Fitting intumescent strips: 4 steps that keep your fire door certified

You fit intumescent strips into grooves cut along the edges of a fire door or its frame, where they expand under heat to seal the gap and contain smoke and flames. Correct installation maintains the door's fire rating and ensures the seal activates when it matters. This guide walks you through the preparation, cutting, fixing and checks that make the job reliable, explains where the strips belong, and shows you how to recognise when replacement is due.

Understanding what intumescent strips do for your door

An intumescent strip is a length of material containing chemicals that swell when exposed to heat. In a fire, the strip expands to many times its cold width, filling the gap between door and frame to prevent smoke and flame passing through. Fire doors rely on this seal to achieve their rated performance, whether that is half an hour or a full hour of resistance.

The strip sits in a narrow groove, typically routed into the door edge or the frame rebate. Some strips combine intumescent material with a brush or flexible blade that provides a smoke seal even when the door is cold, giving you protection against fumes before the fire reaches the door itself. You choose a strip whose cold width and expansion rating match the door's certification, and you never mix components from different systems unless the manufacturer confirms compatibility.

Gathering the tools and materials you need

You will need a router with a suitable bit to cut the groove, a tape measure, a sharp knife, a hammer or staple gun, and the intumescent strip specified for your door's rating. The strip's packaging states the groove dimensions required: depth, width and the distance from the door edge. Write these down before you start.

Check the door's certification label or the manufacturer's data sheet to confirm which strip the door was tested with. Using a different product voids the rating, even when the dimensions appear similar. Where the original specification is lost, consult a fire door specialist rather than guessing. If you are working on Fred Duncombe guidance or similar resources, cross-reference the strip type against current standards.

Preparing the door and frame for installation

Remove the door from its hinges and lay it on a stable surface. Inspect the edges for old strip, adhesive residue or damage to the timber. Scrape away any remaining material and sand lightly to create a clean base. Measure the door's height and both vertical edges, then the top edge; you will cut separate lengths for each.

Mark the groove position using the distances given in the strip's instructions. Most grooves sit a set distance in from the door face, ensuring the strip aligns with the frame rebate when the door closes. Check both sides of the door match, and measure twice before routing. A misplaced groove cannot be easily corrected and may require a new door.

Following the four steps that secure the strip

1. Rout the groove. Set the router to the depth and width specified, and cut along the marked line in a single smooth pass. Keep the router base flat against the door edge and move steadily to avoid burning the timber. Clear the groove of dust and splinters with a vacuum and a brush.
2. Cut the strip to length. Measure each edge separately and cut the strip square using a sharp knife. Do not stretch or compress the material whilst cutting. For mitred corners, cut at forty-five degrees so the ends meet neatly without a gap.
3. Fix the strip into the groove. Press the strip into the groove, ensuring it sits flush with the surface and does not twist or buckle. Some strips have an adhesive backing; others require small pins or staples at intervals along the length. Space fixings as the manufacturer directs, avoiding over-compression that could prevent expansion.
4. Check the fit and seal. Rehang the door and close it gently. Inspect the gap around the perimeter: it should be consistent and narrow, with the strip just touching the frame or sitting a hair's breadth clear. Open and close the door several times to confirm it does not bind or catch on the strip.

Choosing where the strip sits: door edge or frame

Intumescent strip can be fitted to the door's closing edges or to the frame rebate, but not both in the same location. Most installations place the strip on the door, where it is easier to inspect and replace. Frame-mounted strip suits situations where multiple doors meet a single frame, or where the door edge is too narrow to accommodate a groove.

The door's certification will state which method was tested. Switching from door-mounted to frame-mounted strip, or vice versa, changes the door assembly and invalidates the rating unless a new test certificate covers the change. When in doubt, replicate the original configuration exactly.

Recognising when your strip needs replacement

Intumescent strip has no set lifespan, but it degrades through use and environmental exposure. Inspect the strip every few months for signs of damage: cracks, compression that does not recover, sections pulled free of the groove, or discolouration from moisture. A door that has been exposed to heat—even from a radiator or direct sunlight—may have partially activated the strip, reducing its effectiveness.

Replace the strip if you see any of these faults. Prise the old material out of the groove, clean the channel thoroughly, and fit a new length following the steps above. For guidance on identifying other compliance issues, see how to check your fire door's overall compliance.

Comparing strip types and their installation

Avoiding the mistakes that compromise the seal

Do not paint over intumescent strip. Paint forms a skin that can prevent expansion or delay activation, and it is not included in the door's fire test. If the strip has been painted, remove it and fit a new length. Similarly, avoid getting filler, varnish or sealant into the groove before fitting the strip.

Never compress the strip by over-tightening fixings or forcing it into an undersized groove. The material must be free to expand, and excessive compression can cause it to buckle or fracture under heat. Check that the groove dimensions match the strip's specification before you begin routing.

Where a door edge has been planed or trimmed, the groove position may no longer align with the frame. Measure carefully and rout a new groove if necessary, or consult a fire door specialist if you are uncertain. A misaligned strip leaves gaps that smoke and flame will exploit. For related maintenance, see how to fix a sticking fire door without affecting the seal.

Maintaining certification through correct practice

Every component of a fire door contributes to its rating, and changing one element can invalidate the whole assembly. Fit only the strip specified in the door's certification, and keep a record of the product used, including the manufacturer's name and the strip's model or reference. This documentation proves compliance if you are ever asked to demonstrate it.

When you replace a fire door or upgrade internal doors to a fire-rated specification, ensure the new door arrives with intumescent strip already fitted, or fit it before hanging the door. A fire door without its seal is not a fire door, regardless of the timber and ironmongery used. For broader compliance checks, see understanding FD30 and FD60 ratings.

#Intumescent strip #Fire door #Smoke seal #Installation

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