Two different failures get called sag. One is the blanket crushed flat where the sheeting meets the purlin. The other is the blanket drooping into the roof space between purlins. They have different causes and different fixes, and only one of them is solved by buying a thicker blanket. Crushing at the purlin is the expensive one, because that line runs the length of every purlin on the roof.
In short
- Bulk insulation only delivers its rated R-value at full recovered thickness.
- The purlin line is where the sheeting pinches the blanket, and it repeats across the whole roof.
- A spacer lifts the sheet clear of the purlin so the blanket keeps its loft over the support.
- Safety mesh supports the blanket between purlins. It does not stop crushing at the purlin.
- Spacer height comes from the blanket thickness, not from the roof pitch.
This is one of those problems that costs money quietly. The roof looks finished, the invoice says R2.3, and nobody finds out that a good part of that R2.3 was never delivered. It is worth ten minutes at the ordering stage.
Two problems, one word
| Crushing at the purlin | Drooping between purlins | |
|---|---|---|
| What it looks like | Blanket squeezed to nothing in a line along every purlin | Blanket bellying down into the roof space between supports |
| Cause | The sheeting is fixed straight down onto the purlin with the blanket in between | Nothing underneath holding the blanket up over the span |
| Fix | A spacer or batten that lifts the sheet off the purlin | Safety mesh, or a support system under the blanket |
| Cost if ignored | Lost R-value in a repeating line across the whole roof | Untidy, and it can pull the blanket away from the laps |
Most conversations about sag are really about the first one, even when people describe the second. Getting the diagnosis right decides which product you need.
Crushing at the purlin
Bulk insulation works by holding still air in the fibre. Squeeze the air out and the thermal resistance goes with it. That is why every product data sheet ties the rated R-value to the recovered thickness of the blanket.
Now picture the roof. The blanket is laid over the purlins, the sheeting is screwed down through it into the purlin, and at every purlin line the blanket is pinched between two pieces of steel. On a roof with purlins at 1200 mm centres, that is a compressed line every 1200 mm for the length of the building.
A blanket rated at R2.3 that is squeezed flat at each of those lines is not delivering R2.3 across the roof, and buying a thicker blanket does not fix it. A thicker blanket squashed to the same nothing at the purlin performs like a thinner one, and you have paid more for the privilege.
That is the entire argument for a spacer, and it is why we mention it on nearly every roof job.
Spacers
A spacer is a formed steel section that sits on the purlin and holds the roof sheet clear of it. The blanket passes underneath and keeps its full thickness right across the support instead of being pinched.
The Roof Razor spacer we stock is galvanised steel, 1200 mm long to match a standard blanket width, and comes pre-loaded with its fasteners. Six pieces per pack gives 7.2 lineal metres. Its dead load capacity is 205 kg between the legs regardless of span, and because it is steel it adds no fuel load to the roof.
Two details worth knowing before it goes on the order:
- The purlin has to suit it. The standard spacer needs a minimum 1.2 BMT purlin with a full 50 mm flat surface, because a narrower flange will not take the fixing pattern.
- Wind region matters. The standard version covers wind regions A and B. There is a cyclonic version for regions C and D.
- Timber battens are a different order. If you are fixing to timber rather than steel, order the version without pre-loaded screws.
Top hat battens
The other way to get the sheet off the purlin is to run a batten system over the top, which lifts the whole roof plane rather than just the fixing line.
It changes the job in a few ways. You gain a continuous cavity above the insulation rather than a local lift at each purlin, which suits build ups where an air gap is part of the design, for example where a reflective face needs a gap in front of it to work at all. You also add a layer of material, labour and height to the roof, and the fixings get longer.
The choice usually comes down to what else the build up is doing. Where the only problem is the pinch at the purlin, a spacer is the cheaper and faster answer. Where the design wants a ventilated or reflective cavity across the whole plane, battens earn their cost.
What mesh does and does not do
This is where the two problems get confused, so it is worth being precise.
Roof safety mesh is a fall protection system first. On a commercial roof it also carries the blanket, which means it solves the second problem: the blanket sits on the mesh and does not belly down into the space between purlins.
What it does not do is stop crushing. The mesh runs across the purlins and is fixed to them, so at the purlin line the blanket is still between the sheeting above and the structure below. In fact a roof with mesh under the blanket can make the pinch worse, because the sheeting presses the blanket down onto the mesh at every support.
So on a commercial roof you often need both: mesh to hold the blanket up over the span, and a spacer to keep the sheet off the purlin. They are not alternatives. Our guide to roof safety mesh requirements covers the fall protection side.
Choosing the spacer height
Take the height from the blanket you are using, not from the roof pitch. This trips people up because pitch feels like it should matter and it does not.
| Spacer height | Suits blanket | Typical EcoRoof |
|---|---|---|
| 60 mm | 75 to 80 mm | 75 mm at R1.8 |
| 85 mm | 100 to 120 mm | 100 mm at R2.3 |
| 115 mm | Thicker build ups | 130 mm at R3.2 |
If you are still choosing the blanket, our guide to roofing blanket R-values works through the steps, and metal roof insulation in Australia shows where the spacer sits in the layer order.
What to check before you order
- Purlin type and thickness. Steel or timber, and at least 1.2 BMT with a 50 mm flat for the standard spacer.
- Purlin centres. They set how many lineal metres of spacer the roof needs.
- Wind region. A and B take the standard version, C and D need the cyclonic one.
- Blanket thickness. This picks the spacer height.
- Is there mesh? If yes, you are solving both problems and you need both products.
- Is a reflective face involved? A foil face needs an air gap in front of it to do anything, which may push the answer toward battens.
What goes wrong
- Buying a thicker blanket to beat the pinch. It gets crushed at the same line and costs more.
- Assuming mesh solves compression. It solves the span, not the purlin line.
- Sizing the spacer off the roof pitch. It comes from the blanket thickness.
- Ordering pre-loaded screws for timber battens. Different version, and it holds up the install.
- Ignoring the wind region. Regions C and D need the cyclonic spacer, and that is not a preference.
- Leaving it to the roofing day. Spacers go on before the sheets. After that it is a strip and re-lay.
We supply the package, your roofer lays the blanket
AWBM supplies roofing blanket, spacers, safety mesh and thermal break across Australia. Blanket and spacers go on with the sheeting, so they are supply items and your roofer fits them. Batts, wall wrap and sarking we install. Send the roof area, the purlin centres and the wind region and we will work out the spacer height, the quantities and a price.
Or browse roofing blankets, roofing and safety mesh and accessories.
Frequently asked questions
How do you stop insulation sagging under a metal roof?
It depends which failure you have. To stop the blanket being crushed at the purlin, use a spacer or a batten system that lifts the sheeting clear of the support. To stop it drooping between purlins, use safety mesh or another support under the blanket. Commercial roofs usually need both.
What is a roof insulation spacer?
A formed steel section that sits on the purlin and holds the roof sheet clear of it, so the blanket underneath keeps its full thickness across the support instead of being pinched between sheeting and purlin.
Does safety mesh stop insulation being crushed?
No. Mesh holds the blanket up between the purlins, but at the purlin line the blanket is still caught between the sheeting above and the structure below. A roof with mesh can make the pinch worse, so mesh and a spacer do different jobs.
What size spacer do I need?
Take it from the blanket. In the Roof Razor range, 60 mm suits 75 to 80 mm blanket, 85 mm suits 100 to 120 mm, and 115 mm is for thicker build ups. Roof pitch does not decide it.
Will a thicker blanket fix compression at the purlin?
No. A thicker blanket squashed flat at the same line performs like a thinner one, so you pay more and gain little. Lift the sheet off the purlin instead.
Can I fit spacers to timber battens?
Yes, but order the version without pre-loaded screws, because the pre-loaded fasteners are for steel purlins.
Do I need a different spacer in a cyclonic area?
Yes. The standard version covers wind regions A and B, and there is a cyclonic version made for regions C and D.
Can spacers be added to an existing roof?
Not without lifting the sheeting, because they sit between the sheet and the purlin. It is a decision that has to be made before the roof is closed.
Can AWBM work out what my roof needs?
Yes. Send the roof area, the purlin centres and type, the wind region and the blanket you are using, and we will come back with the spacer height, the quantities and a price.
Sources
- Australian Steel Institute, Building with Steel Factsheet #3, Blanket and Foil insulation in Metal Roofing
- AS/NZS 4859.1, Thermal insulation materials for buildings, for declared R-values at recovered thickness
- AS/NZS 4389:2015, Roof safety mesh
- Spacer dimensions, heights, purlin requirements, wind region versions and load capacity from the Roof Razor product data
- Blanket thicknesses, R-values and roll coverage from the EcoRoof product data
General information. Spacer selection depends on the purlin section, the wind region and the roof design, so confirm the structural requirements and the manufacturer's current specification for your own roof before ordering.