Under-tile mats, in-screed cable & in-slab systems

Underfloor Heating Installation Sydney

Underfloor heating is a heat output calculation long before it is a product choice. Two numbers decide whether the floor ever actually feels warm: how much of it can take heating cable, and what you put on top.

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You are buying heated square metres, not room square metres

Almost every underfloor heating conversation starts with the wrong number. Somebody measures the room, gets 4.3 square metres, and everything after that is priced and sized against a figure the system will never see. Heating cable does not go under fixed things. Not under a vanity, not under a floor mounted pan, not under a shower base or a bath, and not under fixed cabinetry, because heat trapped beneath a solid object has nowhere to go and the cable sits there cooking itself.

It is also held back from the walls. Manufacturers put that setback between 50 and 100 mm depending on the system, with roughly 50 mm of clearance kept around each fixture so nothing touches. None of those gaps sound like much on their own. Added together around a small bathroom they are the difference between the room you measured and the floor you can actually heat, and that second number is usually a lot closer to half than to all of it.

This would be a pedantic distinction if the cable could be trimmed on the day. It cannot. A heating element is a fixed resistance sized to its length, and cutting it raises the watts per metre until it burns out, which is why manufacturers put it in the warnings rather than the fine print. The cold tail back to the thermostat can be shortened. The sensor lead can be shortened. The element that makes the heat is the length it is, forever.

So the sequence matters. Measure the floor, take out the fixtures and the setbacks, and order against what is left. A quote that prices your bathroom by its room dimensions, without a set-out showing what comes out for the shower and the vanity, is either padded or about to come up short on site with a roll of cable nobody can adjust.

The measurement

What is left once the fixtures come out

This is a real small bathroom drawn to scale, with a 900 mm shower base, a 900 mm vanity and a floor mounted pan. Everything amber can take cable. Everything else cannot.

Scroll to see the full diagram →

What it means: the room is 4.32 square metres and the heatable floor is 2.23, which is a little over half. That is not a typical figure to quote back at anyone, it is what this particular floor plan produces, and yours will produce its own number. Change the assumptions and it moves: a wall hung pan gives back most of that rectangle, a shower with a tiled floor rather than a moulded base can change the picture again, and a bathroom with a bath along one wall loses more. The set-out above uses a 75 mm wall setback, mid range for the systems we install. What does not change is the principle: the heated area is the number that gets ordered against, and it is worth seeing it drawn before anyone commits to a product.

There is a second reason to care about the ratio, beyond not wasting money on cable. A floor heating system warms a room through its heated area only, and the thermostat stops it once the floor surface reaches setpoint. If the heated portion is well under the room, the system physically cannot be the sole heat source in that space no matter what is installed, because it has run out of both area and temperature headroom. In a bathroom that rarely matters, since a warm floor and a warm towel is the whole ask. In a living room being sold underfloor as its only heating, it matters a great deal, and it is a heat loss calculation rather than a hunch.

If the bathroom also needs the air warm quickly, that is a different appliance doing a different job, and the two work well together rather than competing. Our bathroom heater page covers the wet area zones and the ventilation rules that come attached to it.

What goes on top

The floor covering decides how much heat ever reaches you

Every layer above the cable is thermal resistance. The industry design ceiling is 0.15 square metre kelvin per watt, which is the same thing as 1.5 tog, and it is easier to exceed than people expect.

Floor finish	m²K/W	tog	What it means in practice
Ceramic or porcelain tile, 5 mm	0.00	0.0	Ideal. Effectively transparent to the heat
Vinyl or LVT, 2 mm	0.01	0.1	Near ideal, and check the product is rated for it
Stone or marble, 25 mm	0.02	0.2	Very good. Slower to warm, holds it longer
Engineered timber, 13 mm	0.10	1.0	Works, with the thermostat capped near 27 °C
Solid timber, 22 mm	0.15	1.5	Right on the design limit
Carpet, medium pile	0.15	1.5	Right on the design limit, before underlay
Carpet, deep pile plus 5 mm underlay	0.20	2.0	Past it. Output and response both suffer
Carpet, deep pile plus 8 mm underlay	0.25	2.5	Past it. Not worth installing under

Two different limits get quoted in this field and they are not the same number, which causes a fair amount of confusion. The 0.15 square metre kelvin per watt figure is the design limit from EN 1264, the European underfloor heating standard, and it converts to exactly 1.5 tog. The 2.5 tog figure that turns up on carpet packaging converts to 0.25 square metre kelvin per watt, and the Carpet Institute of Australia applies it to hydronic pipe systems only. For electric cable in a screed the Institute puts the combined carpet and underlay maximum at 1.5 tog. This page is about electric systems, so 1.5 tog including the underlay is the number that matters, and anyone telling you 2.5 tog and 0.15 are the same thing has merged two different rules.

What happens past the limit is not that the system stops working. It is that the floor sensor caps the surface temperature while the resistance layer throttles what crosses it, so the output collapses quietly. Warmup's published worked example is worth knowing: 14 mm hardwood at 1 tog, with 27 degrees held on the underside of the boards in a 20 degree room, delivers about 35 watts per square metre. Push the room to 22 degrees and the same floor delivers 25. The cable underneath is unchanged and running just as often. The heat simply is not getting through.

Timber brings its own cap. Engineered and solid timber floors are generally held at about 27 degrees to avoid cupping and warping, and engineered board handles the cycling considerably better than solid does. It is also worth knowing what not to do afterwards: manufacturers warn against leaving anything above about 2.5 tog sitting permanently on a heated floor, and a thick backed bath mat left in the same spot all winter is comfortably in that range.

There is no Australian standard we are aware of that sets a maximum floor surface temperature. Installers here work to the thermostat limit the manufacturer supplies, and where a design figure is genuinely needed, to EN 1264, which puts an occupied area at 29 degrees and allows 33 in a bathroom. In practice the cap built into the supplied thermostat, usually around 28, is what actually governs your floor.

Three systems

Under tile, in screed, in slab

They are not three price points for the same thing. They are three different machines, and the difference is how much mass sits between the cable and your feet.

Under-tile mat

Cable pre-spaced on a mesh backing, rolled out over the substrate and buried in the tile adhesive bed. It adds almost nothing to floor height, which is what makes it the usual answer for a bathroom being re-tiled. Standard output is 150 watts per square metre, with 200 offered for wet areas and rooms with more heat loss.

Warm underfoot in roughly one to two hours. Because the mat comes in fixed sizes and the cable inside it cannot be cut, the floor has to be measured and set out before anything is ordered.

In-screed loose cable

A single length of cable clipped out by hand and covered by a screed, usually 20 mm or more. Spacing is the control: a 17 watt per metre cable gives about 170 watts per square metre at 100 mm centres and about 230 at 75 mm, and heavier cables go higher again. Because it is loose rather than pre-spaced, it follows an awkward floor plan into corners and around fixtures where a mat simply will not fit.

Slightly slower to respond than a mat, since there is more material above it, but the extra material also holds the heat longer once the thermostat drops out.

In-slab cable

Cable fixed to the reinforcing mesh before the concrete goes in, with a minimum cover over it. This is a new build or a major slab job, not a retrofit. What you get for it is thermal mass: the slab becomes the radiator and the temperature swing across a day is very gentle.

What you also get is lag. Manufacturers put a cold slab somewhere between half a day and two days from switch-on to temperature, which is why a slab is run continuously through the cold months rather than on a daily timer, and why it is the one form of floor heating where a controlled load supply genuinely makes sense.

What all three need

A thermostat with a floor sensor, and the sensor run in flexible conduit so it can be pulled out and replaced without lifting the floor. It sits evenly between two runs of cable, in a representative part of the room, away from doorways and water pipes.

Then 30 milliamp RCD protection, a check that the existing circuit can carry the load, and testing at three separate points during the install. See switchboard upgrades if the board cannot give the circuit a home.

Worth knowing

Four things people get told that are not right

Two of these cost money and two of them save it. All four are checkable.

A bigger wattage mat gives you a warmer floor

It does not. The thermostat measures the floor and cuts the cable off at setpoint, and a supplied thermostat is usually factory limited around 28 degrees, with timber capped lower still.

A 200 watt system and a 150 watt system settle at the same floor temperature. The extra wattage buys faster warm-up and more reserve in a room that loses heat quickly. It does not buy a hotter floor, and it does cost more to run.

The cable can be trimmed to fit on the day

Never. The heating element is a fixed resistance sized to its length, and shortening it raises the watts per metre until it destroys itself. Manufacturers are unusually blunt about this.

The cold tail that runs back to the thermostat can be shortened, and so can the sensor lead. The element cannot. That is precisely why the heated area has to be measured properly before ordering.

Insulation underneath is an upsell

It is the opposite. Without it a good share of the heat goes down into the substrate before it comes up to you, and that is heat you pay for twice: once to make it and once to wait for it.

Warmup claims insulation can cut overall running cost by as much as half, and that its high performance boards reach the same comfort temperature on about 12 per cent less energy than an ordinary board. Those are manufacturer figures rather than independent ones, but the direction is not in dispute.

A controlled load meter will halve the running cost

For a slab, worth discussing. For a bathroom mat, mostly pointless. Ausgrid does list space heaters, underfloor heating named specifically, as eligible for both controlled load tariffs.

The catch is the supply. Controlled load 1 can be connected for as little as six hours in any 24, at times the network chooses. A slab's thermal mass rides straight through that. A mat you want warm at 7am cannot.

Cost and timing

What it costs to run, and one rule you may not have to follow

Running cost falls straight out of the heated area, which is the third reason to get that number right. At 150 watts per square metre, the 2.23 heated square metres in the diagram is about 335 watts, well under a hair dryer. At typical Sydney retail electricity rates that is under 10 cents an hour with the cable drawing full power, and a floor sensor thermostat spends a good part of its time switched off once the floor is up to temperature. An hour of warm tiles before work is small change. Holding a large area warm all day is a different scale of number, and that is where the tariff conversation actually starts.

Which is worth having if you are heating a slab. Ausgrid lists space heaters, with underfloor heating named specifically, as an eligible appliance on both of its controlled load tariffs. Controlled load 1 is usually connected for at least six hours in any 24 and its switching moved toward the middle of the day in July 2024 to soak up solar. Controlled load 2 gives at least sixteen hours including four between 7am and 5pm. A slab with tonnes of thermal mass rides through that comfortably. A bathroom mat you want warm at seven in the morning cannot live on a supply somebody else switches, so for a single room this is generally not worth pursuing. The same tariffs are covered in more depth on our hot water installation page, since that is where most people meet them.

There is also a National Construction Code requirement here that gets applied more widely than the Code asks. NCC 2022 Housing Provisions clause 13.2.6(4) requires a slab on ground with an in-slab or in-screed heating system to have insulation of at least R1.0 around the vertical edge of its perimeter. That is real, it is mandatory, and on a whole floor system it should be in the scope. But clause 13.2.6(7) carves out an exception in the same breath: the requirement does not apply to an in-screed system used solely in a bathroom, amenity area or the like. If you are heating one bathroom floor and a quote carries perimeter slab insulation as a line item, that is worth a question, because the Code does not require it for that job.

The last piece of cost is timing, and it is the largest one. An under-tile or in-screed system lives underneath the finished surface, so retrofitting into a finished room means the surface comes up first, and that cost has nothing to do with the heating. Put the same system in while the floor is already open for a renovation or a re-tile and the heating is a much smaller line on the invoice. If bathroom work is anywhere on the horizon, the decision belongs before the tiler starts rather than the winter after.

Compliance

The parts of this that are not optional

Licence 387609C Level 2 accredited & insured

Floor heating is common enough to have earned its own clause in the wiring rules. AS/NZS 3000:2018 clause 4.10 covers electric heating cables for floors and ceilings, alongside clause 2.6.3.2.2, which requires 30 milliamp RCD protection on every final subcircuit in a home. A hardwired floor heating circuit is one. In commercial and common property work the equivalent trigger is clause 2.6.3.2.3, and a wet area heating circuit falls inside it anyway.

A bathroom job brings a second one that gets missed constantly. Clause 5.6.2.5 requires conductive reinforcing within a concrete floor or wall of a room containing a shower or bath to be bonded to the earthing system of the installation. Building Commission NSW flags this as a frequent finding, and the usual tell is a single earth cable leaving the meter panel. If a bathroom floor is open for heating, that is the moment to check it rather than the moment to look past it.

Testing is what separates a system that lasts from one that fails inside a wall of tiles. The element is tested at 500 volts out of the box, again once it is laid and before anything covers it, and again after the tile or screed goes down, with the resistance held inside a tolerance of about minus 5 to plus 10 per cent of its nameplate value. AS/NZS 3000 sets one megohm as the floor for insulation resistance; manufacturers ask for far more than that and their figure is the one the warranty runs on. Skipping the middle test is how a cable damaged by a trowel becomes a demolition job.

The electrical work is certified. We lodge a Certificate of Compliance for Electrical Work within 7 days of testing, which since 1 July 2026 goes through the BCNSW eCert portal.

Good to know

Underfloor heating questions

How much of my floor can actually be heated?

Less than you would expect, and it is the number your quote should be based on. Heating cable does not go under fixed things: not under a vanity, a floor mounted pan, a shower base or a bath, and not under fixed cabinetry. It is also kept back from the walls, typically 50 to 100 mm depending on the system, with about 50 mm of clearance around each fixture. On a 2.4 by 1.8 metre bathroom with a 900 mm shower, a 900 mm vanity and a floor mounted pan, that leaves about 2.2 square metres of heatable floor out of 4.3. Ask for the heated area before anyone orders anything, because the cable arrives cut to length and cannot be shortened on site.

Can I put it under carpet or timber?

Under timber yes, with a temperature cap. Under carpet it depends entirely on what is beneath the carpet. The thermal resistance of everything above the cable is what decides how much heat reaches the room, and the design ceiling used across the industry is 0.15 square metre kelvin per watt, which is 1.5 tog. Tile and stone are almost transparent to heat. A 22 mm solid timber floor sits right on the limit. A deep pile carpet with an 8 mm underlay is around 2.5 tog and well past it. Timber and engineered timber also need the thermostat capped at about 27 degrees to avoid cupping and warping, and engineered boards handle it better than solid.

Can underfloor heating be my only heat source?

In a bathroom, usually. In a living area, only if the heated area is close to the room area and the building is reasonably insulated. The moment the heatable floor drops well below the room floor, the system is being asked to heat the whole room through half its area, and a floor sensor will stop it before it gets there because the surface temperature is capped. Whether the sums work is a heat loss calculation for your room, not a rule of thumb, and it is worth doing before you commit to underfloor as the only heating.

How long does it take to warm up?

It depends on how much mass sits between the cable and your feet. An under-tile mat sitting in the adhesive bed is warm in roughly one to two hours. Cable in a screed is similar, a little slower with a thicker screed. Cable in a structural slab is a different animal: one manufacturer puts it somewhere between half a day and two days from properly cold, because you are heating tonnes of concrete. That is why a slab is run continuously through the cold months rather than switched on with a daily timer, and why it suits a controlled load supply while a bathroom mat does not.

Can it be retrofitted without lifting the floor?

Not for under-tile or in-screed systems. The heating element lives underneath the finished surface, so putting one in means the surface comes up first. That is the honest answer and it is why the timing matters so much: the same system is far cheaper to install while a floor is already open for a renovation or a re-tile. If you are planning bathroom work in the next couple of years, decide about the heating before the tiler starts, not after.

What does it cost to run?

A standard under-tile system draws 150 watts for every square metre of heated floor. On the worked example above, 2.2 heated square metres is about 335 watts, well under a hair dryer. At typical Sydney retail electricity rates that is under 10 cents an hour with the cable drawing full power, and a floor sensor thermostat spends a good deal of its time switched off once the floor is up to temperature. Running it for an hour each morning is a small number. Heating a whole floor area continuously is a much larger one, which is where the tariff conversation starts.

Does it need its own circuit and an RCD?

It needs 30 milliamp RCD protection. In a home, clause 2.6.3.2.2 of AS/NZS 3000 requires it on every final subcircuit and a hardwired floor heating circuit is one. In commercial and common property work the trigger is clause 2.6.3.2.3, and a wet area heating circuit lands inside it anyway. Whether it gets a dedicated circuit is a load question rather than a rule, and on an older board with one circuit already carrying half the house the answer is usually yes. It also needs a thermostat with a floor sensor, and that sensor has to sit in flexible conduit evenly between two runs of cable so it can be replaced later without lifting tiles. The sensor is the component most likely to need replacing over the life of the system, and the conduit is what makes that a small job instead of a demolition.

How does your pricing work?

Attending your job is a chargeable callout. We confirm what it costs when you book, and the price for the work itself is agreed before we start, so there are no surprises on the invoice.

Where we work

Underfloor heating across the North Shore

Based in Castlecrag, covering all of Sydney. A few of the suburbs we work in most:

Related work: warm air rather than a warm floor is a bathroom heater, and the exhaust that goes with it is an exhaust fan. Controlled load tariffs are covered on hot water installation. A new circuit and RCD protection is a switchboard upgrade, and where the supply itself is the limit, consumer mains. Floors opened for a renovation are often the right moment for rewiring or data cabling. Apartments and common property are strata electrical. See the full list of locations we service.

Planning a floor, or freezing on the one you have?

Upfront pricing, with the price agreed before we start. We measure the heated area and show you the set-out before anything is ordered.

Call 0401 427 270 Request a quote