Measuring a Fire Exit Door: Four Checks That Save You a Refit

You measure for a fire exit door by recording the exact frame opening, the wall thickness, the floor-to-head height and the swing clearance before you order anything. Get any one of these wrong and a certified door can arrive and still fail to fit, or fit but lose its rating once the installer trims it to make it work. Fred Duncombe put this guide together so you can take accurate measurements once, order with confidence and avoid the most common fitting mistakes. Below you will find the order to measure in, what each figure actually affects, and a short checklist you can take to the opening with you.

Why measuring a fire exit door differs from an ordinary door

An ordinary door has some tolerance built in: a gap can be packed, a sticking edge can be planed down, and nobody checks the result against a standard. A fire exit door does not have that freedom. Its fire rating depends on the door, the frame, the intumescent strip and the gaps between them working together as a tested set, so trimming a factory-made leaf to fit a rough opening can quietly undo the certification it left the factory with.

That is why measuring comes first rather than being treated as a formality before the real work starts. Get the opening right and the door, frame and seals arrive as a matched unit that needs fitting, not reshaping. Get it wrong and you are choosing between a door that does not close properly and one that no longer performs the way it was tested to.

What changes if the opening is slightly out of true

Door openings settle over the years, especially in older buildings, so very few are perfectly square. A frame that is a few millimetres wider at the top than the bottom, or a floor that slopes slightly across the threshold, will throw off a single measurement taken in one spot. The practical answer is to measure in more than one place and use the smallest figure, which is covered in the steps below.

Taking the width measurement correctly

Measure the width of the opening at three heights: near the top, in the middle, and close to the floor. Use the smallest of the three figures as your working width, since a door sized to the widest point may not fit through the narrowest. Measure between the finished faces of the surrounding wall or existing frame, not between any trim or beading that will be removed before the new frame goes in.

If you are replacing an existing frame rather than fitting into a bare opening, measure the frame's outer dimensions as well as the clear opening inside it. Suppliers will usually ask for both figures, because the frame itself needs to seat correctly against the surrounding wall before the door is hung within it.

Taking the height measurement correctly

Measure height from the finished floor surface to the underside of the head, again at more than one point across the width of the opening. Account for any flooring that will go down after the door is fitted, such as tiling or flooring boards, since measuring to a subfloor that is later built up will leave the finished door too tight at the bottom.

• Measure close to each jamb and once in the centre
• Note the finished floor level, not the current bare one, if flooring is still to be laid
• Record whether the threshold will sit level or include a small rise or ramp
• Check the head of the opening for any downlighting or ductwork that reduces usable height

Checking the depth of the wall and frame

The wall thickness, or the depth of the existing frame if one is being kept, determines which frame profile will actually seat properly. A frame cut for a stud wall will not sit flush in a thicker masonry opening, and the reverse is just as true. Measure this at a few points along the opening, since wall linings and old plaster can vary the depth from one side to the other.

Whilst you are at it, check whether the wall either side of the opening is itself part of the fire separation, since this affects what kind of frame fixing is appropriate and whether the junction between frame and wall needs its own fire-stopping detail. This is one of those points where it helps to understand when an internal wall needs the same fire protection as the door within it, since the two are often decided together.

Allowing for clearance gaps around the door leaf

A fire door needs a small, specific gap around its edges, and this gap is part of what was tested when the door earned its rating. Too wide a gap lets smoke and flame past the seals before they can do their job; too tight a gap risks the door binding in its frame, particularly once intumescent strips begin to swell in the early stages of a fire. The clearance is not something you choose on site; it is specified by the door and frame manufacturer, and your opening measurements need to allow for it rather than fight against it.

Where a door needs to close against a smoke seal or a drop seal at the threshold, that gap allowance at the bottom edge is tighter still, since the seal has to make contact along its full length to work. If your exit route includes one of these, it is worth reading about how a drop seal protects the gap under a fire door before you finalise the threshold measurement, since it changes what "correct clearance" means at floor level.

Measuring for the door swing and approach

Beyond the opening itself, measure the clear space the door needs to swing through and the space in front of it on the escape route side. A fire exit door has to open fully without obstruction, and furniture, radiators or even a change in floor level can quietly eat into that arc without anyone noticing until the door is hung. If panic hardware is going on the door, allow extra thought for the handle height and the clear run beneath it, which is worth checking against guidance on when a panic bar is the right hardware for an exit route.

Recording the measurements you will actually need to order

Once you have taken every figure, write them down in one place rather than trusting memory or a single scrap of paper. A supplier or joiner will ask for the same core set of numbers, and having them ready in a clear format speeds up ordering and avoids a second site visit.

Steps to follow before you place an order

1. Measure the width at three heights and record the smallest figure
2. Measure the height at both jambs and the centre, to the finished floor level
3. Measure the wall or existing frame depth at several points
4. Check the clear swing arc on both sides of the door for obstructions
5. Note whether a threshold seal or drop seal will need its own clearance
6. Write every figure down together before contacting a supplier or joiner

Getting it checked before the door is cut

If there is any doubt over a measurement, have it checked before a door leaf is cut to size, since a certified fire door loses its rating the moment it is altered outside the manufacturer's instructions. A joiner experienced in fire door installation, or the supplier's own technical team, can usually confirm whether your figures and the intended frame will work together. In short: a second look at the numbers costs far less than a door that has to be replaced.

Start this week

You do not need every answer before you pick up a tape measure; you only need to start in the right order.

• Measure the opening width at three heights today and write down the smallest
• Check the finished floor level against the current bare floor before you measure height
• Walk the door's swing arc and clear away anything that sits within it
• Call a supplier with your full set of figures rather than a single estimate

This article is for general information only.

#Fire door

Download PDF: Fire Exit Door Measuring Checklist PDF, 29 KB