Laying system boards for underfloor heating

A system board does three jobs at once. It holds the pipes at a precise spacing so the floor warms evenly. It provides thermal insulation directly under the pipes, so the heat goes into the screed. Its top film, with overlaps and interlocking edges, also stops wet screed from pushing into the joints and forming cold bridges under the pipes.

The most common type is the studded board, often called a castellated or knob board. It is made of rigid expanded polystyrene with raised studs, and the pipe is simply pressed in between them with your foot. The studs sit on a regular grid, usually at 50 mm, so the heating installer can lay pipes at 100, 150 or 200 mm centres without measuring. The top is usually laminated with a tough plastic film that protects the polystyrene from foot traffic and from moisture in the screed.

Why not just lay the pipes on ordinary insulation board? You can, but the pipes then have to be fixed with clips or rails, and they tend to lift when the screed is poured. A system board simplifies this work and gives a more predictable result.

Studded board with integrated insulation

The standard choice for new builds and larger renovations. Under the studs there is a layer of insulation, roughly 10 to 30 mm according to the manufacturer. It goes on top of a further insulation layer that provides the main thermal protection of the floor. The advantages are fast laying and firm pipe retention. When choosing, check the pipe diameter the board is designed for and its compressive strength. In living rooms the standard strength from the manufacturer is usually enough. Under a heavy kitchen run, a wood-burning or tiled stove, or in a garage, you need a stronger grade.

Studded film without insulation

A thin moulded film with studs that is laid over insulation already in place. It is used where the designer wants the insulation to come from a different board, for example one with higher compressive strength or better thermal performance. Laying takes one step longer because insulation and film go down separately, and the film must lie flat without waves.

Flat tacker board for clip systems

A board with a laminated film and a printed grid, onto which the pipes are fixed with plastic staples using a hand tool called a tacker. It is more flexible in irregularly shaped rooms and under sloping walls, and the pipe can run off the grid where needed. On the other hand, only the staple holds the pipe, and careless screeding can pull it loose.

Low-profile boards for renovations

When an older flat is refurbished, there is often not enough height for the full floor build-up. Thin boards exist for smaller-diameter pipes, finished with a thin self-levelling compound instead of a thick screed. They have little or no insulation, so they suit floors above heated rooms. Over an unheated cellar or on a ground-bearing floor they are not suitable without additional insulation.

If several variants appear on the same price-list page, the difference usually lies in the board type, the insulation thickness, and whether the item also covers the lower insulation layer and the edge strip. Before ordering, clarify exactly what the item includes.

Most time is lost on sites where the base is not ready. Before the crew starts, these things should be in place:

• A flat, clean base. A concrete slab or floor structure without humps, mortar lumps or rubble. Deviations should stay within the tolerance the board manufacturer gives, usually just a few millimetres under a 2 m straightedge. Larger unevenness must be levelled first.
• Damp-proofing on ground floors. On a ground-bearing floor the damp-proof membrane must be complete and undamaged. The system board does not replace it.
• Services in the floor. Cables, water and drainage pipes running in the floor must be finished and fixed. Ideally they sit within the lower insulation layer, cut out around them, or in a levelling fill.
• Plastering and windows. Internal plastering should be done so the boards do not get dirty and the edge strip has a finished wall to sit against.
• A floor level plan. You need to know where the finished floor level will be and how thick the floor covering and screed will be. The insulation thickness follows from that.
• Manifold position. The manifold cabinet should be fitted so the board can be cut neatly around it.

If you are working in a flat, inform the building management or owners' association, especially if the underfloor heating is meant to connect to the building's communal heating. This is often not allowed, or needs consent and a technical assessment. Whether changes to the floor and heating need a permit or notification depends on local building regulations, so check with your local building authority. Do not simply assume that nothing is needed.

Perimeter edge strip

The first step is a foam edge insulation strip, stuck around the whole perimeter of the room and around columns, door frames and pipe penetrations. It is usually about 8 to 10 mm thick and tall enough to stand above the future screed. It lets the screed expand as it heats up, and it separates the floor from the walls, which also helps reduce impact sound. The strip normally has a film skirt that is later laid over the system board.

Lower insulation

Where the design calls for it, a lower insulation layer goes down first, either thermal or acoustic. The boards are laid in a staggered bond, with offset joints, tightly butted together. With several layers, the joints of one layer must not line up with those of the next. Around services the insulation is cut out, or the gaps between them are filled with a levelling fill, so the system board is supported over its whole area.

Laying the system boards

Boards are laid from one corner of the room, usually from the manifold wall or the longest straight wall. They join with film overlaps or interlocking edges so the surface forms a continuous area without gaps. The offcut from the end of one row often starts the next, which saves material and staggers the joints at the same time.

Boards are cut with a sharp knife or a fine-toothed saw. Cuts at door frames, pipes and corners need to be precise, because every gap is a place where screed can run under the board. Small gaps are filled with offcuts or taped according to the manufacturer's instructions.

Edge strip skirt and movement joints

The film skirt of the edge strip is laid onto the boards and taped down, so screed cannot get between the strip and the board. Movement joint profiles go in doorways and wherever the design divides the floor into separate bays. A bay should usually be neither too large nor too long and narrow. A common guideline is roughly up to 40 m² with no side longer than about 8 m, but the exact figures come from the design or the screed manufacturer.

Handing over to the heating installer

The heating installer then lays the pipes on the finished surface, carries out a pressure test, and the floor is ready for screed. If no work is happening for a while, protect the boards from foot traffic, especially from people carrying heavy loads and from wheelbarrows pushed across without boards laid down as a runway.

The amount of work on site varies a lot with conditions. Things go quickly and smoothly where:

• Rooms are large and rectangular, with few corners and penetrations
• The base is flat and clean and needs no levelling
• The material is on site, kept dry and easy to reach
• Services in the floor are finished and their routes are clear

The job takes longer with:

• Small rooms, narrow hallways and bathrooms with lots of cuts
• Curved or angled walls, columns, staircases and alcoves
• Many pipe and cable conduit penetrations through the floor
• An uneven base that needs levelling fill or a smoothing compound
• Multi-layer insulation combining thermal and acoustic layers
• Poor access, for example carrying material up several floors without a lift
• Work in a partly occupied home, where other rooms need protecting and daily clean-up

Laying the system board sits in the middle of a chain. Before it come the builder, electrician, plumber and whoever installs the damp-proofing. After it come the heating installer with the pipes, the screed contractor and finally the floor layer. The most important coordination point is heights. If the insulation thickness is chosen without regard to the screed and floor finish, you end up with doors that are too low, awkward thresholds or a step between rooms.

The screed cover over the pipe depends on the screed type. A sand-cement screed usually needs a thicker cover, roughly around 45 mm, while a calcium sulphate (anhydrite) flowing screed can often be thinner. The instructions of the screed manufacturer and the design always take priority. With a liquid flowing screed it is especially important that all film joints are sealed, otherwise the liquid gets under the boards and can even lift them.

The laying itself does not take long. In a house with a normal layout, an experienced crew usually manages it in one to two days, longer in rooms with many angles and penetrations. What takes longer is what follows. The screed needs to dry, a cement screed usually for several weeks, and before the floor covering goes down there is a heating-up procedure following a set protocol. Laying the board faster does not shorten any of these periods.

Most problems only appear once the floor is finished, when they are hard to fix.

• Uneven base. The board rocks, flexes underfoot and the screed above it varies in thickness. This shows up as cracks and a floor that warms unevenly.
• Missing or interrupted edge strip. The screed pushes against the wall as it heats, cracks, or carries noise into neighbouring rooms. In blocks of flats this is a frequent cause of complaints about footstep noise.
• Gaps between boards. Screed runs under the board and forms a cold bridge. Heat escapes downwards and the floor warms more slowly.
• Wrong insulation strength. Soft polystyrene under heavy furniture or in a garage compresses over time and the screed sinks.
• Too little insulation on the ground or over unheated space. The heating works, but it also heats the ground or the cellar.
• Services under the board without levelling. The board rests on the pipes like on a ridge, gets damaged, and a crack forms in the screed above the services.
• Forgotten movement joints in doorways. The screed cracks exactly at the thresholds between rooms. Pipes crossing a movement joint need a protective sleeve, which the heating installer takes care of.
• Trampled studs. The pipe will not hold where studs are crushed and lifts when the screed is poured.

Laying a system board is a job a handy homeowner can do. It does not involve gas or electricity and needs no special tools. Leave the pipes, their connection to the manifold and the pressure test to the heating installer, though. Before you start, agree with them on the pipe spacing, the insulation thickness and where the movement joints go.

1. Check the flatness of the base with a long straightedge and mark any areas that need levelling.
2. Sweep and vacuum the base, and remove mortar lumps and rubble.
3. Inspect the damp-proof membrane on ground floors and have any damage repaired.
4. Stick the edge strip around the entire perimeter, around door frames and penetrations.
5. Lay the lower insulation with staggered joints and cut it accurately around services.
6. Start laying the system boards from the corner by the manifold or from the longest straight wall.
7. Join the boards with their overlaps or interlocking edges so that no gaps remain.
8. Use the offcut from the end of each row to start the next one, so the joints are staggered.
9. Cut the boards around door frames, pipes and the manifold cabinet with a sharp knife.
10. Lay the film skirt of the edge strip over the boards and tape it down.
11. Fit movement joint profiles in doorways and according to the bay layout.
12. Walk over the whole area, tape small gaps and replace any damaged pieces.
13. Cover the area with boards or keep off it until the heating installer arrives.

If you are unsure about the heights or the insulation strength, check with the designer or the heating installer first. A wrong decision in this layer can later only be put right by breaking out the screed.

The right moment to check the work is straight after laying, and once more after the pipes are in. Take photos of the whole area. They are very useful years later when you need to know where pipes or services run. When accepting the work, go through these points:

• The edge strip runs around the whole perimeter, door frames and penetrations without any breaks
• The edge strip skirt lies on the boards and is taped down
• The boards meet tightly, with no visible gaps or open joints
• The boards do not rock or flex when you walk on them
• The studs are not crushed and the film is not torn
• Lower insulation joints are staggered and there are no humps over services
• The surface height matches the floor level plan
• Movement joint profiles are in the doorways and at the places shown in the design
• The board is cut neatly around the manifold cabinet
• The area is clean, without rubble or leftover material
• Photographs of the whole area are taken and stored