Floor milling lets you fit water-based underfloor heating without breaking out the floor. A contractor cuts channels into the existing screed, lays the pipe in them and fills them in. Whether it works in your home depends on one thing above all: the screed you already have.
This guide explains what milling involves, which floors suit it, what a quote should cover and how it compares with dry and low-profile overlay systems. It is about water-based underfloor heating, not electric mats. Specialists also call the method "in-cut", "chase-cut" or "routing" underfloor heating.
If you have not chosen a method yet, start with retrofitting water-based underfloor heating: what to measure first. Its worksheet for floor levels and layers is what a milling decision rests on too.
What milling involves
This is how the UK and Irish milling specialists whose pages we read describe the job. Details vary from firm to firm.
- Checking the floor. The contractor assesses the screed, from photos and drawings or on site, before quoting.
- Cutting the channels. A milling machine cuts channels in the pattern of the loops. An attached vacuum takes the dust.
- Laying the pipe. The pipe goes into the channels and is connected to the manifold.
- Pressure testing. The pipework is tested under pressure before it is covered.
- Filling the channels. The channels are filled, for example with a cementitious latex compound, back to the floor level.
- Floor finish. Tiles, engineered wood, vinyl or carpet suitable for underfloor heating go on top.
The specialists say milling usually takes one or two days per room. Allow time for the filler to cure and for the floor finish to be laid to its manufacturer's instructions.
Every loop has to reach the manifold, so channels also run through the hall and across door thresholds. Where the manifold goes changes how long those runs are; see where to put an underfloor heating manifold.
When milling works: the screed, its depth and its condition
Milling depends entirely on the existing screed. The table below sums up what the specialists we read ask for. These are their requirements, not a standard, and your contractor may set different ones.
| What to check | What the specialists say | What to ask |
|---|---|---|
| Type of floor | Concrete and sand and cement screed; some also mill anhydrite screed. Not suspended timber | What your floor is made of, and whether that comes from drawings or from opening it up |
| Depth | A UK guide to the method and one firm give roughly 75 to 100 mm of slab and screed combined; another firm asks for at least 35 mm of screed | What depth the contractor measured and what they need for their channels |
| Condition | Not weak, damp, contaminated or structurally compromised | What they will do if the screed turns out weak once they start |
| Channels and pipe | One firm cuts channels about 16 to 17 mm wide and 20 mm deep for 16 mm pipe | Which pipe, what spacing and what depth of channel for your floor |
| What is below | Rarely addressed on their pages | What is under the screed, and what they propose if there is no insulation |
Do not take depths and conditions on trust. Ask for the floor to be opened up or test-cut in one place before you sign, and note the result in the measurement worksheet in the retrofit guide.
What is under the screed: insulation, services and asbestos
Milling does not change what lies under the screed. If there is no insulation, more of the heat goes down, into the ground or the room below, instead of into the room. In England, Approved Document L asks for new underfloor heating to have insulation that limits downward heat loss: on ground floors and floors exposed to the outside, over unheated rooms, and between storeys (Approved Document L, Volume 1, paragraphs 6.28 to 6.31). Ask your installer what is under your floor and how that guidance applies to a milled retrofit. In Wales, Scotland, Northern Ireland and Ireland, ask which local rules apply.
Water pipes, waste pipes and cables may run through the screed. Find out where they are before any cutting, from drawings or with the contractor.
In a flat, the screed usually sits on the building's structural floor. The channels must stay in the screed, so check the lease or building rules and ask the managing agent before any cutting.
In older buildings, floor tiles, their adhesive and some screeds can contain asbestos. HSE lists PVC and thermoplastic floor tiles and screed materials among the places it was used (HSE, locations of asbestos), and in Ireland the HSA says it can be in any building built or refurbished before 2004 (HSA asbestos FAQs). Do not mill a floor until you know what it is made of, and leave any removal to a specialist.
If the screed shows damp or cracks, or the floor moves, have it assessed before you choose a method. Neither this guide nor HeatAlgo can assess the condition of a floor.
What floor milling costs
We found no UK or Irish milling specialist that publishes a price per square metre. The specialists quote per project, after a survey or from photos and drawings; the one online estimator we found, on a Devon installer's site, covers jobs of up to two rooms and is a guide only. One UK guide to the method reports the same: none of the providers it researched publishes a standard rate (underfloorheating.info, updated 21 August 2026). General cost guides give figures for wet underfloor heating retrofits, but they do not describe milling, so we do not repeat them here.
A figure per square metre also tells you little on its own. Ask for a quote that lists:
- the milling, dust extraction and clearing up,
- the pipe and laying it,
- the manifold, its cabinet and the connection to your heat source, and a mixing unit if your system runs hotter than the floor needs,
- filling the channels and levelling,
- the pressure test and commissioning,
- lifting the old floor finish and laying the new one,
- VAT, and travel if it is charged separately.
To compare the scope of two quotes, use the worksheet in the underfloor heating design guide.
Milling, a dry system or a low-profile overlay?
Milling is one of three ways to avoid laying a new, deep screed. Each suits a different floor. Compare them on the same measurements and with the same floor finish.
| Method | Where the pipe sits | What it needs | Insulation under the pipe | Can HeatAlgo calculate it |
|---|---|---|---|---|
| Milling | In channels cut into the existing screed | A sound screed of enough depth; no new heating layer | Whatever is already under the screed | No, you need the contractor's or pipe manufacturer's data |
| Dry system | In grooved insulation boards with metal spreader plates, under dry boards | Height for the whole board build-up and finish; no wet screed | Its own insulation board | No, you need the system manufacturer's data |
| Low-profile overlay | In a thin layer laid on top of the existing floor | Height for that layer and the finish, from the manufacturer's data | Depends on the system | No, you need the system manufacturer's data |
Milling has the edge where the screed is deep and sound and the floor cannot rise, for example because of doors and thresholds. A dry system or an overlay is the more usual route where the screed is too thin or weak and the floor can go up a little. The retrofit guide's worksheet shows how much height you really have, and each method's contractor or manufacturer supplies its output for your floor finish.
What HeatAlgo can calculate for a milled floor
Before you pay for milling, find out how much heat each room needs. In an older home the floor cannot always heat a room on its own, because the output per square metre has a ceiling. In HeatAlgo you can work out room-by-room heat loss to EN 12831-1. A HeatAlgo report is not an MCS heat load calculation. Divide a room's heat loss by the floor area that can take pipes, and you know how many watts each square metre of floor has to deliver. That is the figure to put to the milling contractor.
HeatAlgo cannot calculate the milled floor itself. It calculates the output of floors with the pipe embedded in a cement screed laid over insulation (type A in EN 1264), with 30 to 100 mm of screed over the pipe and a 16×2 or 17×2 pipe. In a milled floor there is a much thinner layer over the pipe, and old screed, with whatever lies beneath it, under the pipe. The app has no way to describe that build-up.
You can still draw loops on your floor plan in the underfloor heating module, but the pipe spacing, output and flow temperature in the report are calculated for a pipe in screed, not for channels in an old screed. Do not treat them as a design for a milled floor. Ask the contractor or the pipe manufacturer for output data for a milled floor with your finish. If they have none, ask before you sign who will work out the floor's output, and on what basis.
Questions for the contractor before you sign
- How deep is my screed, and what condition is it in, from an opening-up or a test cut?
- What is under the screed, and what do you propose if there is no insulation?
- Which pipe, what spacing and what channel depth, and what will fill the channels?
- Is the screed separated from the walls by an edge strip, and does it need movement joints at doorways once it is heated?
- What output will this floor give with my floor finish at the water temperature my heat source supplies, and how do you know?
- What does the price include: milling, pipe, manifold, connection, filling, levelling, pressure test, travel? Is VAT included?
- Who carries out and records the pressure test before the pipes are covered?
- What will you do if the old floor has vinyl or thermoplastic tiles or adhesive that could contain asbestos?
- When can the floor finish go down, and who commissions the system?
For everything worth gathering before you ask for quotes, see the underfloor heating planning checklist.
Start with the heat loss
Milling only makes sense if the floor can give each room as much heat as it loses. HeatAlgo works that out room by room and gives you a floor plan with the figures to take to a contractor. If you choose a new screed with the pipes embedded in it instead, you can also design the loops in the app and get a report with the spacing, loop lengths and flow temperature.
See how to prepare a plan and report