Rotten weatherboards or just failed paint? How to tell them apart

Push a screwdriver into the bare timber with firm thumb pressure. If the tip sinks in easily and the wood feels soft, spongy or stringy, that board has decayed and no coating will change it; if the timber is hard and dry and the blade skids off a grey weathered surface, you are looking at coating failure over sound wood.

That single test settles most of it, and you can walk a whole house doing it in twenty minutes. Everything below is about where to push, and what to do with the answer.

What does failed paint over sound timber look like?

Sound timber under failed paint is grey or silvery where it has been exposed, hard to the touch, and still shows crisp arrises at the board edges. Tap it with the handle of a screwdriver and you get a sharp, solid note. The paint may be peeling, flaking or blistering, and the board may look terrible, but the wood itself has not lost anything.

This is by far the most common finding, and it is good news. It means the job is preparation: scrape and sand back to a firm edge, spot-prime the bare timber, and rebuild the coating. The board stays where it is.

What does rot actually look and feel like?

Decay changes the wood itself, so the signs are physical rather than visual:

• The surface gives under a screwdriver or even a thumbnail, and stays dented.
• Fibres lift like felt or come away in soft strings rather than crisp splinters.
• Tapping gives a dull thud instead of a sharp knock.
• The timber is darker than its neighbours, or bleached and crumbly at an end grain.
• Paint stays stuck to a surface that moves independently under it, which is the odd sensation of a sound coating over dead wood.
• Board ends and bottom edges crumble away when you pick at them.

Timber does not decay while it is dry. Decay needs wood that is persistently wet rather than merely exposed, which is why rot follows water and not weather in general. Something is feeding that board water, and on weatherboards the usual culprit is a coating that has stopped keeping water out, so the timber gets wet and stays wet.

Where should I check first?

Do not survey the whole wall. Go straight to the places water collects, which is where nearly all of it starts:

• The bottom edge of each board, particularly the lowest two or three boards near ground level.
• Board ends at external corners, scribers and stop ends, where end grain drinks water.
• Directly under window sills and along the bottom of joinery.
• Behind and below downpipes, and anywhere a gutter joint has been dripping.
• Where a deck, path, planter or fence meets the cladding and holds moisture against it.
• Behind shrubs that have been left to grow against the wall.

Under the eaves is usually the healthiest timber on the house, which makes it a good control sample. If a board under the eaves feels the same as a board at ground level, you are probably fine.

How do I tell them apart on the wall?

Sign	Failed paint, sound timber	Rot
Screwdriver test	Blade skids off, no penetration	Sinks in, leaves a soft dent
Sound when tapped	Sharp and solid	Dull thud
Colour of exposed wood	Even silver-grey	Dark brown patches, or bleached and crumbly
Broken fibres	Crisp splinters	Soft strings, felty lift
Board edges	Sharp and defined	Rounded, crumbling, missing material
Where it is	Whole elevation, mostly the weather sides	Localised, always near a water source
What it needs	Preparation and recoating	Cutting out and replacing, then coating

Does the elevation tell me anything?

Yes. The south and west walls take the wind-driven rain here, so those elevations run ahead of the rest of the house on coating failure. Rot follows a different logic: it turns up wherever water is being delivered and held, so a shaded south wall with moss growing on it and a fence hard against it can be worse than an exposed but fast-drying west wall. Look for the wet spot, not the windy one.

Can rot be repaired, or does the board come out?

It depends how far it has gone and where it is.

• Shallow and localised. A small soft patch on an otherwise sound board can be cut back to firm timber, treated, and made good with a two-part timber repair filler. That is a legitimate repair, not a bodge, provided the wood around it is genuinely hard.
• Board end or bottom edge. Cut back past the decay to sound timber and splice in a new length, primed on all faces including the cut ends before it goes up.
• Decay running through much of the board, or reaching the framing. The board comes out and the framing behind it gets checked. If the framing is soft, that is a builder's job and it needs doing before anything decorative happens.

What does not work is filler over structurally soft timber. It looks right for a season and then fails on exactly the same schedule as the paint over it, and you have paid twice.

Is a moisture meter worth buying?

A cheap pin meter is useful for comparison rather than for verdicts. Take a reading on a board under the eaves, then the same board at ground level, then the suspect area. What you are looking for is the difference. A suspect board reading well above a known-dry one tells you water is being delivered there, even if the surface looks acceptable. Do not treat any single number as a diagnosis.

What about the lead paint?

If the building was built in the 1980s or earlier, assume the old coatings contain lead. That matters here because working out whether a board is rotten involves scraping, and repairing it involves a lot more. Dry sanding, power sanding and burning off all produce lead dust that settles in the soil, in the ceiling space and on the vegetable garden. WorkSafe's guidance is that if a building was built in the 1980s or earlier, it is best to presume it has been painted with lead-based paint. Have it tested before anyone sands anything, and until you know, work in a way that keeps the dust down. If you are poking at an older villa or bungalow, poke gently and keep children and food gardens away from the debris.

When is it a builder rather than a painter?

If soft timber runs behind the cladding into framing, if the bottom plate is affected, if there is decay around a window frame rather than just the board beside it, or if the affected area is more than a handful of boards, get a builder to look before you book any painting. A good painter will tell you this themselves rather than quote around it.

What to do next

Walk the house with a screwdriver, start at the bottom edges and the board ends, and mark anything soft with chalk. If nothing gives, you have an exterior repaint and a preparation-heavy one at that. If a few spots give, note them and get them repaired before the paint goes on, because painting over rot is money set on fire.

Get in touch if you would like us to walk it with you and tell you which boards are actually a problem.