Installing a Roofing Blanket: Step by Step

Roofing blanket goes on in one window: after the structure is up and before the sheets go down. The method is not complicated, but almost everything that goes wrong on a metal roof is decided in that window, and none of it can be fixed afterwards without lifting sheets. This is the sequence, the details that matter, and what to check before the sheeter starts.

This is a practical guide, not a safe work method statement. Working at height is high risk construction work and the controls for your site come from the WHS regulations and the SWMS for the job, not from an article. Read our guide to roof safety mesh requirements alongside this.

Before you start

Five things decide whether the day runs smoothly, and all of them are checkable the week before.

1. Is the whole package on site? Blanket, mesh, spacers, thermal break and tape. Missing one item stops the sheeting crew.
2. Do the spacer heights suit the blanket? Take the height from the blanket thickness, not the roof pitch.
3. Do the spacers suit the purlins? The standard Roof Razor needs a minimum 1.2 BMT purlin with a full 50 mm flat, and there is a separate cyclonic version for wind regions C and D.
4. Steel or timber? Pre-loaded screws are for steel purlins. On timber battens, order the version without them.
5. Is there somewhere dry? Blanket is light and very bulky, and it must not get wet before it goes up.

The sequence

On a residential roof without mesh the sequence is shorter but the principle is identical: everything under the sheet goes on before the sheet.

Laying the blanket

Foil face down. The foil goes to the inside of the building. It is doing two jobs there: it faces the air space so the reflective surface can work, and it sits on the warm side where it can keep interior moisture off the cold underside of the sheeting.

Run with the roof. Roll out across the purlins, working so each run laps over the one before it in the direction water and gravity would take it, not against.

Do not stretch it and do not compress it. A blanket only delivers its rated R-value at its recovered thickness. Pulling it tight over the purlins to make it look neat is exactly the wrong instinct.

Keep it clear of wiring. The foil facing conducts electricity. This is a live hazard during the build, not a theoretical one.

Cut cleanly around penetrations. Skylights, vents, flues and plant platforms all want a proper cut and a sealed edge rather than a torn hole. This is where roofs leak air and vapour.

Laps and sealing

The lap is where the blanket becomes a continuous layer instead of a set of separate strips, and it is the step most often rushed.

Our EcoRoof blanket is 1200 mm wide with a 150 mm foil lap on every run. Lap the foil edge over the previous run, then seal it. An unsealed lap is a gap in the vapour layer, and on a roof that gap runs the length of the building.

Two consequences worth planning for:

• Coverage is not roll coverage. Every run loses 150 mm to the lap, so order against effective width.
• Tape is part of the order. It is not an accessory, and it is the difference between a thermal layer and a working vapour control layer.

Where a separate reflective membrane is used instead of a faced blanket, BlueScope's guidance is to drape it between the battens with at least 20 mm of air space above and below rather than pulling it tight to the sheeting, and to use a membrane with a vapour barrier classification to AS/NZS 4200.1:2017. Installation of membranes is covered by AS 4200.2:2017.

Spacers and the purlin line

The purlin line is where the roof either delivers its R-value or quietly does not.

Without a spacer, the sheeting is fixed straight down onto the purlin with the blanket in between, so the blanket is pinched flat in a line at every support. A blanket rated R2.3 crushed at each of those lines is not delivering R2.3 across the roof, and a thicker blanket squashed to the same nothing does not fix it.

The spacer sits on the purlin and holds the sheet clear, so the blanket keeps its loft right across the support. Match the height to the blanket: 60 mm suits 75 to 80 mm blanket, 85 mm suits 100 to 120 mm, and 115 mm is for thicker build ups. The full treatment is in stopping insulation sag.

If the roof needs a thermal break under NCC 2022 Housing Provisions clause 13.2.3(7), that is a separate item on the purlin line and the spacer does not replace it.

Weather and storage

This is the most common way a good roof package goes wrong, and it happens on the ground rather than on the roof.

• Store it dry and off the ground. Blanket is bulky, so it takes more covered space than people allow for.
• Do not lay ahead of the sheeting crew. Exposed blanket and an afternoon storm is a write off, not a delay.
• Wet blanket is replaced, not dried. It loses its loft, does not recover its R-value, and holds water against the steel.
• Watch the wind. A 1200 mm wide roll is a sail. It is a handling problem before it is a quality problem.
• Stage large deliveries. If the site cannot store the whole roof dry, split the delivery to the sheeting program.

What to check before sheeting

Ten minutes here is worth more than anything else on this page, because it is the last moment anything can be changed.

1. Mesh continuous over the whole area to be roofed, laps correct for the purlin spacing, no missed fixings.
2. Blanket foil face down, no runs laid the wrong way up.
3. Every lap lapped and sealed, including the awkward ones at hips and valleys.
4. Nothing compressed: no pulled runs, no blanket pinched where a spacer should be.
5. Spacers on every purlin line, at the right height, correct version for the frame material.
6. Thermal break in place where the clause applies.
7. Penetrations cut properly and sealed rather than torn.
8. Nothing wet.

What goes wrong

• Foil face up. Easy to do on the first run and very expensive to correct after sheeting.
• Unsealed laps. A vapour layer with a continuous gap in it is not a vapour layer.
• No spacers. Crushed at every purlin, across the whole roof.
• Laid too far ahead of the sheeters. One shower and the package is compromised.
• Torn penetrations. The sealed edge matters more here than anywhere else on the roof.
• Ordering against roll coverage. Every run loses width to the lap.
• Leaving the thermal break to the sheeting day. It goes under the sheet, so there is no later.

Frequently asked questions

Sources

General information, not a safe work method statement. Work at height is high risk construction work and the fall protection, SWMS and licensing requirements for your site are set by the WHS regulations in your state or territory. Follow the manufacturer's installation instructions for the products you are using.