Why You Should Never Waterblast an Old Decramastic Roof Without Asbestos Testing

August 21, 2026

Why You Should Never Waterblast an Old Decramastic Roof Without Asbestos Testing

Waterblasting an old roof might seem like a straightforward way to remove moss, lichen and years of built-up dirt. But if the roof contains asbestos, that simple cleaning job can turn into a much larger contamination problem.

WorkSafe New Zealand has recently highlighted incidents involving waterblasting of asbestos-containing roofs, including a residential property with Decramastic roof tiles where asbestos-containing debris was spread around the property and into the home.

It is an important reminder: before cleaning, repairing, painting or otherwise disturbing an older roof, find out what it is made from.

What Are Decramastic Roof Tiles?

Decramastic roofing is a type of pressed metal roofing tile commonly seen on older New Zealand homes.

The metal tiles were coated with a bituminous material and stone-chip finish. Some older Decramastic roofing products contain chrysotile (white) asbestos within the bituminous coating .

You cannot reliably determine whether a particular roof contains asbestos simply by looking at it.

If there is any doubt, representative sampling and laboratory analysis can confirm whether asbestos is present.

Why Is Waterblasting Such a Problem?

Asbestos-containing materials that are in good condition and left undisturbed may present a relatively low risk.

The situation changes when the material is aggressively disturbed.

A high-pressure waterblaster can damage and strip the weathered coating from Decramastic tiles. If that coating contains asbestos, fragments and fibres can be carried with the water and debris away from the roof.

Instead of having asbestos contained within one building material, you can potentially create contamination across other areas of the property.

Where Can the Contamination Go?

Water and debris coming off a roof do not necessarily stay in one place.

Depending on the property and how the work is carried out, asbestos-containing debris may potentially reach:

  • Gutters and downpipes

  • Exterior walls and building surfaces

  • Paths, decks and driveways

  • Lawns, gardens and surrounding soil

  • Roof spaces

  • Open windows and doors

  • Interior areas of the home

WorkSafe advises that gutters and downpipes associated with asbestos-containing roofs may themselves contain asbestos contamination from the roofing material.

Once asbestos-containing debris has been spread around a property, assessing and cleaning up the contamination can become considerably more complicated than dealing with the original roofing material.

A Cheap Roof Clean Can Become an Expensive Problem

This is one of the biggest reasons to identify suspect materials before work starts .

Having a representative sample professionally collected and analysed is a relatively straightforward process.

Discovering afterwards that asbestos-containing material has been waterblasted across a property is a very different situation.

The affected areas may need to be assessed, access may need to be restricted, and specialist asbestos remediation may be required.

The cost and disruption can potentially be far greater than simply identifying the roofing material before work begins.

WorkSafe’s Position on High-Pressure Waterblasting

WorkSafe New Zealand states that high-pressure water blasting must not be used to clean asbestos-containing roofing or cladding.

WorkSafe defines high-pressure water spray in this context as equipment capable of pressures greater than 350 kPa (approximately 50 psi).

This isn’t simply a recommendation about protecting an old roof. High-pressure water blasting can disturb asbestos-containing material and spread asbestos fibres and debris into the surrounding environment.

Don’t Assume an Old Decramastic Roof Is Asbestos-Free

Not every Decramastic-style roof will necessarily contain asbestos.

But appearance alone isn’t enough to confirm that a roof is asbestos-free.

If you’re planning to clean, repair, repaint, replace or otherwise disturb an older Decramastic roof, identifying the material before the work begins can prevent a much bigger problem later.

The same principle applies to many materials in older New Zealand buildings: test before you disturb.

Need an Asbestos Roof Tested?

Asbestos Safe Consultancy provides independent asbestos inspections and sampling throughout Hawke’s Bay, Gisborne, Taupō and the Manawatū.

If you have an older Decramastic roof and aren’t sure whether it contains asbestos, we can inspect the material, collect representative samples where appropriate and arrange analysis through an IANZ-accredited laboratory.

Knowing what you’re dealing with before work starts is far easier than finding out after the material has already been disturbed.

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