Think of it as a layered system
From the outside, a roof looks like a single surface of tiles or steel. Underneath, a well-built roof is a stack of layers, each doing a different job. When people talk about roof underlayment and insulation, they usually mean three things:
- Sarking, sometimes called reflective foil or roof membrane, laid directly under the roofing.
- A roof blanket, an insulation layer laid over the battens under steel sheeting.
- Ceiling insulation, usually batts, lying on top of the ceiling inside the roof space.
These layers sit on either side of the roof space, which itself needs some airflow. Together they shape how hot your house gets in February, how loud the rain sounds in July, and whether moisture builds up where you cannot see it.
Sarking: the second line of defence
Tiles are not a sealed surface. Wind-driven rain, dust and the odd ember can work their way between them, especially during the strong north-westerly and south-westerly winds that come with Perth's winter cold fronts. Sarking is a flexible membrane that sits under the tiles, catching that water and guiding it into the gutter.
Many sarking products have a reflective foil face. When installed with an air gap, that foil reflects radiant heat back towards the roof, which helps keep the roof space cooler in summer. Sarking is also used under some steel roofs, particularly where a roof blanket is not used.
When sarking may be required
Sarking is not always mandatory, but there are situations where it may be needed. In bushfire-prone areas mapped by DFES, new roofing work may need to meet a Bushfire Attack Level under AS 3959, which can include requirements for sarking and ember protection. Some roof pitches and tile types also call for it. Your roofer and your local government can confirm what applies to your property.
Roof blankets: made for steel
A steel roof heats up quickly in the sun and loses heat quickly at night. Without anything underneath, that can mean a hot roof space in summer, condensation on the underside of the sheets in winter, and noticeable noise in heavy rain or hail.
A roof blanket deals with all three. It is typically a layer of glasswool insulation with a foil facing, rolled out over the battens just before the sheets are fixed. The insulation slows heat transfer, the foil adds reflective benefit and acts as a vapour barrier, and the bulk of the blanket absorbs much of the drumming sound of rain.
Roof blankets come in different thicknesses and ratings. Thicker blankets generally insulate better but need care at the edges so sheets still sit flat and gutters still align. Most of the contractors we work with fit a roof blanket as standard on residential COLORBOND roofing jobs, and it is far easier to install during a re-roof than at any other time.
Ceiling batts: the layer closest to you
Ceiling insulation sits on the plasterboard, between the ceiling joists. Its job is to slow heat moving between the roof space and your living areas. In summer, it keeps the hot air of the roof space from radiating down into bedrooms; in winter, it holds warmth in.
Batts are usually installed separately from the roof itself, and they are not strictly part of a re-roof. But a re-roof is a good moment to have someone check them. Years of trades crawling through the roof space, pests and leaks can leave batts flattened, damp or patchy, which reduces how well they work.
How the layers compare
| Layer | Where it sits | Main jobs |
|---|---|---|
| Sarking or reflective foil | Directly under tiles or steel | Catches wind-driven water, reflects radiant heat, can help with embers |
| Roof blanket | Over battens, under steel sheets | Insulates, reduces rain noise, limits condensation |
| Ceiling batts | On top of the ceiling | Slows heat moving into and out of living areas |
Moisture, condensation and airflow
Perth's dry summers can make it easy to forget about moisture. Winter is different. Most of the year's rain falls between May and September, and on cold, still nights the underside of a steel roof can drop below the temperature at which water condenses out of the air. Without a roof blanket or proper ventilation, droplets can form and drip onto ceilings and insulation.
Signs worth investigating include:
- brown stains or spotting on ceilings that are not directly below an obvious leak;
- damp or mouldy patches on batts;
- rusting fasteners or marks on the underside of steel sheets;
- a musty smell in the roof space.
Good underlayment works hand in hand with roof-space airflow. Our article on roof ventilation in Perth explains how whirlybirds, ridge vents and eave vents fit in. If you are already seeing stains, our guide to roof leaks and water damage can help you work out whether it is a leak or condensation.
Heat: where the layers really earn their place
Long runs of very hot days are a normal part of a Perth summer, and strong UV beats on roofs for months. Roof colour matters, and lighter COLORBOND colours with Thermatech solar-reflectance technology reflect more of the sun. But even a light roof gets hot. What stops that heat reaching your rooms is the combination of reflective layers, bulk insulation and a roof space that can shed hot air.
Tile roofs behave a little differently. Concrete and terracotta tiles soak up heat through the day and release it slowly into the evening, so a tiled house can stay warm well after sunset. Reflective sarking under the tiles, combined with ceiling batts and some way for hot air to escape the roof space, helps take the edge off that late-afternoon build-up.
If you are switching from tiles to steel, the change in how the roof handles heat is a good reason to think about the full stack of layers rather than just the sheeting. A light-coloured roof over a decent blanket and well-laid batts will usually feel noticeably different from a dark roof with nothing underneath.
Safety notes for older roofs
Some older Perth homes may have roofing or other materials in the roof space that contain asbestos, particularly homes built between the 1950s and 1980s. Do not disturb old sheeting, lagging or unknown loose-fill material yourself. WorkSafe WA requires a licensed asbestos removalist for removing more than 10 square metres of non-friable asbestos, and an assessment before work starts is sensible. Our asbestos roof replacement page explains how this is usually handled.
Working in roof spaces is also hot and risky in summer. Leave it to trades who know how to move safely across ceiling joists.
What to ask when getting quotes
When you compare re-roof quotes, check that each one is clear about the hidden layers:
- Is sarking or a roof blanket included, and which product or rating?
- Will any existing sarking be replaced or patched?
- How will the roof space be ventilated?
- Will ceiling batts be inspected, topped up or left alone?
For a sense of how these items affect the overall price, see our roof replacement cost guide.
Plan the layers with your roofer
The best time to get underlayment and insulation right is while the roof is off. Roofing Perth can arrange a free on-site quote from an independent local roofer who will look at your roof space and suggest the right combination for your home. Request a quote.