How to Identify a Fire Door on Site

For: Joiners, caretakers, inspectors, apprentices, fitters, and building managers.

Why This Guide Matters

Not every fire door is clearly marked. On live sites, in older buildings, or during refurb works, you’ll often come across doors without labels or paperwork. Being able to identify a fire door through its physical characteristics is a valuable skill — especially when paperwork is missing or unreliable.

This guide outlines what to look for.

Common Indicators of a Fire Door

1. Door Thickness

Most fire doors are 44mm thick (FD30) or 54mm (FD60). Standard internal doors are usually 35mm. A quick measurement with a ruler or digital caliper can help you spot the difference.

2. Weight and Feel

Fire doors are heavy. They’re made from solid cores — either solid timber, composite, or particleboard. If the door feels unusually light when opened or removed, it’s unlikely to be a compliant fire door.

3. Intumescent Seals

Look for heat-activated seals around the edge of the door or the frame. These may be visible as a slim groove housing a rubbery strip, typically 10–15mm wide. Sometimes painted over, but usually still visible on close inspection.

4. Door Closers

Fire doors are required to close automatically. This can be via an overhead closer, concealed jamb closer, or floor spring. If there’s no closer, or it’s visibly damaged, it’s a red flag.

5. Hinge Count and Quality

You should see a minimum of three hinges. These must be steel, marked with the CE or UKCA symbol, and firmly secured. Lightweight brass hinges or anything loose suggests it’s not a fire door.

6. Edge Construction

Check the top or side edges of the door. Fire doors often show signs of a laminated or solid hardwood core. If the paint is chipped or the veneer is peeling, you might get a look at the internal construction.

7. The Frame

A proper fire door frame is typically made from hardwood and may be rebated. Gaps between door and frame should be consistently 2–4mm. Large gaps or softwood, non-rebated frames suggest standard joinery.

8. Hardware Upgrades

Look for signs of fire-rated ironmongery: intumescent pads behind hinge blades, fire-rated latches, or metal push plates. These are often subtle clues that you're looking at a door with fire protection intent.

What Suggests It’s Not a Fire Door

• Hollow core (drum-like sound when knocked)

• Less than 3 hinges, or visible brass/cheap hinges

• No visible intumescent strips

• Missing or broken door closer

• Gaps around frame larger than 4mm

• Decorative glass without fire-rated markings

• Lightweight frame or poor fixings

• Door edge thinner than 40mm

Practical Tip

When in doubt, document what you see: take measurements, photos, and notes. Then pass the information to a certified fire door inspector or responsible person for a formal assessment.

Optional Tools to Carry

• Digital callipers (for measuring thickness)

• Magnet (some fire doors have steel cores)

• Torch or inspection light

• Gap gauge or feeler gauges

• Screwdriver (to check hinge fixings if safe and appropriate)

• Small step ladder to check to of door for manufacturers labels