How much clearance under an internal door is enough?

How much clearance under an internal door do you need?

There is no single gap that suits every internal door. The useful measurement is the clear space between the bottom of the door and the finished floor, including carpet or another floor covering. You also need enough room for the door to swing without catching. Whether the door provides an air path or forms part of a fire-resisting assembly changes what you should check before adjusting it.

For homes in England, Approved Document F gives guidance on ventilation, including air transfer beneath internal doors. Its guidance describes a 10 mm undercut above a finished floor for relevant internal doors, with an allowance when the floor finish has not yet been fitted. Treat that as a ventilation check in its stated context, not as a universal trimming instruction. Building standards differ across the UK, so check the requirements for your nation and project.

Measure the existing gap at the right places

Close the door gently and measure vertically from the finished floor to the door's bottom edge in millimetres. Check near the hinge side, at the centre and near the latch side; a sloping floor or door can give different readings. A ruler or a stack of known-thickness spacers is easier to read than an angled tape measure. Record the smallest gap, the floor covering present and where each measurement was taken.

Next, open the door slowly through its full swing and watch the bottom edge. Check the route over carpet pile, doorway transitions and any raised part of the floor, not just the position beneath the closed door. Note where it rubs or comes closest, because that may be the limiting point for everyday use. If the door has recently begun catching, also check for loose hinges before assuming the bottom needs cutting.

Allow for the finished flooring

If flooring is about to change, measure the complete proposed build-up above the current surface. Include underlay, boards, adhesive or a threshold component where each will sit in the door's path. Ask the installer for the finished height rather than relying on a product's nominal thickness alone. A carpet sample can help you assess pile, but the fitted height is what matters for the final check.

Subtract the rise in finished floor height from the current clearance at the same point. For example, a floor that rises by 6 mm reduces a measured 14 mm gap to an estimated 8 mm gap. Repeat the calculation for the lowest point across the swing, since a single closed-door reading can miss a higher patch of floor. Keep the estimate separate from the final measurement: installation tolerances may change the result.

Check ventilation before deciding on a gap

Some internal doors form part of the route by which air moves through a home towards extract ventilation. In England, Approved Document F sets out guidance for air transfer beneath internal doors; the finished floor level matters when checking the undercut. A thick new carpet can reduce an existing air path even if the door still opens freely. Check the applicable guidance for your work rather than judging the gap by appearance alone.

First establish whether the door is intended to provide an air path, then compare its finished-floor clearance with the relevant specification. Look for a ventilation design or ask the installer if you are changing floors as part of a wider refurbishment. Do not assume that adding a draught strip is harmless: it can close an air path that the ventilation arrangement relies on. Equally, a larger gap is not automatically a better result if the door has another specified function.

Identify doors that need separate advice

Before trimming, check whether the door is a fire door or part of a specified door set. A label, certification mark, closer, smoke seal or intumescent strip may be a clue, but absence of a visible label does not prove it is an ordinary door. A fire door's permitted gaps and alterations depend on its tested assembly and installation instructions. Ask the manufacturer, installer or a competent fire-door professional to assess it instead of applying a general internal-door measurement.

Also check doors with drop seals, acoustic seals, raised thresholds or manufacturer-specified clearances. These components may need adjustment or replacement rather than a change to the door leaf. If you rent or live in a flat with managed common areas, establish who is responsible for the door before authorising work. Record any identification marks and instructions so the person carrying out the work can check the correct assembly.

Choose and confirm the adjustment

Use the measurements to state the problem precisely: the door catches at a particular point, the proposed floor reduces the gap, or a required air path needs checking. Have the installer inspect hinge alignment and the finished floor before discussing trimming. If trimming is appropriate, check the door maker's limits, especially for hollow-core or engineered doors with restricted cutting allowances. Agree the intended finished clearance in millimetres and identify who will verify it after the flooring is fitted.

Measure again only when the new floor, underlay and threshold are in their final positions. Test the full swing several times and check the gap at the same marked points used for the initial measurements. Confirm separately that any specified ventilation route, seal or door-set requirement still works as intended. Keep the before-and-after readings with the flooring details for later maintenance.

Final verification checklist

A useful result records more than one number. Write down the smallest closed-door gap, the closest point during the swing and the finished flooring used for each check. If a planned floor has not yet been installed, label the calculated gap as an estimate. This makes it clear what remains to be verified on site.

Recheck the door after normal use has settled the floor covering, particularly if carpet pile was involved. If it rubs, a seal no longer closes or a ventilation gap has changed, ask the installer to assess the cause. Do not repeatedly trim a door without checking its construction and purpose. Keep any fire-door or ventilation instructions alongside your measurements.

• Measure in mm above the final floor finish at the hinge side, centre and latch side.
• Test the entire swing and note the point with the least clearance.
• Confirm whether ventilation guidance or a door-set specification applies.
• Record the final readings and resolve rubbing or seal problems before signing off.

Scope and limitations

This note helps householders organise measurements and understand the calculation. It does not replace an on-site decision by a designer, installer, contractor, or manufacturer. Fabrication, structure, electrical work, gas, waterproofing, and personal safety require suitably qualified verification.