Dark exterior colours, heat and what it does to timber

A dark colour on timber runs far hotter in the sun than a light one, and that heat is what drives resin bleed, board movement, checking and early coating failure. This is why timber cladding suppliers set a minimum light reflectance value for the paint that goes on their boards, and why the figure for your own cladding is worth reading before you choose a colour.

If you want to go darker than your cladding supplier allows, the answer is not to ignore the number. It is to use a heat-reflective version of the same colour, and to check the cladding manufacturer’s rules before you commit.

What is LRV, and why 45?

Light reflectance value is a measure of how much visible light a colour bounces back, on a scale where zero is black and 100 is white. Every paint colour chip carries the number, and it is the fastest way to compare two colours that look similar on a screen.

Timber cladding suppliers draw their line at 45. The Envira weatherboard system guidelines, for example, call for a colour with an LRV of 45 or higher and a gloss level of at least 10 per cent, and state plainly that timber painted in dark colours below LRV 45 may produce resin bleed.

Note what that is: a cladding requirement, not a paint requirement. The paint itself may be perfectly happy. Going darker can put you outside the guidelines of the product your house is clad in, which is a different conversation and often a warranty one.

What actually goes wrong on dark timber?

• Resin bleed. Heat drives resin out of the timber, particularly at finger joints and around knots, and it appears as sticky amber or brown patches coming through the paint film. It is cosmetic, it is persistent, and it is very hard to fix permanently once it starts.
• Movement. Heavy heat and moisture cycling makes boards cup, shrink and open at the laps. Sealant in the joints gets worked back and forth until it lets go.
• Checking and splitting. Repeated expansion and contraction opens fine splits along the grain, and once the film is broken, water gets into the board rather than running off it.
• Coating stress. The film is worked harder every sunny day, so it chalks, loses gloss and fades sooner than the same product would on a lighter wall.
• Visible fade. Dark pigments show their fade far more obviously than light ones, so a bleached west elevation next to a sheltered south one is impossible to miss.

Does heat-reflective paint actually work?

Yes, and it is measurable rather than marketing. The technology uses pigments that reflect infrared while looking the same to the eye, so the visible LRV stays exactly where it was but total solar reflectance goes up considerably. Resene’s testing of a reflective formulation of one of their darker colours recorded a temperature 12 degrees lower than the standard version of the same colour over a standard five-minute test.

It does not make a black weatherboard behave like a white one. What it does is move the substrate back inside the range the timber can live with, which is why cladding guidelines that allow darker colours usually pair them with a total solar reflectance threshold. The Envira guidelines, again as an example, ask for a TSR rating of 40 per cent or more on the darker shades.

Which substrates care, and how much?

Substrate	How much dark colour matters	What to check first
Timber weatherboard, especially finger-jointed or pre-primed	A great deal	The supplier’s stated LRV minimum, and any TSR requirement for darker shades
Timber joinery, doors and garage doors	A great deal, worst on north and west faces	The same limits, plus accept a shorter maintenance interval
Fibre cement weatherboard and sheet	Less, but not nothing	Manufacturer colour guidance, which still exists
Solid plaster and concrete block	Less for the substrate, more for cracking risk	Movement joints and any existing cracking
Steel roofing and metal cladding	Moderate	Existing coating type, and whether it sits over a habitable space
PVC, composite and cellular products	A great deal, and usually non-negotiable	The manufacturer’s stated LRV limit, which tends to be strict

Where does Wellington make it worse?

Two things, working together. The housing stock here is full of timber weatherboard villas and bungalows from the 1900s to the 1930s, which is precisely the substrate that cares most. And on a steep section, a north or west elevation can sit in unshaded sun all afternoon with no neighbouring building in the way, which piles the heat load onto the one wall.

The awkward part is that the same house often has a south elevation getting hammered by wind-driven rain that rarely dries out properly. One house, two entirely opposite problems, and usually one colour applied across both. Deciding your scheme by looking at the sunny side alone is how the shaded side gets neglected.

Dark colours also show coastal grime more than people expect. Salt deposits and dust read as a light bloom on a dark wall, so a dark house near the harbour or the south coast needs washing down more often to keep looking like the colour you chose.

Does dark cost more?

Not usually to apply, though deep colours often need an extra coat to reach uniform coverage over a light substrate. Where dark costs you is ownership: the recoat interval on a hot, exposed elevation is shorter than on a sheltered one, so you get back to the next exterior repaint sooner.

A sensible compromise most people are happy with is dark where you look and lighter where it cooks. Dark on a sheltered south wall, a recessed entry, joinery sitting under a deep eave. Lighter on the unshaded west elevation that takes the afternoon sun. That is a normal way to design a scheme rather than a concession.

When is dark genuinely fine?

• On fibre cement or plaster rather than timber
• On small areas: a front door, window reveals, a single feature panel
• On sheltered elevations, or anything living under a deep eave
• Where the cladding supplier permits it and you are using a heat-reflective formulation
• Where you know the recoat interval will be shorter and have decided it is worth it

And when it is not: on finger-jointed timber below the specified LRV without a reflective formulation, on any cladding still under a warranty you have not read, and on an unshaded west wall on an exposed site where what you actually want is the longest possible gap between repaints.

One last option people forget. If what you want is dark timber rather than dark paint, a stain is a different product with a different maintenance cycle, and on some surfaces it is the better answer.

If you have a colour in mind and want to know whether your cladding will tolerate it, send us the colour and the cladding type and we will tell you straight. Get in touch.