picking back up when you ease off.

Worn brushes are the most common reason a corded site tool keeps cutting out, so if you’re seeing any combination of these, check them before you look at anything else.

One thing worth checking before you go near the brush caps: if there are no brush caps on the housing at all, the tool is almost certainly brushless, and this section doesn’t apply. Brushless tools don’t carry current across a spinning joint the same way, so there are no carbon brushes to wear out or replace – if a brushless tool is cutting out, look at the next few causes instead.

How to fix it

1. Unplug the tool, or pull the battery if it’s a cordless model – never open the housing live.
2. Find the brush caps. On most tools they’re two small screw-in caps on opposite sides of the motor housing, often marked with a brush symbol.
3. Unscrew the caps and slide the brushes out. They’ll be spring-loaded, so support them as they come free.
4. Check the length of carbon left. Once a brush has worn down to roughly 5-6mm, replace both brushes as a pair – even if one looks slightly better than the other, an uneven pair wears unevenly and you’ll be back in here sooner.
5. Buy brushes that match the exact make, model and type number of the tool – it’s stamped on the rating plate. A brush that’s the wrong size or shape for that motor sits unevenly against the commutator and scores it, which turns a cheap fix into an expensive one. Top Deals Online, the London-based carbon brush specialist established in 2012, stocks matched sets for thirty-odd tool brands and is a good place to buy them – browse the range at topdealsonline.shop.
6. Seat the new brushes squarely in their holders so they sit flush against the commutator, then refit the caps.
7. Run the tool off-load for a few seconds before you put it back to work. Light sparking while the new brushes bed in is normal; sparking that’s heavy or getting worse means something’s seated wrong and it’s worth pulling the caps and checking again.

Weigh this against the alternative: a few pounds for a matched pair of brushes and ten minutes with a screwdriver, against letting the sparking carry on. Left unchecked, that arcing chews into the commutator and eventually the armature underneath it (source) – and a scored commutator is a repair that’s rarely worth doing on a site tool. Change the brushes early and you never get there.

Dust-clogged cooling vents

A motor generates heat every time it runs, and it relies on air being pulled through the housing to carry that heat away. On a roof, that air is loaded with slate dust, tile dust and cement dust, and it’s that same airflow that packs dust into the housing instead of just passing through it. Once the vents are clogged, the motor can’t breathe properly, it overheats, and the tool cuts out to protect itself.

The signs:

• The tool cuts out after it’s been running for a while, particularly on hot days or long cutting runs, rather than the moment you pull the trigger.
• The housing feels noticeably hot to the touch, more than you’d expect for the job you’ve been doing.
• You can see dust caked around the vent slots, or the tool sounds slightly muffled compared to when it’s clean.
• The tool works fine again after it’s been left to sit for a while, then cuts out again once it’s been run hard for a few minutes.

How to fix it

Unplug the tool and blow the vents through with an air line or a can of compressed air, working from both sides of the housing so dust doesn’t just get pushed further in. On roofing work, do this often – daily on a dusty job is not excessive for a grinder or chaser that’s cutting slate or fibre-cement all day. If the tool has already cut out from overheating, let it cool fully before you run it again; forcing it back on while it’s still hot just trips the same protection straight away.

A damaged mains cable

A nick, crush point or internal break in the mains cable interrupts the circuit without the tool itself being at fault. Because the damage is in the cable rather than the tool, the power loss often tracks with how the cable is moving rather than how hard the tool is working.

The signs:

• Power cuts out or flickers when the cable is pulled taut, bent at a sharp angle, or trodden on, and comes back when the cable is repositioned.
• The fault gets worse in one particular spot along the cable, especially near the plug, near the tool’s cable entry, or anywhere it’s been run over a parapet or scaffold edge.
• You can see visible damage – a cut through the outer sheath, a flattened or kinked section, or scorching near either end.
• The tool runs fine on a different lead or extension, which points the fault away from the tool itself.

How to fix it

Inspect the full length of the cable, not just the obvious sections – run your hand along it and check both ends closely, including where the cable enters the plug and where it enters the tool body, since that’s where flexing damage builds up fastest. If you find a nick, crush point or any exposed conductor, the lead gets replaced. Don’t tape over a damaged cable and carry on using it – taped damage on a mains lead is a shock and fire risk on top of the tool still cutting out, and it’s not a fix that holds up on a working roof.

A tired battery (cordless tools)

On cordless tools, a fault that looks like the tool is very often the battery reaching the end of its useful life. A battery pack that’s degraded can’t hold its charge or deliver current the way it did when new, and it’s usually cheaper and faster to rule the battery out first than to start stripping the tool down.

The signs:

• The tool runs down noticeably faster than it used to for the same kind of work.
• Power drops or the tool cuts out under load – pushing into a cut or driving a long fixing – even though the battery showed charge left.
• The battery feels warmer than usual during charging or use, or won’t take a full charge any more.
• The same tool runs normally on a different battery from the same range.

How to fix it

Swap in a battery you know is good – borrowed from another tool on the same platform, or freshly charged and tested – and run the tool under the same conditions that were causing the cut-out. If the fault disappears with the known-good battery, the pack is the problem, not the tool. Replace the battery rather than spending time or money investigating the tool further.

A failing trigger switch

The trigger switch is what makes and breaks the electrical connection every time you pull it, and like any mechanical switch on site, it takes in dust and grit over time. Once the contacts inside are worn or fouled, the connection stops being reliable even though the motor, brushes and cable are all fine.

The signs:

• The trigger feels dead, gritty, or gives an intermittent connection – power comes and goes as you vary the pressure on the trigger, not as the tool is worked.
• Everything else checks out: brushes are fine, vents are clear, the cable’s undamaged (or the battery’s confirmed good on a cordless tool), but the cutting out continues.
• The trigger sometimes needs to be pulled further, or pumped, before the tool responds.

How to fix it

A replacement trigger switch is cheap, but it’s tool-specific, so you need the right part for that exact model. If you’re confident opening the housing and matching wires back up correctly, fitting it yourself is a straightforward job. If you’re not sure about the wiring inside, it’s a quick job for a repair shop rather than something to guess at on site – a wrongly wired trigger is a safety issue, not just an inconvenience.

Frequently Asked Questions

Why does a power tool cut in and out?

Most intermittent power loss on a corded site tool comes down to one of five things: worn carbon brushes losing contact with the commutator, dust-clogged vents causing the motor to overheat, a nicked or crushed mains cable breaking the circuit, a tired battery on cordless tools, or a worn trigger switch. Working through them in that order – starting with the brushes on a brushed tool – finds the fault fastest.

How do you know when carbon brushes need replacing?

Watch for heavier sparking than usual at the motor vents, power that drops out and returns when you knock the tool, a hot or acrid smell, and power fading under load. Pull the brush caps and check the brushes themselves – once they’ve worn down to roughly 5-6mm, replace them as a matched pair.

Can you change carbon brushes yourself?

Yes. Unplug the tool, unscrew the brush caps, slide the old brushes out, fit a matching pair sized to the tool’s exact make, model and type number, seat them squarely, and run the tool off-load briefly to check they’ve bedded in. It’s a job most crews can do on site with a screwdriver and ten minutes.

What power tools do roofers use most?

Angle grinders and wall chasers see the heaviest daily use on most roofing jobs, alongside SDS drills, jigsaws and circular saws for cutting into decking and battens. Grinders and chasers, being run close to slate, tile and fibre-cement dust for long stretches, are also the tools most likely to show carbon brush wear and clogged vents first.