Chemical delignification

Your roof battens are quietly going soft. Not rotting — dissolving. In Perth, whether that counts as a major structural defect is worth thousands of dollars, because it decides whether the seller has to fix it.

What is actually happening up there

Timber is fibre held together by lignin, a natural polymer that acts as the glue and gives the wood its rigidity. Airborne salts and pollutants, with moisture, break that lignin down. The fibres come unglued.

The surface goes frayed and stringy. Inspectors call it hairy timber; the older term is defibrosis. Fibres shed and drift down onto the insulation below. Push a thumbnail into an affected batten and the surface crushes like felt.

Is it a major structural defect?

AS 4349.1 names it. Appendix F of the standard, which sets out types and examples of defects, lists delignification severe enough that the strength of the member has been reduced as an example of a material deterioration defect. That is the standard's own wording category, not an inspector's opinion.

Whether a particular instance is a major defect turns on the definition in clause 1.4.10: a defect of sufficient magnitude that rectification has to be carried out to avoid unsafe conditions, loss of utility, or further deterioration of the property. Battens hold the tiles up. Where they have lost strength, all three limbs of that definition are live.

This matters because the answer decides who pays. It is also why we put our position in writing rather than leaving it vague, and why the position is signed by a registered building contractor — BC105819.

Where it happens, and why

125 of the 338 suburbs we cover sit within 5 km of the coast, the band where salt-laden air moves through roof spaces year round. That is 37% of the service area. Exposure falls away inland, but falls away is not the same as stops.

The coastal band is also where tie-down straps corrode, where mortar frets, and where wall ties give up. Salt does not attack one component at a time.

The forty closest to the coast, of 125 classified HIGH. Every suburb page on this site opens with its own distance and exposure band.

What we do about it

Where there is safe access we enter the roof space and look at the battens directly, because this defect cannot be seen from the ground and cannot be inferred from the tiles. From the street a roof with failing battens looks exactly like a roof without.

Affected timber is photographed and located. Where the extent cannot be established from the accessible part of the roof space, the report says so rather than extrapolating.

We do not walk affected roofs, and we would advise you not to either. Where the fixings have lost their hold, foot traffic is how a suspected problem becomes a repair bill.

Common questions

Is chemical delignification a major structural defect?

AS 4349.1 lists delignification severe enough to reduce the strength of the member as an example of a material deterioration defect (Appendix F). Whether any particular instance meets the standard's definition of a major defect depends on whether rectification has to be carried out to avoid unsafe conditions, loss of utility or further deterioration.

What causes it?

Airborne salts and pollutants, with moisture, break down the lignin that binds timber fibres together. The fibres come unglued and the surface goes soft and stringy. It is not rot and it is not termite damage.

Which Perth suburbs are affected?

Exposure is highest within about five kilometres of the coast. 125 of the 338 suburbs we cover sit inside that band.

Can you walk on an affected roof?

Not safely, and not without risking further damage. Where battens are breaking down the fixings lose their hold, and foot traffic is how a suspected problem becomes a hole.

This comes up on every pre-purchase inspection and building and timber pest inspection we carry out in the coastal band. It also intersects with roof tie-downs, because the same salt that softens a batten corrodes the strap next to it.