Timber Cladding in the Scottish Highlands

[North80]

The premature failure of timber cladding on some houses built in the Scottish Highlands over the last 15 to 20 years is a notable issue. The rapid decay seen in these instances, particularly when compared with the longevity of traditional stone and render, suggests a complex problem stemming from a combination of environmental challenges, material science, building practices, and economic factors.

 

This report investigates the core reasons behind the instability of some timber cladding systems in this region.

 

1. The Challenge of the Highland Climate

A primary factor is the uniquely harsh climate. Exposed coastal and highland areas of Scotland are subject to some of Europe's most challenging weather conditions for building materials:

 

High Rainfall: Annual rainfall frequently exceeds 2000 mm, creating a constantly damp environment.

Wind-Driven Rain: Strong, persistent winds drive moisture into every crack and joint of a building's exterior, testing the water resistance of the cladding far more than simple vertical rainfall.

High Humidity: Ambient humidity is consistently high, slowing the drying process. Timber that gets wet stays wet for longer, creating ideal conditions for rot and fungal growth.

Salt Spray: In coastal areas, airborne salt is a significant factor. It is hygroscopic, meaning it attracts and holds moisture on surfaces, and can accelerate the degradation of both the timber fibres and the metal fixings used to secure it.

Any timber used in this environment must be exceptionally durable or robustly protected to ensure a long service life.

 

2. Timber Selection: Fast-Grown vs. Slow-Grown Wood

A crucial factor in the longevity of timber cladding is the selection of the wood itself. A significant difference exists between fast-growing timber common in the UK and the slow-growing wood often imported from Scandinavia.

 

Fast-Grown Domestic Timber: The most widely available and commercially important softwood grown in the UK is Sitka Spruce. It is fast-growing and can be harvested relatively cheaply. However, untreated Sitka Spruce has a low natural durability (Class 4 'Slightly Durable' or Class 5 'Not Durable'). Its rapid growth can lead to a higher proportion of 'juvenile wood,' which is less dense, less stable, and more prone to moisture-related movement and decay than mature wood. If used for cladding without significant preservative treatment or a robust protective coating, its service life in a damp climate can be very short.

 

Slow-Grown Scandinavian Timber: In contrast, softwoods like Scots Pine (often sold as Scandinavian Redwood) or Siberian Larch are frequently sourced from colder climates. The shorter growing seasons result in slower growth, producing timber with tighter growth rings. This timber is generally denser, more stable, and more naturally resistant to decay (e.g., Siberian Larch is typically Class 3 'Moderately Durable'). This density and stability contribute to a superior quality of wood, characterised by what might be described as dense cells.

 

While high-quality, durable timbers like Larch, Douglas Fir, and Western Red Cedar are available from Scottish suppliers, the selection of lower-cost, less durable options to meet budget constraints is a significant risk factor for premature failure.

 

3. The Critical Role of Treatment and Detailing

The practice of leaving timber cladding unpainted, as is sometimes seen in the Highlands, contrasts with traditions in Scandinavia and is a key performance factor. While leaving timber to weather to a natural silver-grey is a popular aesthetic, it is a practice only suitable for species that are inherently durable or have been appropriately treated.

 

To Coat or Not to Coat: In Scandinavia, there is a long-standing tradition of protecting timber buildings with robust paint systems or high-quality stains, which provide a vital barrier against moisture. The trend in Scotland to leave timber uncoated is viable for very durable species, but can be problematic for non-durable timber like untreated Sitka Spruce or even lower-grade Larch. If a protective coating is omitted for cost or aesthetic reasons, the timber is left exposed to the elements, promoting decay.

 

Installation Detailing: Even the best timber will fail if installed incorrectly. Best practice, as outlined by bodies like the Timber Decking and Cladding Association (TDCA), can be overlooked, leading to systemic failures:

 

Ventilation Gaps: A ventilated and drained cavity behind the cladding is essential. This allows moisture that gets past the cladding to drain away and lets air circulate to dry the back of the boards. Closing this gap or installing it incorrectly traps moisture, leading to rot from the inside out.

Fixings: In coastal environments, Grade 316 stainless steel fixings are required to prevent corrosion. The use of cheaper, lower-grade fixings can lead to rust staining and premature failure of the fastenings.

Ground Clearance: Timber cladding should terminate at least 200 mm above ground level to prevent it from soaking up moisture from the ground.

4. Economic Drivers and Workmanship

Finally, economic drivers can influence material choices and workmanship. While high-quality, correctly installed timber cladding is an excellent, long-lasting building material, it is often more expensive upfront than a standard block and render wall.

 

This creates a financial incentive for developers or builders working to a tight budget to reduce costs. This can be achieved by:

 

Selecting cheaper, less durable timber.
Omitting or skimping on preservative treatments or protective coatings.
Rushing the installation, leading to poor detailing.

Timber cladding might be perceived as less complicated to install than rendering, potentially requiring less specialised labour. However, this belies the technical skill needed to get the detailing right.

 

In conclusion, the premature rotting of some timber cladding in the Scottish Highlands appears not to be a failure of the material in principle, but a failure of specification, protection, and installation. It is the likely result of using timber that is not durable enough for the local climate, failing to provide it with adequate protection, and not adhering to the critical details of its installation, often driven by a desire to reduce initial building costs.

 

[North80]

I asked Google Gemini AI to use the deep research function on the topic of wood cladding in the highlands, interesting what it came up with. 

[JohnMo]

Choose of cladding material is like any other choice you need to make, you select an appropriate material chooses based on environment. Simple softwood just don't last in most locations, exposed locations have little chance.

 

You really don't need to AI to make that call, just read any best practice guide on cladding installation and design.

 

Softwood ok for your shed - full stop.

 

We used Scottish Larch, no issues 4 years on

[Kelvin]

Scottish Larch factory treated with SiOO:X. It’s only been on for two years so who knows but I’m not expecting any problems. We’re in East P&K so less wet (generally) than elsewhere. 
 

There’s a house near us that has Scottish Larch fitted as board on board like ours. I went to see it before we decided on board on board. I noticed that the cladding had failed but it was the underbuild that had failed not the larch. It had been on for 25 years though. Why it failed was in the detailing as there was a lack of suitable ventilation behind it as far as I could tell. The screws had mostly rusted too so that also contributed to it failing as some were broken. 
 

We went belts and braces carefully following the guidelines and also added a plastic breather strip from SIGHA which makes the cladding sit off the straps and used somewhat dear SIGHA fixings. 

[North80]

This rainfall map illustrates the point perfectly. There appears to be a clear threshold around the 1500mm mark, above which timber cladding seems much more vulnerable to premature failure unless specified to an exceptionally high standard.

 

This highlights the problem with a 'one-size-fits-all' approach from some planning departments, an approach that doesn't fully account for the dramatic climatic differences across the Highlands. A timber-clad house suitable for the drier conditions of the Moray Firth coast is facing a completely different challenge on the west coast, where higher rainfall, persistent humidity, and salt-laden winds are the norm.

 

It's little wonder that many on the west coast live in traditional stone houses that are over a century old and still performing well, while many modern timber-clad homes may struggle to last 30 years without major intervention.

 

This raises questions about the focus of the planning and building standards process. While there is a heavy emphasis on thermal performance, measured through SAP calculations, there seems to be less focus on the long-term durability of the materials used on the exterior of buildings. Timber frame construction can perform very well in these energy calculations, which may encourage its use without giving equal weight to ensuring the external cladding is specified to withstand the local environment for decades to come.

 

The recent news about The Wee House Company entering administration is also interesting in this context. Given they installed timber-clad houses on the west coast, and noting some anecdotal online discussions about cladding issues, one has to wonder if the long-term cost of addressing potential widespread cladding failures became a contributing factor.

 

 

[scottishjohn]

you said it precisely 

why anyone wold use anything other first quality western red cedar or well seasoned larch is asking for trouble

 

you want to do it cheaper then use made man cladding that looks like wood 

 

 I do not think this is just the highlands either 

 

any wooden structure in my area --s/w scotland will go green on any side not facing south or west 

those not near the sea anyway

 

 

its just not a sensible choice for cladding in scotland 

 

especially with the crap timber that is supplied --even the so called pressure treated stuff is a joke 

 

only properly pressure treated wood you will see is telegraph poles --cut one of them and its all the way through -hence they last 50years +

 

cut one end  of treated wood from builders merhcants and you will see the treatment is not right through just first 3-5mm 

 

  we used to have a chalet park  and it was sweedish built chalets --  but they needed to be recoated with preservative every 3years - and if that was done they lasted for ever -

 they were 20years old when we took over  and were still fine 15 years later

 

-next owner did not do that with   few years they were all rotten and then the idiot replanked them in pine -another 5years and no treatment and they look pretty shabby

 

we used xyladec almost clear and very thin  and in the 70,s it was £30 per 5 litres--- when they needed doing the panels ,whiich had expansion joints every 3m or so would be about 1" wide 

 

 

recoat them and it would close up to next to nothing --which just showed what was happening over time the wood was drying pout and needed the prservative oily fluid to keep them good

 

 

If we had cold dry winters like canada and scandinavia --fine --but not constantly damp and warm ones  like we get here 

 

Edited 6 hours ago by scottishjohn