How to Measure a Fire Door Correctly for Replacement

Practical Guide #4

For: Joiners, site carpenters, surveyors, and contractors replacing fire doors in residential or commercial settings.

Why Getting the Measurements Right Is Critical

Replacing a fire door isn’t the same as swapping out a standard door. A fire door forms part of a tested system — and that means the new door leaf must be an accurate match to the frame, wall construction, and installation method.

Inaccurate measuring can lead to oversized gaps, invalid certification, and non-compliance under UK fire safety law.

This guide will help you get it right first time.

Step 1: Confirm What’s Being Replaced

Before you start measuring, confirm exactly what you're dealing with:

• Is it just the leaf that’s being replaced?

• Or will the entire doorset (leaf + frame) be renewed?

  
  

This guide focuses on leaf replacement only, which is more common in maintenance and retrofit scenarios.

Step 2: Measure the Door Leaf Itself

If the existing leaf is available and still in reasonable shape:

• Measure height (top to bottom) — in millimetres

• Measure width (side to side) — ideally at top, middle, and bottom to check for warping

• Measure thickness — FD30 doors are typically 44mm, FD60 doors are 54mm (use a digital caliper or measure from a clean edge)

  
  

Also note:

• Hinge positions (measure from top of leaf to centre of each hinge mortice)

• Lock or latch height

• Any vision panel sizes and positions

  
  

Record all these measurements — they’re vital when ordering a compatible replacement.

Step 3: Measure the Frame Opening

If the existing door is missing or unsalvageable, you'll need to measure the frame instead.

• Clear height: Measure from floor (finished floor level) to underside of head

• Clear width: Measure the opening width at top, middle, and bottom

• Rebate depth: If the frame is rebated, measure how deep the leaf will sit in

Pay attention to:

• Any twist or bow in the frame (use a straight edge or level)

• Uneven floor levels (if the gap under the door varies)

• Presence of a threshold or drop seal — you’ll need to account for this in undercut

Step 4: Check Fire Door Ratings and Compatibility

While measuring, confirm or record:

• The existing FD rating (FD30, FD60)

• The certification body (BM TRADA, Certifire, IFC)

• The frame type and condition

• Whether seals are in the frame or on the leaf

• The door handing (left or right hinge side)

This ensures you order a replacement that is like-for-like or better — and that the certification still applies.

Step 5: Record Everything Clearly

Use a standard fire door replacement survey form or simple spreadsheet. Suggested fields include:

• Door number / location

• FD rating

• Existing condition

• Leaf size (HxWxT)

• Hinge positions

• Lock / handle height

• Vision panel size/position (if applicable)

• Notes on frame, seals, thresholds, or issues

Common Pitfalls to Avoid

• Measuring to the frame stops instead of clear opening

• Guessing thickness without tools

• Not checking for a drop seal or threshold variation

• Assuming hinge positions are standard — they often aren’t

• Failing to note certification info from old label

Tip

If you’re unsure about tolerances or compatibility, speak to your fire door manufacturer before ordering. Many will accept photos and site measurements for assistance.

Let me know when you're ready for Guide #5:"How to Install an FD30 or FD60 Doorset" — it'll be slightly longer but packed with practical advice.