Concrete Roof Insulation: stop the slab cooking the rooms underneath it
A flat concrete roof needs a screed laid to falls whether you insulate it or not. Lay that screed in SUNPERL perlite instead of sand and cement and the layer you were already paying for starts insulating, keeping heat out of the slab instead of letting it bake all day and release into the rooms at night. From R474 per m² incl VAT at 62.5 mm average thickness, laid to falls.
A bare concrete roof is barely insulation at all
Concrete conducts. A 250 mm roof slab with a plastered soffit and a normal sand-cement screed on top has a U-value of about 2.49 W/m²K. On a Highveld afternoon the top of that slab sits well past 50 degrees, and the heat it stores all day comes down into the rooms through the evening. That's why upstairs bedrooms stay hot hours after sunset.
Lay the same screed in perlite and that U-value drops to about 0.99. Same thickness, same falls to the drains, about 60 per cent less heat coming through.
How much it helps depends on what's already under the slab, and we'd rather show you all three cases than just the flattering one.
| 250 mm slab, 62.5 mm screed | Sand-cement screed | SUNPERL perlite 6:1 | Heat through the roof |
|---|---|---|---|
| Plastered soffit, no insulation | U 2.49 | U 0.99 | 60% less |
| Ceiling board, no insulation | U 1.59 | U 0.81 | 49% less |
| Ceiling with 100 mm Isotherm | U 0.34 | U 0.28 | 17% less |
Our calculation for summer heat flowing down: 250 mm reinforced concrete at 1.7 W/m·K, perlite 6:1 at the SABS-tested 0.0962 W/m·K, 12 mm ceiling board over an unventilated void, and Isotherm's published R 2.33 for 100 mm. Your roof will differ, so treat these as a guide, not a guarantee.
Where the insulation sits matters too
If your ceiling already has 100 mm of insulation, perlite trims the heat by about a sixth, and that's the honest number. But ceiling insulation sits under the slab, so the slab itself still bakes all day and still has heat to give back at midnight.
Perlite sits on top of the slab, so the concrete runs cooler in the first place. A U-value calculation doesn't capture that, which is why we won't put a number on it, but it's the difference you notice in the rooms directly under the roof.
The moment to do this is when the waterproofing comes up
Flat roof waterproofing has a life. Torch-on gets ten to fifteen years, liquid systems less. When it reaches the end, everything on top of the slab comes off anyway.
That's the window. The screed is already being stripped, the drains are already exposed, the scaffolding is already up, and the roof is already off limits. Relaying the falls in perlite instead of sand and cement at that point costs you the difference between two screeds, not the price of an insulation project.
Do it at any other time and you're paying to remove a perfectly good screed and redo waterproofing that had years left. We'll tell you to wait.
So if a roofer has told you the waterproofing is due, that's the call to make before they quote the reinstatement. Once the new membrane is down, the next window is ten years away.
To be clear about who does what. We lay the screed. Your waterproofing contractor lays the membrane over it. We're not a waterproofing company and we won't pretend to be one, so we'll work to your roofer's programme and hand them a surface laid to correct falls. If you haven't got a roofer yet, get their quote before ours, because their programme sets the dates.
When this is the right call, and when it isn't
Right for you if
- You've got a flat or low-pitch concrete roof and the rooms under it are the hottest in the house
- The waterproofing is due for renewal, so the screed is coming off anyway
- You're building new and the roof needs a screed to falls regardless
- Your engineer is watching dead load on a long span or an added storey
- You want something that can't burn sitting under the membrane
Not right for you if
- Your waterproofing was done in the last few years. Wait for it. We'd rather quote you in eight years than waste your money now
- You've got a pitched roof with a ceiling void. Board or bulk insulation in the void is cheaper and works better
- You're hoping perlite alone will make the roof comply with SANS 10400-XA. It won't. It closes the last gap on a roof with 100 mm ceiling insulation, it doesn't replace that insulation
- The slab has structural cracking or the drainage design is wrong. Fix that first, a screed hides nothing
What perlite roof screed costs
Laid to falls from the high point down to the perimeter drains, so materials and installation are quoted on average thickness rather than the thickest point. Here's a real 100 m² roof.
100 m² roof, 80 mm at the high point, 45 mm at the drains
62.5 mm average thickness at our 6:1 roof mix, supplied and laid to falls. R47,386 for the roof.
- SUNPERL perlite, 42.5N cement and SBR bonding liquid
- Screed laid to falls, high point to drains
- Perimeter fillet and edge detailing, 43 m
- Drain point detailing, 8 points
- R430/m² if your roof has no fillets or drain detailing to speak of
We work off 80 mm at the high point as a baseline and go thicker where the falls or the span call for it. Thickness drives the rate, and thickness comes from your falls, your span and where the drains are. A thin screed carries proportionally more labour, so a 250 mm deep fill works out at about R4.91 per millimetre per m² while this 62.5 mm job is nearer R7.58. That's why we measure before we quote rather than publishing a rate per millimetre.
Laid thick enough for SANS 10400-XA. At 100 mm high and 55 mm at the drains, which is what it takes to bring a roof with 100 mm ceiling insulation up to R 3.70, the same 100 m² roof is R52,132, or R521 per m² incl VAT at 77.5 mm average.
SANS 10400-XA and concrete flat roofs
If you're a homeowner you can skip this. It's the compliance case, and it's the reason perlite ends up on drawings.
SANS 10400-XA:2021 sets a minimum total R-value for the whole roof assembly, measured across every layer from the outside air film to the inside one. The requirement depends on the energy zone.
| Energy zone | Includes | Minimum roof R-value |
|---|---|---|
| Zones 1, 2, 3, 4, 6 and 7 | Gauteng, Cape Town, the Karoo and the interior plateau | R 3.70 m²K/W |
| Zone 5 and 5H | The KwaZulu-Natal coastal belt | R 2.70 m²K/W |
What perlite does to a typical concrete roof
A 250 mm slab with a 12 mm ceiling board and 100 mm Isotherm in the void. Only the screed changes.
| Screed to falls | Average thickness | Roof R-value | Against R 3.70 |
|---|---|---|---|
| Sand and cement | 62.5 mm | R 2.96 | Short by 0.74 |
| Perlite, 80 mm high, 45 mm at drains | 62.5 mm | R 3.56 | Short by 0.14 |
| Perlite, 100 mm high, 55 mm at drains | 77.5 mm | R 3.72 | Right on the line |
R-values for the perlite rows are area-weighted across the falls, not taken at the average thickness, because the thin end near the drains insulates less. Calculated on the same assumptions as the table above. Compliance on your roof has to be confirmed by your competent person, and we'll supply the SABS test report for their calculation.
Here's the honest version. Perlite roof screed is not a compliance solution on its own. On a bare slab you'd need about 320 mm of it to reach R 3.70, which nobody is going to build. What it does is turn a layer that's going onto the roof anyway into about R 0.6 of insulation, where sand and cement gives you almost nothing.
On a roof with 100 mm ceiling insulation, that's the difference between failing and passing, but only just. Laid at 100 mm high and 55 mm at the drains, our calculation puts the roof at R 3.72, which is right on the line rather than comfortably over it, so treat that as the minimum and let your competent person confirm it. On a 100 m² roof that works out at R521 per m² incl VAT, about R47 per m² more than the thinner 62.5 mm example in the pricing section, which falls short.
Plastered soffit roofs are different. With no ceiling void, all the insulation has to go on top of the slab, usually as rigid board under the membrane. Perlite doesn't replace that board, but laying the falls in perlite trims it by roughly a fifth, about 18 mm of XPS, and gives you a screed that's non-combustible and a quarter of the weight.
A quarter of the weight
31 kg/m² at 62.5 mm against 131 for sand and cement. On a 200 m² roof that's 6.2 tonnes instead of 26.2, so 20 tonnes off the structure. On long spans, added storeys and existing slabs of unknown provenance, that changes what has to be designed for.
Non-combustible under the membrane
Expanded volcanic glass and cement. Nothing to ignite, no smoke, no contribution to fire load. Foam boards under a torch-on membrane are a known risk during application and a known fuel afterwards.
It takes the falls with it
Board comes in flat sheets, so a board specification means a screed to falls and then boards on top, two layers and two trades. Perlite is the falls and the insulation in one pour, and it goes round drains, upstands and penetrations without a cut.
No thickness ceiling
XPS is made in South Africa in a single density range up to 80 mm, so anything thicker means laminating two boards and doubling the joints. A poured screed has no maximum thickness and no joints at all.
The SABS numbers for the roof mix
Tested at the SABS by heat flow meter under SANS 8301:2010. Report numbers are on this page so you can ask for them.
| Mix | 6:1, roofs and deep fill | 4.5:1 | 3:1, floors |
|---|---|---|---|
| Dry density | 500 kg/m³ | 600 | 800 |
| Wet density | 650 kg/m³ | 800 | 1000 |
| Thermal conductivity | 0.0962 W/m·K | 0.1046 | 0.1390 |
| Compressive strength | About 5 MPa | About 10 MPa | About 15 MPa |
| SABS report | BCT-170523-00052 | BCT-170523-00052 | BCT-170515-00034 |
We use 6:1 on roofs because the strength you need under a membrane and the occasional foot is far less than a floor needs, and the leaner mix insulates better. 5 MPa is still roughly fourteen times the compressive strength of the highest-density XPS board on the South African market.
Perlite screed against rigid board on a flat roof
Including the row where board wins, because you'll find it out anyway.
| SUNPERL perlite 6:1 | XPS board | PIR board | Sand-cement screed | |
|---|---|---|---|---|
| Thermal conductivity | 0.0962 W/m·K | 0.030 | 0.022 | 1.40 |
| R-value per 80 mm | R 0.83 | R 2.67 | R 3.64 | R 0.06 |
| Can be laid to falls | Yes, it is the falls | No, flat sheets | No, flat sheets | Yes |
| Needs a screed under it | No | Yes | Yes | n/a |
| Weight at 80 mm | 40 kg/m² | 2.8 kg/m² | 2.6 kg/m² | 168 kg/m² |
| Compressive strength | About 5,000 kPa | 310 to 360 kPa | About 150 kPa | Very high |
| Burns | No, volcanic mineral | Yes | Yes | No |
| Joints to open up | None, one pour | Every board edge | Every board edge | None |
| Thermal value in year 20 | Unchanged | Down 5 to 10% | Down 5 to 10% | Unchanged |
| Max thickness made in SA | No limit | 80 mm | Varies | No limit |
Where board wins, and what we actually recommend
Read the second row. At the same 80 mm a PIR board gives you more than four times the thermal resistance of perlite screed. If you need to reach SANS 10400-XA, board or ceiling insulation does most of the work, and perlite is not a substitute for it.
What we recommend on a new build is both. Lay the falls in perlite rather than sand and cement, then size the insulation for what's left. On a roof with 100 mm ceiling insulation, perlite laid at 100 mm high and 55 mm at the drains is what brings the assembly up to R 3.70. You also get 20 tonnes off the structure on a 200 m² roof and a non-combustible layer under the membrane.
What the roof looks like in year twenty
A roof is the hardest place in a building to go back to. Whatever goes under that membrane has to still be doing its job when the membrane comes off.
The R-value doesn't fall
XPS and PIR are blown with a gas that diffuses out over time and is replaced by air, taking roughly 5 to 10 per cent of the R-value with it over 10 to 15 years. There's a standard for it, Long-Term Thermal Resistance, because the industry knows the number on the box isn't the number later. Perlite has no blowing agent and nothing to lose.
No joints to open
Boards are laid as boards and a roof moves more than any other part of a building, expanding and contracting through forty degrees of daily swing. Every board edge is a line that can open and a cold bridge waiting to form. One bonded pour has no edges.
Nothing to lift
Board on a flat roof has to be mechanically fixed or ballasted against wind uplift, and every fixing is a penetration through the layer you just installed. A screed is bonded to the slab across its whole area. There's nothing for wind to get under.
It survives the next roofer
When the waterproofing is replaced in fifteen years, someone will be up there with torches, blades and a wheelbarrow. A cement screed takes that. Foam board under a fresh torch-on application is a different conversation.
From roof survey to a surface your waterproofer can work on
Roof survey
We measure the area, the existing falls, the drain positions and the perimeter. On a retrofit we look at the slab condition once the old screed is off, because that's when problems show. Free, and it's a survey, not a callout for leak repairs.
Fixed quote
Priced on average thickness with the falls worked out from your high point and drain levels. Fillet, cove and drain detailing come in as their own lines so you can see them.
Slab preparation
Surface cleaned back, laitance removed, SBR slurry bond coat applied so the screed keys into the slab rather than sitting on it.
Screed laid to falls
Mixed on site to 6:1, laid from the high point down to the drains, fillets formed at upstands and parapets, drain points detailed. One continuous pour, no joints.
Cure
Seven days before it takes the waterproofing, per the SUNPERL data. We'll tell your roofer the date rather than leaving the two of you to work it out.
Handover to waterproofing
You get a surface laid to correct falls, free-draining and ready for the membrane. Perlite is porous, so the membrane isn't optional, and we won't leave a roof without a waterproofing system specified over it. That work is your roofer's, not ours.
Roof questions, answered with figures
How do you insulate a concrete roof?
There are two places to put it. Under the slab, as ceiling insulation such as Isotherm laid over the ceiling boards, or on top of the slab under the waterproofing, which is called a warm roof. On top is better for comfort, because the slab itself stays cooler instead of baking all day and giving the heat back at night. A warm roof normally means a screed laid to falls, then rigid insulation boards, then the membrane. Lay that screed in SUNPERL perlite and the screed becomes insulation too. On a roof that already has 100 mm of ceiling insulation, a perlite screed averaging about 78 mm brings the roof up to the SANS 10400-XA requirement of R 3.70 without a separate board layer.
How do you make a concrete roof cooler?
Stop the sun heating the slab in the first place, which means insulation on top of the concrete rather than only in the ceiling underneath it. A perlite screed laid to falls does that, and on a roof with a plastered soffit and no insulation it cuts the heat coming through by about 60 per cent. A light-coloured or reflective waterproofing finish helps too, by reflecting some of the sun before the roof absorbs it. Ceiling insulation still helps, but on its own it leaves the slab hot and releasing heat into the evening.
Is it better to insulate the ceiling or the roof?
On a concrete roof they do different jobs, and most roofs need both. Ceiling insulation, such as 100 mm Isotherm, is the easiest place to add a lot of R-value, and in most of South Africa you'll need it to reach the SANS 10400-XA roof requirement of R 3.70. Insulation on top of the slab keeps the concrete itself cooler, which is what stops upstairs rooms staying hot late into the night. If you can only do one and the roof has a ceiling void, start with ceiling insulation. If the waterproofing is due anyway, that's the moment to lay a perlite screed on top.
What type of insulation is best for a concrete roof?
Per millimetre, rigid boards like PIR and XPS insulate three to four times better than perlite screed, and we won't pretend otherwise. But boards can't form the falls, need a screed laid under them, have joints that open over time and, apart from mineral types, will burn. Perlite screed is the falls and the insulation in one bonded, non-combustible layer, at about a quarter of the weight of sand-cement. On a new build we'd use both: perlite for the falls, then as much board or ceiling insulation as your energy zone needs on top of that.
How thick does roof insulation need to be?
It depends on your energy zone and what's already in the roof. SANS 10400-XA asks for a total roof R-value of 3.70 in most of South Africa, including Gauteng and Cape Town. On a 250 mm concrete roof with a sand-cement screed, that typically means a 145 mm ceiling insulation blanket such as Isotherm, because 100 mm leaves the roof at about R 2.96. Keep the 100 mm in the ceiling and lay the screed in perlite at about 78 mm average, and the roof reaches about R 3.72 instead. Your competent person confirms the calculation for your specific roof.
How much cooler will the rooms actually be?
It depends on what's under your slab. On a concrete roof with a plastered soffit and no insulation, perlite cuts the heat coming through by about 60 per cent. With a ceiling board but no insulation, about half. If you've already got 100 mm of ceiling insulation, about 17 per cent. What that does to room temperature also depends on your windows, ventilation and orientation, so anyone quoting you a degrees-cooler figure is guessing. What we can say is that perlite keeps the slab itself cooler, which is the heat you feel in the rooms under it late at night.
Does perlite roof screed meet SANS 10400-XA?
Not on its own, but it can get a roof there. SANS 10400-XA:2021 requires a total roof R-value of 3.70 m²K/W in energy zones 1 to 4, 6 and 7, which covers Gauteng and Cape Town, and 2.70 in zone 5. On our calculation a 250 mm concrete slab with 100 mm ceiling insulation and a sand-cement screed comes to about R 2.96. Lay the screed in perlite at 100 mm high and 55 mm at the drains and it comes to about R 3.72, right on the line, at about R521 per m² incl VAT on a 100 m² roof. Your competent person has to confirm compliance for your roof, and we supply the SABS test report for that calculation.
Which SANS 10400-XA energy zone am I in?
Most of the country, including Gauteng and Cape Town, falls in the zones that need R 3.70 for a roof. The KwaZulu-Natal coastal belt is zone 5 and needs R 2.70. Zone boundaries can split a single municipality, so check your exact site on the CSIR energy zone map or ask your architect before the roof is specified.
Can you lay it over my existing roof?
Only when the existing screed and waterproofing are coming off, which usually means the membrane is at the end of its life. Laying new screed over old waterproofing traps whatever's underneath and adds weight to a build-up nobody has assessed. If your waterproofing has years left, wait. We'd rather quote you in eight years' time than take money for the wrong job now.
Does it need waterproofing over it?
Yes, always. Perlite is a porous material and the screed is not a membrane. It's the substrate the membrane goes onto, laid to correct falls so water actually reaches the drains. We won't sign off a roof that doesn't have a waterproofing system specified over it.
How long before the waterproofing can go on?
Seven days' cure, per the SUNPERL data. We give your roofer the date when we start so the two trades don't end up arguing on site.
What does it weigh compared to a normal screed?
31 kg per square metre at 62.5 mm average, against 131 for sand and cement. On a 200 square metre roof that's 6.2 tonnes instead of 26.2, so 20 tonnes off the structure. On a new build your engineer designs for less. On an existing slab it means you can add a layer without adding load.
What does perlite roof screed cost?
From R474 per square metre including VAT at 62.5 mm average thickness laid to falls, which is what a 100 square metre roof at 80 mm high and 45 mm at the drains works out to, including perimeter fillets and drain detailing. Roofs with simple perimeters come in nearer R430. Laid at 100 mm high and 55 mm at the drains, which is what SANS 10400-XA compliance takes on a roof with 100 mm ceiling insulation, the same roof is R521 per m² incl VAT. Thickness drives the rate and thickness comes from your falls, so we measure before we quote.
Why 6:1 on a roof when floors get 3:1?
Strength you don't need costs you insulation. Perlite floor insulation goes in at 3:1 because a floor carries furniture, tiling and people. A roof screed carries a membrane and occasional foot traffic, not furniture and tiling, so the leaner 6:1 mix is right. It still tests at about 5 MPa, roughly fourteen times the compressive strength of the highest-density XPS board sold in South Africa.
What it's like working with us
These are general reviews of Synergy rather than roof jobs specifically. We're not going to dress them up as something they aren't.
"What a pleasure to deal with Synergy, it's a company with experience and after-care service. Jacques always takes my calls during and after the installation, always willing to assist. Our site was always clean, they were always on time."
"Synergy Energy Solutions is an excellent company, very good workmanship as requested. Thank you also for the over-and-above tasks that were performed for the preparation of the installation."
"Thank you Jacques and Jaco from Synergy Energy Solutions. Your professionalism and excellent service resulted in a great installation and solution. We are very happy and would recommend you to anyone."
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Where roof insulation fits in the rest of the build
Get a fixed price on your roof screed
Tell us the roof area, roughly where the drains are and whether the waterproofing is due. We'll survey it, measure the falls and quote on average thickness. Free, and it's a roof survey, not a leak callout. If your waterproofing has years left, we'll say so and you can call us back when it doesn't.
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