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Hot Works Best Practice

Hot works cause an estimated 15% of UK construction fires — and most are started not by the cut or the weld itself, but by the unwatched fifteen minutes after it.

By Watkins Academy Updated April 2026 7 min read

15% UK construction fires caused by hot works each year

60min Minimum fire watch after the last spark — non-negotiable

10m Standard exclusion zone to clear or shield combustibles

£25kavg Typical uninsured excess for a hot-works-caused site fire

Sources: Fire Protection Association, HSE, CICAIR data — figures are indicative UK construction-sector averages.

What counts as hot works?

If it sparks, burns, melts or heats material above ambient ignition temperature, it’s hot works and it needs a permit. The confusion usually happens at the edges — the “just a quick cut” or the “it’s only a heat gun”. Those are the jobs that end up in claims reports.

Welding & cutting

MIG, MMA, TIG, oxy-acetylene cutting. Creates molten metal spatter and sustained ignition temperatures in the thousands of degrees.

Grinding & disc-cutting

Angle grinders, disc cutters, abrasive wheels. Showers of hot sparks travelling 8–10m, easily lodging in clothing and debris.

Brazing & soldering

Copper pipework, plumbing, HVAC connections. Lower temperature than welding but still ample to ignite insulation or dust.

Blow-torches & heat guns

Paint stripping, shrink-wrap, pipe thawing. Concentrated flame or radiant heat that can ignite hidden timber or voids behind plaster.

Bitumen boilers & roofing

Felt-roofing torches and hot bitumen. Responsible for a disproportionate share of serious commercial fires — often ignite timber decks under new felt.

Thermic lances & exothermic cutting

Demolition, concrete cutting. Extreme ignition risk — molten particles travel well beyond standard 10m exclusion zones.

The permit-to-work process

A hot works permit isn’t paperwork for paperwork’s sake — it’s the discipline that makes sure every person involved has consciously signed off on the same set of controls, in sequence. Skip a step and the whole chain collapses.

1

Pre-work survey
Walk the area. Identify all combustibles within 10m — insulation, packaging, timber, dust, flammable liquids. Remove what you can. What you can’t remove, shield with fire blankets or non-combustible boards. Check voids, ducts and floor penetrations that sparks could fall into.
2

Issue the permit
Authorised signatory only. Names of operative and fire watcher, precise location, start and end time, specific control measures. No copy-paste permits from last week — every one is for this task, this area, this shift.
3

Sign-on
The operative signs to confirm they’ve read the permit, understand the controls, and have the PPE listed. The fire watcher signs to confirm they understand what ‘watching’ actually means and won’t leave the post.
4

Perform the work
Fire watcher in attendance, extinguisher (CO₂ or dry powder) within reach, water on standby. If any control fails — missing PPE, extinguisher moved, combustibles uncovered — work stops.
5

Sign-off & fire watch
Work ceases. Permit signed off. Fire watch continues for a minimum 60 minutes after the last spark. In high-risk environments (roofs, voids, warehouses full of combustibles) extend to 2–4 hours or longer.
6

Final check
Authorised person re-inspects at the 2-hour mark. Thermal imaging is worth its weight in gold here — confirms no smouldering in insulation, voids or debris before the site is left unattended.

The 60-minute rule: slag and metal particles can smoulder invisibly in insulation, dust or debris for hours before flashover. Cutting a permit short to end the shift has caused more major UK site fires than the ignition event itself.

The fire watcher — the most important job on site

Fire watcher isn’t a job for the apprentice, the nearest spare pair of hands, or someone who can “keep an eye on it while you grab a brew.” It’s a focused role with specific competencies.

The fire watcher must…

Remain in the immediate work area throughout the task and the full 60-minute post-work watch.
Have clear line of sight to the operative and the opposite side of any wall being cut or welded.
Have an extinguisher in hand (CO₂ or dry powder) and know how to use it.
Carry a means of raising the alarm — radio, phone, hooter.
Stop work immediately if any control measure fails or a new hazard appears.
Not be assigned any other task — no lifting, no radio calls, no fetching tools. Watching is the whole job.

Before / during / after — the site check

The permit makes the plan. The check makes sure it happened.

Before you strike the arc

Permit signed, dated and visibly displayed
Combustibles cleared or shielded within 10m
Voids, ducts and penetrations checked and sealed
CO₂ or dry-powder extinguisher within arm’s reach
Fire watcher briefed, in position, nothing else to do
Water / damping available for adjacent material
PPE matches the task (visor, gauntlets, fire-retardant overalls)

Stop work — immediately — if…

Wind is carrying sparks outside the protected zone
A void or cavity is discovered mid-task
The fire watcher is called away, for any reason
A fire alarm sounds anywhere on site
Permit duration has been exceeded
Combustible material has shifted into the exclusion zone
You smell smoke, char or hot insulation

What a hot works permit must contain

There’s no legally-mandated template — but insurers, the FPA and HSE all point to the same minimum set of fields. If your current permit is missing any of these, it’s not fit for purpose.

Work description & exact location. Not “north wing” — the specific room, floor or plot.

Operative and fire watcher named. Both signed on before work starts.

Start and end times. Permit is dead at end-time — no grace period.

Authorised signatory. Named person who has walked the area and confirmed the controls.

Identified combustibles. What’s been removed, what’s been shielded, what remains.

Control measures. Extinguisher type and location, fire blankets, PPE list, water source.

Post-work fire-watch duration. Minimum 60 minutes — extended where relevant.

Final inspection sign-off. Separate space for the 2-hour post-work check.

The three failures behind most hot-works fires

Walk through any major insurance-claim report on hot-works fires and you’ll find the same three root causes over and over.

The 60-minute shortcut

Fire watch cut short to finish the shift. Slag embedded in insulation smoulders for hours before flashover — the single biggest cause of hot-works fires in the UK.

No-one watching

Fire watcher called away “for two minutes” to help lift, collect a tool, or answer a call. Work continues. This is the scenario every insurance claim report describes.

Hidden voids

Sparks find cavities you didn’t know existed — floor voids, cable risers, false ceilings. If you can’t see behind a wall you’re cutting near, assume there’s combustible material there.

What site leaders can lock in this week

If you’re the SHEQ lead, supervisor or site manager, these are the moves that immediately lower your hot-works risk — without waiting for the next audit cycle.

Standardise your permit template. One permit across all sites and all subcontractors. No “their version” vs “your version” — one pack, always the same fields.
Publish a Hot Works exclusion list. Areas where no hot works are allowed under any circumstance (plant rooms, roof voids, loaded racking, storage yards). Get signage up.
Ring-fence the fire watcher. The watcher is off all other duties for the full task plus 60 minutes. Make that explicit in your permit sign-on.
Invest in a thermal imager. One camera per site. Mandatory on all 2-hour final checks where insulation, voids or concealed cavities are in the work area.
Train every authorising signatory. Not just “the SHEQ manager” — anyone who might sign a permit needs formal training. Our e-learning covers exactly this.

Hot Work Training (online)

15-minute certified course with end-of-course assessment and digital certificate — for operatives, watchers and authorising signatories. Hub readers get 10% off — use code HUBHOT10 at checkout.

View the course