How to install underfloor heating in screed with clip rail or tacker clips

Clip rail and tacker clips (also called staples) are the two classic ways to hold water underfloor heating pipe in place before a screed is poured. Both fix the pipe to the insulation, both let you set any pipe spacing you like, and both are cheap. The difference is speed and feel on site: staples are fired in with a tacker gun as you walk the loop, while rails are set out first and the pipe is pushed in afterwards.

This guide walks through a screeded install from bare slab to screed, with the fixing distances, bend limits, test pressure and screed depths that UK manufacturers publish. It assumes you already have a loop design. If you are still sizing loops, read our guide to underfloor heating pipe centres first.

Clip rail or tacker clips: which should I use?

Use tacker clips for most rooms because they are faster and hold the pipe tight at every bend. Use clip rail where the insulation will not hold a staple well, or where you want the spacing fixed before the pipe goes down.

Point	Tacker clips (staples)	Clip rail
How it fixes	Barbed staple pushed into the insulation over the pipe	Rail fixed to the insulation, pipe clicked into it
Typical fixing	About every 500mm, three per bend	Rails about 1m apart across the runs
Tools	Tacker gun	None beyond a knife
Layouts	Spiral or serpentine	Best for serpentine (straight runs)
Best for	Fast work on large floors	Plain floors where you want spacing set out first

You can mix the two. Many installers use rail to set out the long straight runs and a few staples to hold the return bends and the pipe tails near the manifold.

How do I prepare the floor before laying pipe?

Start with a clean, flat slab with its damp proof membrane in place, then lay the insulation, edge strip and any membrane before the pipe. Gaps here are where screed gets under the boards and heat escapes.

1. Lay the insulation. One leading manufacturer's guide says the insulation must keep downward heat loss to no more than 10W/m², as BS EN 1264 requires. Stagger the board joints and tape them. Our guide to insulation under underfloor heating covers thicknesses.
2. Fit the edge strip. Run it round every wall and round columns, steps and door frames. Lay its plastic skirt flat over the insulation so screed cannot run under the edge. One leading manufacturer advises starting mid-wall, overlapping the joint at the end and slitting the skirt at corners.
3. Add a membrane if the boards are plain. One leading manufacturer's staple guide calls for polythene of at least 0.15mm, with joints lapped by at least 80mm, over plain insulation. Gridded, foil-faced boards do not need it.
4. Mark out. A printed grid on the board or membrane lets you set 100, 150 or 200mm centres without measuring every run.

Leave the edge strip standing proud of the finished floor. It is trimmed flush after the screed and floor finish are down.

How do I fix pipe with tacker clips?

Fire a staple over the pipe about every 500mm on straight runs and put three on each return bend, one at the apex. Keep the pipe tight to the board as you go.

The published figures vary a little between makers, so treat 500mm as the usual working figure:

• One maker: a staple about every half metre on straights, three at each turn including one at the apex, and the pipe held within 5mm of the board.
• Another maker: staples at a minimum of 500mm centres, with one box of 300 staples for about 150m of pipe.
• A third maker: at least one fixing per metre and up to six round a loop, with more needed on plain insulation.

Work from the manifold end. Leave a tail long enough to reach the manifold, then lay the loop and finish back at the manifold. A pipe decoiler lets the coil turn as you pull, so the pipe lies flat and does not fight the staples. Two people work fastest: one lays the pipe, the other follows with the gun.

Do not bend the pipe too tightly. One leading manufacturer says never bend tighter than a 225mm diameter. Another manufacturer gives 8 times the pipe diameter for a free-hand bend and 5 times for a supported bend, which is 128mm and 80mm radius for 16mm pipe. If the pipe kinks, a third manufacturer's instruction is that the coil must be replaced, not straightened.

How do I fit clip rail?

Lay the rails across the direction of the pipe runs at about 1m intervals, then press the pipe into the clips at your design spacing. The rails hold the spacing, so the runs stay straight and parallel.

1. Set out the first rail. One leading manufacturer leaves about 800mm round the room edges for the return bends and the tails to the manifold.
2. Fix the rails. Self-adhesive rail sticks straight down to a clean, dry, dust-free surface. Butt or interlock the lengths end to end. One leading manufacturer's rails do not need to overlap.
3. Space the rails. About 1m apart across the room. One leading manufacturer allows roughly 1m of rail per m² of floor.
4. Clip in the pipe. Press each run into the rail at the design centres, working in a serpentine pattern.
5. Secure the returns. The 180° bends sit outside the rails, so add an extra clip or staple where a return wants to lift.

Clip rail suits serpentine layouts, where the pipe runs back and forth in straight lines. For a spiral layout, staples are easier because the pipe changes direction all over the room.

Which pipe layout should I use?

Use a serpentine (back and forth) layout for most rooms and a spiral where you want an even floor temperature across a large, square room. Both work with staples, and serpentine works best with rail.

• Serpentine: One leading manufacturer advises running the pipe parallel to the room's longest walls and starting nearest the manifold.
• Spiral: lay the first pass at twice the final spacing, then fill in between on the way back. One leading manufacturer's example lays 400mm first to finish at 200mm centres.
• Loop size: One leading manufacturer limits a 16mm circuit to 100m and about 22m² at 200mm centres. Closer spacing covers less floor per loop.
• Fixtures: keep pipe out from under permanent fixtures such as kitchen units and bath panels.

Every loop goes back to its own pair of manifold ports, so plan the loop order before you start laying. Mark each loop on the drawing with its room, length and port number, and label both ends of the pipe as you lay it.

How do I protect the pipe at doorways, joints and the manifold?

Sleeve the pipe in conduit wherever it passes through a wall or doorway, crosses a screed joint or rises to the manifold. Hard edges and screed movement are what damage pipe after the floor is finished.

• Conduit: One leading manufacturer fits a 400mm length at internal walls, doorways, through the edge strip, at the manifold rise and across expansion or day joints, and allows about 2m per circuit.
• Expansion joints: BS EN 1264-4, as quoted in manufacturers' installation guides, recommends a joint in stone or ceramic finished screeds at least every 40m², with no bay longer than 8m and a length to width ratio of no more than 2:1.
• Crossing joints: One leading manufacturer allows only flow and return pipes to cross an expansion joint, sleeved for about 0.3m.
• At the manifold: use pipe support bends so the pipe turns up to the manifold without flattening.

How do I pressure test and screed over the pipe?

Fill and purge every loop, test at 6 bar for an hour, and keep the system at pressure while the screed is laid. Then screed to the depth your screed type needs.

1. Test. One leading manufacturer raises the system to 6 bar and holds it for 60 minutes. If it drops, check the joints and test loops one at a time. Our guide to pressure testing underfloor heating covers the method and records.
2. Hold pressure while screeding. Manufacturers' guides keep the system at 6 bar during the pour, so any damage shows straight away.
3. Screed. One leading manufacturer quotes BS 8204-1: at least 65mm of sand and cement screed for domestic and light commercial floors. For a flowing screed, use the screed maker's figure. One calcium sulphate maker, for example, asks for at least 25mm cover over the pipes.
4. Wait, then heat slowly. Do not use the heating to dry the screed. One leading manufacturer waits at least 21 days after a cement screed, then runs at 20 to 25°C for at least 3 days before raising to the design flow temperature.

Our guide to screed depths and drying times has the full drying rules and moisture tests.

Products for this job

• High Grip Barbed Pipe Clips, 40mm or 60mm
• Tacker Gun for 40mm and 60mm clips
• Self-Adhesive Pipe Clip Rail
• Underfloor Heating Grid Membrane, printed for 100, 150 and 200mm centres
• Edge Insulation Foam Roll

Frequently asked questions

How far apart should underfloor heating staples be?

About 500mm apart on straight runs, with three staples on each return bend. Some makers allow one per metre on gridded boards, but plain insulation needs more.

How far apart should clip rails be?

About 1m apart, laid across the pipe runs. Leave around 800mm at the room edges for the return bends and the tails to the manifold.

Do I need a membrane over the insulation?

Over plain insulation, yes. One maker calls for polythene of at least 0.15mm with 80mm laps so screed cannot get into the joints. Foil-faced, gridded boards with taped joints do not need it.

What pressure should the pipe be under during screeding?

Makers keep it at 6 bar while the screed is laid, after a 6 bar test held for an hour. A drop during the pour tells you the pipe has been damaged before it is buried.

How thick should screed be over underfloor heating pipe?

At least 65mm of sand and cement screed in a home, under BS 8204-1. Flowing screeds can be thinner, but use the screed maker's minimum for floors with heating pipes.

Can I use the underfloor heating to dry the screed?

No. Let the screed cure first, then start the heating at a low flow temperature and raise it slowly. One maker waits 21 days for a cement screed before the first heat-up.

What do I do if I kink the pipe?

Replace the loop. A kinked pipe should not be straightened and buried. Use a decoiler and bend no tighter than the pipe maker allows to avoid kinks.

Sources: flowing screed manufacturers' data sheets and installation guidance to BS EN 1264