A warehouse roof is not a big house roof. It is one enormous volume with wide purlin spacing, everything is installed from above before the sheets go on, and every decision is multiplied by thousands of square metres. Get the build up right and it is a straightforward job. Get the sequence wrong and the only fix is lifting sheets over an area the size of a football field.
In short
- Mesh, blanket and spacers all go under the sheeting. One window, before the roof closes.
- Wide purlin spacing changes the mesh lap rules and the amount of material you need.
- Effective coverage, not roll coverage, is what you order against once laps are counted.
- A big volume traps a lot of moist air, so ventilation is part of the roof, not an extra.
- Commercial specifications name fire classifications as well as R-values. Both have to be met.
This guide covers the roof itself. For how a commercial specification differs from a residential one, and what documentation a builder will want, see commercial vs residential insulation specification.
What makes it different
| House roof | Warehouse roof | |
|---|---|---|
| Purlin or batten spacing | Close, often under a metre | Wide, commonly well over a metre |
| Roof space | Divided by hips, valleys and bulkheads | One continuous volume, often the whole building |
| Insulation position | Usually at ceiling level, sometimes under the sheeting | Under the sheeting, because there is no ceiling |
| Fall protection | Risk assessed | Safety mesh is standard practice |
| Consequence of an error | A few square metres | Thousands of square metres |
The last row is the one that matters commercially. On a house you can absorb a mistake. On a 5,000 square metre roof, ordering against the wrong coverage figure or missing the thermal break is a five figure problem.
The standard build up
Working from the inside out, a typical insulated steel warehouse roof runs:
- Safety mesh laid across the purlins and fixed, providing fall protection and carrying the blanket.
- Roofing blanket rolled out over the mesh, foil face down into the building.
- Spacers on the purlin line, holding the sheeting clear so the blanket keeps its thickness at every support.
- Thermal break where the code requires it between sheeting and metal purlin.
- Sheeting fixed through the lot.
Every one of those layers is above the purlin and below the sheet, which is why the whole package has a single install window. The order is not adjustable and neither is the timing.
Options and trade-offs
| Decision | Options | What it turns on |
|---|---|---|
| Blanket thickness | 55 mm at R1.3 through to 130 mm at R3.2 in our EcoRoof range | The specification, and whether the spacer height suits the thickness |
| Faced or unfaced | Foil faced blanket, or a separate membrane | Whether the foil is doing the vapour control, and whether it is visible from below |
| Blanket or separate membrane | One product doing both, or two doing one each | Cost against control. A separate membrane can be a higher vapour class |
| Spacer or top hat batten | A spacer at each purlin, or a batten system across the plane | Whether the design wants a continuous cavity or just a local lift |
| Ventilation | Wind driven, powered, or hybrid | Site exposure, and whether the building has a management system to tie into |
The one people underestimate is the spacer. A blanket rated R2.3 pinched flat at every purlin is not delivering R2.3 across the roof, and on a wide span roof that compressed line repeats hundreds of times. Our guide to stopping insulation sag works through it.
Quantities at scale
This is where commercial ordering goes wrong, and it is always the same mistake: ordering against roll coverage instead of effective coverage.
Mesh. A 1800 mm wide roll with a 150 mm side lap gives about 1650 mm of effective cover per run, so a 90 square metre roll covers roughly 82 square metres once lapped. On wide purlin spacings needing a 300 mm lap, allow closer to 1500 mm effective width. AS/NZS 4389:2015 ties the side lap to the purlin spacing, so the wider the span, the more of your roll goes into laps.
Blanket. EcoRoof is 1200 mm wide with a 150 mm foil lap on every run, so the same arithmetic applies. Roll coverage steps down as thickness goes up: 18 square metres at 55 and 75 mm, 12 square metres at 100 and 130 mm. A high R specification means noticeably more rolls for the same roof, not just a dearer roll.
Spacers. These are ordered in lineal metres, driven by purlin centres and roof length rather than area. Six 1200 mm pieces per pack gives 7.2 lineal metres.
Then add end laps, penetrations, skylights and plant platforms on top. Our guide to working out quantities has the method, and on a job this size it is worth having someone check the take off rather than scaling up a house calculation.
Condensation in a big volume
Large sheds have a condensation problem that houses do not, and it catches people out because the building is not obviously wet.
A metal roof cools at night by radiating to the sky and can end up below the temperature of the air beneath it. In a house, the moisture load is showers and cooking. In a warehouse it can be a wash bay, a commercial kitchen, forklift exhaust, curing product, or simply a very large volume of air with nowhere to go.
What comes out of that is water on the underside of the sheeting, dripping onto stock, and corrosion on a roof that has never leaked. The blanket with a foil face is doing vapour control work as well as thermal work, and where a higher vapour class is required a separate membrane is the answer. Our diagnostic guide is rust, mould and wet insulation.
Ventilation
On a building this size ventilation is part of the roof design rather than an accessory, and the still hot afternoon is the case that decides the product.
A wind driven turbine costs nothing to run and delivers nothing when there is no wind, which is often exactly when a large shed most needs air moved. A powered unit is reliable but runs up energy cost across a big installation. The hybrid answer is a turbine with a motor that cuts in when the wind will not do the job, which is what the Bradford EcoPower 400 mm does with a Hurricane turbine and an EC motor.
On a managed building the control side matters too. Air iQ responds to temperature and humidity, and 0-10 V BMS integration lets the ventilation be part of a strategy rather than an independent hole in the roof. Whichever you choose, the intake has to exist: an extractor can only move air that is getting in somewhere else. Our comparison is in which roof ventilator actually works.
Fire and the specification
Commercial work adds a layer that residential rarely has. The specification will often name a fire classification alongside the R-value: a Group number to AS 5637.1, a spread of flame index, a smoke development index, or a requirement for non-combustible material.
Meeting the thermal number alone is not enough. A product with the right R-value and no Group classification can be rejected, and that is a substitution problem rather than a performance one. NCC 2022 Volume One Part J4 sets the building fabric requirements for the classes it covers, and Total R-value is calculated to AS/NZS 4859.2 rather than read off the pack.
Sequence and program
The roof package has a hard deadline that nothing else in the insulation scope has.
- Frame up. Mesh, blanket, spacers and thermal break should already be ordered and on site or scheduled.
- Mesh laid and fixed. Continuous over the area to be roofed, no unlapped runs, no missed fixings.
- Blanket and spacers. Following the sheeting crew closely, because exposed blanket must not get wet.
- Sheeting. After this, nothing under the sheet can be changed.
Two practical points. Insulation is light but very bulky, so a big roof package needs somewhere dry to store it and often a staged delivery to match the sheeting program. And glasswool that gets rained on before it goes up is a problem you then build permanently into the roof.
What goes wrong
- Ordering against roll coverage. Laps eat a real percentage on both mesh and blanket.
- No spacers. The blanket is crushed at every purlin across the whole roof.
- Missing the thermal break. Easy for a certifier to check and impossible to add later.
- Treating ventilation as an accessory. A big volume with no air movement is a condensation machine.
- Substituting on R-value alone. The specification's fire classification is a separate requirement.
- One delivery for the whole roof. Bulky product, limited dry storage, and a sheeting program that will not wait.
We supply the package, your roofer lays the blanket
AWBM supplies commercial roof packages across Australia: safety mesh, roofing blanket, spacers, thermal break, membranes and ventilators. The roof package is supply, because all of it goes down ahead of the sheeting crew and they lay it. Batts, wall wrap and sarking we install. Send the roof area, the purlin centres and the specification and we will do the take off and quote it, staged to your sheeting program.
Or browse commercial insulation, roofing and safety mesh and roofing blankets.
Frequently asked questions
What is the best insulation for a warehouse roof?
A roofing blanket laid over safety mesh and held at full thickness by spacers, with a thermal break where the sheeting fixes to metal purlins. Thickness comes from the specification, and a separate membrane is added where a higher vapour class is required.
Does a warehouse roof need safety mesh?
Safety mesh is standard practice on commercial and industrial roofs, where purlin spacing is wide and the fall is a long one. It also carries the blanket. It must comply with AS/NZS 4389 and be used with edge protection or fall arrest.
How much mesh and blanket do I need for a large roof?
Order against effective coverage, not roll coverage. A 1800 mm mesh roll with a 150 mm side lap gives about 1650 mm of effective width, so a 90 square metre roll covers roughly 82 square metres. Blanket loses width to a 150 mm foil lap on every run.
Why does my warehouse roof drip when it has not rained?
Condensation. The steel cools at night by radiating to the sky and drops below the temperature of the air below it, and moisture in that air condenses on the underside. In a large building the moisture load can come from wash bays, kitchens, forklifts or the sheer volume of air.
Do spacers really matter on a commercial roof?
Yes, more than on a house, because the compressed line at each purlin repeats across a very large area. A blanket squashed flat at every support is not delivering its rated R-value across the roof.
Can insulation be added to an existing warehouse roof?
Not under the sheeting without lifting sheets. Options for an existing building are limited to what can be applied from below or to improving ventilation, so it is worth getting the build up right the first time.
What fire requirements apply to commercial insulation?
Specifications often name a Group number to AS 5637.1, a spread of flame index, a smoke development index or non-combustible material, alongside the R-value. A product that meets the thermal number but carries no fire classification can be rejected.
When does the roof insulation need to be on site?
Before the roof is sheeted. Mesh, blanket, spacers and thermal break all sit under the sheeting, so once the sheets are down the only way to add them is to lift sheets.
Can AWBM quote a full warehouse roof package?
Yes. Send the roof area, the purlin centres and type, the wind region and the specification, and we will do the take off, work out the staged delivery to suit your program, and quote it. The roof package is supplied for your roofing crew to lay. Where the same building has batts, wall wrap or sarking in it, that part we can install.
Sources
- NCC 2022 Volume One, Part J4 Building fabric
- NCC 2022 Housing Provisions, clause 13.2.3(7) thermal break, and Part 10.8 Condensation management
- AS/NZS 4389:2015, Roof safety mesh, for the side lap rules by purlin spacing
- AS/NZS 4859.2, Thermal insulation materials for buildings, calculation of Total R-value
- AS 5637.1, Determination of fire hazard properties, for the Group number classification
- BlueScope Corrosion Technical Bulletin CTB-11, Roof Condensation and Insulation, October 2022
- Roll widths, coverage, lap allowances and ventilator specifications from the product data for the products listed above
General information. Commercial roof design, fall protection and fire requirements depend on the building class, the structure and the project specification, so confirm the requirements for your own building with the designer, the engineer and the relevant regulator before ordering.