Castellated panels and liquid screed for underfloor heating

Castellated panels and liquid screed are often used together because they solve the same problem from two sides. Liquid screed is runny when it goes down, and it can lift pipe that is only clipped at intervals. A panel that holds the pipe at every boss keeps it where you laid it.

This guide covers how the panels work, how deep the screed should be, how to set up the floor for a pour and what a typical extension needs. For a comparison of all floor systems, see underfloor heating floor systems explained.

What is a castellated underfloor heating panel?

A castellated panel, also called an egg crate or studded panel, is a plastic sheet covered in raised bosses. You push the pipe down between the bosses and it stays there, so there are no clips, staples or clip rails to fit.

We sell two types:

The 1220 x 820mm sheet covers about 1m², so working out quantities is simple: one sheet per m² of floor, plus a few for cuts. MaxHeatSolo covers 0.6m² per panel and has a minimum order of 10 panels (6m²).

Castellated panel, staples or clip rail: which is best for liquid screed?

For liquid screed, a castellated panel is usually the most secure option because it holds the pipe continuously rather than at fixing points. Staples and clip rail work too, but need closer fixings.

If you do use staples or rail under a liquid screed, follow the screed maker's fixing distance. One calcium sulphate screed maker asks for fixings at least every 400mm. Another asks for every 300mm, with more on bends. Our step-by-step install guide covers the staple method in full.

How thick should liquid screed be over underfloor heating?

Liquid calcium sulphate screed over UFH pipe is thinner than sand and cement. One maker's minimum is 25mm cover over the pipe, with 30mm nominal. Another UFH system maker suggests 35mm cover, which gives about 50mm in total over 15mm pipe.

Depth also depends on how the screed sits on the layer below. These are the minimums one calcium sulphate screed maker publishes:

Every screed brand has its own data sheet, so confirm the depth with your screed contractor before you set floor levels. Sand and cement screed is thicker and slower to dry. Compare the two in screed depths and drying times for UFH.

How do I prepare the floor for a liquid screed pour?

You prepare the floor by making it liquid-tight: liquid screed finds every gap, and screed that runs under the insulation or panels can lift them. Work through these steps in order.

1. Insulate the base. Lay the floor insulation your building specification calls for, tight jointed. One screed maker asks for insulation of at least 100kPa compressive strength.
2. Lay a membrane. One maker asks for polythene of at least 500 gauge, lapped 100mm, taped at the joints and turned 100mm up the walls.
3. Fit the edge strip around every wall, column and door frame, and tape its skirt down to the membrane so screed can't get behind it.
4. Lay the castellated panels with the edges interlocked, cutting around obstacles with a sharp knife.
5. Lay the pipe from the manifold flow port, pressing it fully down between the bosses at your design centres, and back to the return port.
6. Pressure test. Fill and purge each loop, then test. One screed maker asks for 6 bar, or the system's maximum operating pressure, held while the screed is placed.
7. Seal the building. Weatherproof openings and cover large glazing to stop direct sun on the fresh screed.
8. Pour. Don't pour in temperatures of 5°C and falling or 30°C and rising, according to one screed maker's guidance.

Check the edge strip thickness your screed maker asks for. One liquid screed maker specifies 8mm for heated screeds and 5mm for unheated. Our edge insulation roll is 100mm high and 5mm thick.

Worked example: a 20m² kitchen extension

Here is a typical single room job: a 5m x 4m extension, castellated panels, 16mm pipe at 150mm centres and a liquid screed. The manifold is about 5m away.

• Panels: 20m² ÷ 1m² = 20 sheets, plus 10% for cuts = 22 sheets.
• Pipe in the floor: 20 ÷ 0.15 = 133m. That is too long for one loop, so split it into 2 loops of about 67m. Add 10m of tails per loop (5m flow, 5m return) for about 77m each, 154m in total.
• Edge strip: perimeter 5 + 4 + 5 + 4 = 18m, so one 25m roll.
• Manifold: 2 loops, so a 2 port manifold or 2 spare ports on an existing one.
• Screed drying: at about 50mm thick, roughly 40 + (10 x 2) = 60 days to dry fully in good conditions.

Prices October 2026. Add the manifold, pump pack and controls, plus insulation and membrane to your building specification. Our guide to manifold ports helps you size the manifold.

Which pipe should I use in a castellated panel?

Use a 16mm pipe with a protected oxygen barrier for screed and castellated systems. Our 5-layer PERT-EVOH pipe has the EVOH barrier sealed inside the wall, so it can't be scuffed as you push it between the bosses. It is rated to 6 bar at 70°C.

• The castellated floor panel takes 15, 16 or 17mm pipe, so standard 16mm pipe fits.
• MaxHeatSolo is for 12mm pipe only. Use 12mm PERT-AL-PERT or 12mm PERT-EVOH.
• Use a pipe decoiler so the pipe comes off the coil without twisting. Twisted pipe fights the bosses and is hard to press in.

Can I use castellated panels with levelling compound instead of screed?

Yes. A castellated panel with 12mm pipe and a self-levelling compound is a low-profile alternative to a full screed, often used in renovations. MaxHeatSolo is made for this: it has a self-adhesive backing that sticks to a primed subfloor.

1. Clean the subfloor and prime it. Ultra Prime IT is a multi-surface primer in 5 litres.
2. Peel and stick the MaxHeatSolo panels, butting them tightly together.
3. Press the 12mm pipe in between the bosses.
4. Pressure test the loops before you pour.
5. Pour the levelling compound over the panels at the depth its maker specifies for covering UFH pipe.

Thin builds have less mass, so they warm up faster than a full screed but store less heat. For tile-direct boards with no pour at all, see low-profile overlay boards.

What do I need to do before tiling liquid screed?

Before tiling, the screed must be dry, the surface laitance removed and the screed primed. Calcium sulphate screed needs more preparation than sand and cement.

• Dryness. One adhesive maker treats calcium sulphate screed as dry enough to tile when a surface hygrometer reads under 75% RH.
• Laitance. The weak top skin must come off. One screed maker recommends sanding with a 60 grit orbital sander with vacuum just before the floor finish goes down.
• Primer. Apply the adhesive maker's primer, usually 2 coats, to separate the calcium sulphate from cement-based adhesives.

You can often speed drying by running the UFH from about 7 days, following the screed maker's force drying schedule. The steps are in our screed drying guide.

Products for this job

• Castellated Floor Panel 1220 x 820mm
• MaxHeatSolo Self-Adhesive Egg Crate Panel for 12mm Pipe
• All castellated and in-screed panels
• 5-Layer PERT-EVOH Pipe 16mm
• Edge Insulation Foam Roll
• Ultra Prime IT Multi Surface Primer 5L
• Pipe Decoiler

Frequently asked questions

Do castellated panels need clips?

No. The pipe is held between the raised bosses, so there are no clips, staples or rails. That is the main reason installers choose them for liquid screed.

Do castellated panels replace floor insulation?

Plan your floor insulation separately, to your building specification, and lay the panels over it. The insulation thickness comes from the Part L U-value calculation. See insulation under underfloor heating.

Can I walk on castellated panels before the screed goes down?

Yes, with care. Use boards to spread your weight on busy routes, and check that no pipe has lifted out of the bosses before the pour.

How long does liquid screed take to dry?

About 1mm a day up to 40mm, then 0.5mm a day, in good conditions. A 50mm screed takes about 60 days. Force drying with the UFH can shorten this if the screed maker allows it.

Can I use sand and cement screed over castellated panels?

Yes. The castellated floor panel can be finished with screed or levelling compound. A sand and cement screed is thicker and slower to dry, so check the floor levels first.

Why does liquid screed lift underfloor heating pipe?

Water-filled pipe is lighter than wet screed, so it tries to float. Panels hold it all along its length, and keeping the pipe under test pressure during the pour helps you spot any problem straight away.

How many castellated panels do I need?

For the 1220 x 820mm panel, one sheet per m² of floor, plus about 10% for cuts. For MaxHeatSolo, divide the floor area by 0.6 and round up, with a minimum of 10 panels.

Sources: Gyvlon flowing screed data pack, Gypsol preparation for liquid flowing screeds, Timoleon staple system data sheet, Instarmac: calcium sulphate screeds