SiteIt's Monday morning. One of your three fitters has gone off sick, and the note says two weeks. The other two say they'll do 12s to cover. You thank them and move on.
Week one goes fine. The premium's welcome and the line keeps running.
Then the two weeks become six. In week four, a coupling guard goes back on with two bolts instead of four. A bearing change that should take an hour takes three, because nobody can find the puller and nobody has the energy to look. One of the two starts talking about a job down the road with no weekends.
Overtime is the fastest cover any site has. The trouble starts when it stops being a stopgap and quietly becomes the plan.
When overtime works
Overtime works when it's short, planned and has an end date.
A shutdown weekend. A fortnight of holiday cover booked months ahead. One person off for a week with a bad back. Your team knows the dates, can plan around them, and gets paid well for it. Most fitters are happy with that deal.
It also works when the people doing it are rested. Two weeks of extra shifts after a quiet month is a very different thing from two weeks after a peak.
When it breaks people
Overtime breaks people when it has no end date. Covering a vacancy is the classic case. The job's been open for four months, so the same two engineers have been doing 60-hour weeks for four months.
HSE's guide to shift work is blunt about it. Overtime, "especially when following long shifts", is likely to increase the risk of ill health, errors and accidents (HSG256). The guide says to limit shifts to a maximum of 12 hours, overtime included. Where you can, it says, avoid overtime by having relief staff to cover absence, vacancies and emergencies.
Then there's the quality of the work. A tired fitter is slower to find a fault, more likely to swap a part that wasn't faulty, and more likely to skip the check at the end. We've written about fatigue on long shifts as a safety risk. It's a breakdown risk too.
And the biggest cost comes later. The engineers who carry the overtime are usually your best ones. They're the people other sites want. Burn them out and you lose them, and then you're covering two vacancies instead of one. Our post on keeping good engineers goes into why they leave.
What the rules say
Your team only has to work overtime if their contract says so (GOV.UK, compulsory overtime). Even then, they can't usually be forced past an average of 48 hours a week*. The average is normally taken over 17 weeks* (GOV.UK, maximum working hours).
Anyone over 18 can opt out of the 48-hour limit. The opt-out must be voluntary and in writing, and nobody can be sacked or treated unfairly for refusing. They can cancel it with at least 7 days' notice*, or up to 3 months* if their agreement says so (GOV.UK, opting out). So an opt-out signed on someone's first day doesn't settle the matter for good.
Night workers are different. Regular overtime counts towards their average of 8 hours in 24*, and they can't opt out of that limit (GOV.UK, night working hours). If your cover plan puts a day fitter on extra nights, check it against that rule too. Our post on working time rules and maintenance overtime covers the detail.
Be fair about who gets it. GOV.UK says you can't discriminate by letting some employees work overtime while stopping others.
The rules are only the floor. HSE's fatigue guidance says the legal duty to manage fatigue is yours, "irrespective of any individual's willingness to work extra hours" (HSE, fatigue). An engineer who's keen for the money still needs you to say no sometimes.
Overtime looks cheaper than it is
On paper, time and a half for your own fitter beats paying for outside cover. Look at the full bill before you decide.
Count the faults that took longer because the person on them was on their sixth 12 in a row. Count the repeat failures from jobs that were rushed. Then count what each extra hour of a stopped line costs you. Our guide to the true cost of downtime shows how to work that out. Add the cost of replacing an engineer who leaves, and the overtime rarely looks cheap.
Overtime also hides the real problem. HSE says problems with overtime "may indicate inadequate resource allocation and staffing levels". If your overtime bill has been high for six months, you're short of an engineer, and the overtime is covering it up.
Our view
Use overtime for short gaps with an end date. For anything longer, bring in cover.
Here's a simple rule. If one absence will need more than two weeks of overtime, or the overtime is covering a vacancy, book outside cover on day one. Keep your own team's overtime for the jobs only they can do, like the machine with the quirks nobody else knows. A cover engineer can take the routine breakdowns and the PPM, so your people get their weekends back.
When you do book cover for longer than a few weeks, know what changes at week 12 for agency engineers. And if you've just lost someone to sickness, our post on one fitter down sets out the first steps.
What to do on Monday
- Pull each engineer's hours for the last 17 weeks. Check who is over 48 on average and whether they have a signed opt-out.
- Set a written limit on overtime and consecutive long shifts, and tell the team.
- Agree the trigger for outside cover: two weeks of overtime, or any vacancy.
- Ask the people doing the most overtime whether they want to keep doing it. Listen to the answer.
* The 48-hour average, 17-week reference period, opt-out notice periods and the 8-hour night limit correct as of 28 September 2026. Check GOV.UK on maximum weekly working hours and GOV.UK on night working hours for the latest.