Who this guide is for
- Owners adding insulation, heating or a new covering to an existing floor
- Anyone whose refurbishment spans several rooms with different floor build-ups
- People planning a wet room, where a graded former adds depth at one end
- Readers who have been told a door will need trimming and want to know what else changes
- Anyone coordinating floor layers between a heating installer, a screeder and a floor layer
Why the sum is never done
Each layer is proposed by a different trade at a different moment, and each of them is individually small. The levelling layer is discussed with the screeder, the heating board with the heating installer, the underlay with the floor layer and the covering with the owner. The total appears for the first time when a door will not close.
Nothing in the ordinary sequence forces anyone to add them up, because no single trade owns the finished level. Somebody has to be given that job explicitly, and the natural moment is before the base is specified rather than after the covering is chosen.
- A levelling or smoothing layer, however thin, is still a rise
- Insulation, a heating board or an isolated deck each add their own depth
- An underlay or lining sheet adds a little more
- The covering itself, and its bedding where it is bonded
- Any graded former in a wet area, which is deepest at one end
The fixed points the floor has to meet
A door leaf was made to an opening and hung to a floor. Raising the floor changes the undercut, and where a door has to be trimmed there is a limit to how much can come off before the leaf stops being the door that was bought. Fire and smoke arrangements attached to a door set are a matter for the responsible professionals and the relevant authority.
A stair is less forgiving still. Treads are set out cumulatively up a flight, so a floor raised at the bottom or the top changes one riser and leaves it different from all its neighbours. A tread that differs from its neighbours is experienced as an irregularity rather than as a step, and stair geometry is a matter for the designer.
Thresholds are a design problem, not a finishing one
The threshold is the bottom of an external door opening and it is walked over, which means everything asked of it has to survive traffic at or near floor level. It is also where several requirements genuinely conflict: keeping water out favours an upstand, level access favours no step, and the insulation line has to continue through a position that is structural and trafficked.
That is why raising an internal floor at an external door is not a trim decision. A level threshold depends on external drainage being built correctly, and the two are one design problem rather than two. Threshold leaks are frequently drainage failures outside the door rather than seal failures at it.
Where two rooms meet, something has to absorb the difference
A doorway between two rooms with different build-ups is where the difference becomes visible. A transition profile resolves it: a flat cover strip, a T section where two coverings are of similar thickness, or a ramped section where one is thicker. Where the difference in level is greater than a trim can reasonably close, the trim is the wrong answer.
The doorway is also where a floating floor's free perimeter has to be interrupted, and pinching that movement gap at the opening is a common way of making a floor buckle somewhere else. A T section in the opening is what lets the gap continue while still giving a finished edge.
The sum has to be done before the base is specified
Almost every product in the build-up has a range of depths, and the choice between them is only free while nothing has been ordered. Once the base is down, the remaining layers have to fit whatever height is left, and the compromise usually lands on the covering or on the door.
The practical sequence is to establish the fixed points first — door leaves, stair geometry, external thresholds, adjoining floors and any appliance opening — then work back to what depth is available, and only then choose the layers. That is the reverse of how it usually happens.
What a change of level does to everything else
Raising a floor moves more than the surface. Skirting heights change, architraves may need easing, radiator pipe tails and fitted furniture plinths sit differently, and an appliance opening under a worktop can become too shallow. In a kitchen a covering laid under units cannot be lifted later, so the decision about what runs under what is taken once.
The same rise reaches the stairs at both ends. A floor raised on a landing changes the top riser; a floor raised at the foot changes the bottom one. Neither is a matter for the floor layer to resolve on the day.
Who decides what
Stair geometry, and whether an altered riser is acceptable, is a matter for the designer and the relevant authority. Level access and threshold drainage are design decisions taken by a qualified professional. Whether the structure can carry the added weight of a build-up is a structural determination. Adding the depths up, however, is nobody's specialism and has to be assigned to someone by name.
Before any layer is added to a floor
- 1Nominate one person to keep the running total of the build-up
- 2Write down the existing finished floor level and where it is measured from
- 3Record every door leaf that opens over this floor and its present undercut
- 4Check the stair at both ends of the floor, top riser and bottom riser
- 5Identify any external door and establish whether level access is required
- 6Ask who has designed the drainage immediately outside that door
- 7Note the floor level of every adjoining room the floor opens into
- 8Check appliance openings, plinths and any fitted furniture affected by a rise
- 9Confirm whether a graded former or a fall changes depth across the room
- 10Ask whether the structure can carry the weight of the proposed build-up
- 11Decide what runs under fitted units, because that cannot be revisited
- 12Settle the transition detail at each doorway before the coverings are ordered
- 13Keep the free perimeter of any floating floor continuous through openings
Common mistakes to avoid
- Letting four trades each add a small layer with nobody holding the total
- Choosing the covering first and leaving the base to fit whatever depth remains
- Treating a door that will not clear as a trimming job rather than a level problem
- Forgetting the stair, where one altered riser differs from all its neighbours
- Raising a floor at an external door without revisiting the drainage outside it
- Using a transition strip to close a difference larger than a trim can reasonably take
- Pinching a floating floor's movement gap at a doorway and finding the buckle elsewhere
When to involve a professional
- Stair geometry, and whether an altered riser is acceptable in this building, is determined by the designer and the relevant authority
- Level access and the drainage immediately outside a threshold are designed together by a qualified professional
- Whether the structure can carry the weight of a proposed floor build-up is a structural determination
- Fire and smoke arrangements attached to a door set are matters for the responsible professionals and the relevant authority
- Where a trim is fixed into a floor containing a membrane, a heating layer or services, the fixing method is confirmed before drilling
Frequently asked questions
Questions readers ask about this topic
Can a door just be trimmed to clear a new floor?
Sometimes, and there is a limit before the leaf stops being the door that was bought. It is also only part of the answer, because the same rise reaches the stairs, the thresholds and the adjoining rooms. Trimming a door treats a symptom of an arithmetic problem.
Why does raising a floor affect the stairs?
Treads are set out cumulatively up a flight, so raising the floor at the top or the bottom changes one riser and leaves it different from all its neighbours. A step that differs from its neighbours is experienced as an irregularity, and stair geometry belongs to the designer.
When should the build-up be added up?
Before the base is specified. While nothing has been ordered the depths are still a choice; afterwards the remaining layers have to fit whatever is left, and the compromise usually lands on the covering or the door rather than on the layer that caused it.
What happens where two rooms have different floor heights?
Something has to absorb the difference at the doorway: a flat cover strip, a T section, or a ramped section where one covering is thicker. Where the difference is greater than a trim can reasonably close, the trim is the wrong answer and the levels need revisiting.
Is raising a floor at an external door a problem?
It changes how water has to be managed outside the door, which makes it a design question rather than a finishing one. Keeping water out favours an upstand and level access removes that option, so the threshold and the external drainage are resolved together.
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