Chemical Delignification can occur to timbers within the ceiling space and the damage can be minor or quite the opposite if the timber becomes defibrated.
It can appear to look like hairy timber and often occurs under old terracotta roof tile battens but can be seen on other structural timber within the roof space. The photo below shows a timber purlin under a metal roof in Port Adelaide.
The lignin, which is the natural glue that holds wood fibres together, can break down due to moisture ingress, or by the chemical environment due to the air quality of the area over time, or just from proximity to the ocean.
When the fibres begin to detach the timber begins to look hairy or defibrated.
Often the hairy timber is just surface damaged and not a structural issue, I have also however in some roof areas, been able to crush the timber with my hand and pull off chunks of timber.
Chemical delignification is not a termite or pest issue, so may not be noted in a termite report, but a good building inspector should note it on a building report, if they fully inspect the ceiling space.
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​Most hot water systems with a glass lined steel tank will generally only last between 8 to 10 years once installed, the reason for the failure is normally due to corrosion of the tank walls, because the sacrificial anode has been depleted. The anodes are steel core rods surrounded with either magnesium, aluminium or zinc and are suspended inside at the top of hot water tanks. Due to a process called electrolysis, the sacrificial anodes are designed to corrode before the steel tank. The anodes should be changed over every 3 to 5 years and the steel tank life may be extended to around 20+ years, but hardly anyone knows that this can be done. Anodes only cost between $50 to $90 to buy and DIY install or a plumber may charge around $150 to supply and install them. It is worth noting that some hot water systems have two anodes, not one. Before attempting to replace the anodes, read the documentation relevant to your heater for instructions and be aware you may void your warranty unless you are a licensed plumber.
Most stainless steel tanks still require a sacrificial anode, even Solahart units have them and recommend servicing them regularly to prolong the useful life of the hot water system.
Finding out the age of the hot water system is often difficult as most companies have a different serial number code system, although some manufacturers have it listed on the pull out panel where you can relight the burner on a gas unit. Often external electric units have a label that has faded or peeled off, due to sunlight or just the weathering of the unit.
Rinnai hot water systems often have a 10 figure serial number, the first two digits are the month, the next two are the year, Rheem can be shown the same way, but this may differ on some models. Thermann show the year first then the month of manufacture. So often it is difficult to work out the date of manufacture. Older houses rarely have the original hot water service and if they do, they probably require replacing.
Instantaneous units have no tanks and should last 20 years minimum. Most houses that have replaced the original tank systems, install an instantaneous system as they are cheaper to run, purchase and do not run out of hot water.
The external wall mounted instantaneous systems also often have no label to show the date of manufacture, as they have peeled off or faded.
So its best to allow for a hot water replacement in your plans, if you're buying an older established home and a hot water tank is still in use.