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Why do you need to know how timber walls are constructed? Surely you’re just looking for a shed? But if you’ve ventured into the wonderful world of timber buildings, you’ve probably discovered that it’s more complicated than this.

Different people look for different types of timber buildings according to what they need from that building. Making that decision involves knowing what you’re getting, and how that building will work. What maintenance will it need? Will it be suitable for your plans?

To help you understand why we construct our buildings the way we do, here’s a comparison of the different construction methods we use for our Sheds, Garages and Garden Rooms

Whether you buy a Gillies & Mackay building or not, having this knowledge will help you evaluate the wide range of products out there, making sure that you know exactly what to expect from your timber building.

How are shed walls constructed?

We build our sheds from one layer of 19mm thick Scandinavian Redwood Tongue and Groove Weatherboard.

Scandinavian Redwood is the species of timber that we use to withstand the Scottish weather best. Tongue and groove refers to the profile of the cladding – the shape that it has from side-on. This profile slots together snugly to create a solid, weatherproof panel of cladding. The term weatherboard also describes the profile, which allows water to travel down the wall panel without gathering anywhere on the wall.

A stack of tongue and groove weatherboard, with the short end of the boards facing the camera, showing off the tongues and grooves of the profile.
A stack of tongue and groove weatherboard, showing the tongues and grooves that slot together.

We slot these 112mm wide boards together horizontally by their tongue and groove profile, then nail them to 70mm x 45mm pressure-treated C24 graded framing. The combination of 19mm weatherboard and 70mm x 45mm C24 framing results in a solid building that’s more than capable of smashing The Shove Test.

How waterproof are shed walls?

A single skin of tongue and groove weatherboard is the traditional formation for shed walls. The combination of the type of timber and the 19mm thickness provides a durable lifespan of 30 years.

It does require preserving with an exterior coat of Sadolin and an internal wash of anti-fungal treatment.

This wall thickness can withstand moderate moisture levels. In extreme cases, the moisture absorption will supersede its ability to keep water out. Continual exposure to wet and damp conditions will cause water will travel up and around the tongue and groove.

There’s always some level of moisture in a single-skinned building – safe levels are up to 22%. Anything more than this means that water ingress can become a damp issue.

How can I prevent damp in a shed?

The wall of a timber building showing signs of water ingress.
A timber building showing signs of water ingress on its framing.

Damp issues happen when moisture is unable to escape from inside the building. In Scotland, buildings often need a helping hand because we experience so much wet weather. Much like windowless bathrooms, the moisture needs somewhere to go. And in single-skinned buildings, it’s your job to help it out of your shed. 

A dehumidifier is the best solution for winter dampness. Powered dehumidifiers remove moisture from inside a building by extracting moisture from the air and collecting it in a tank which you can then empty. Chemical dehumidifier pots are filled with material that absorbs and collects moisture. While these pots are usually only suitable for a single use, refillable options are also available.

Single-skinned buildings are for seasonal use, and although they will retain some level of moisture in the wetter months, they can be managed well. The exterior wall handles the atmospheric moisture and expels it. This is why any paint or surface treatment used on your shed must be microporous. These tiny pores allow moisture in and out of the timber.

If you buy a single-skinned shed, make sure you understand its limitations. There’s only so much 19mm thick weatherboard can do and if you suffocate it with too much paint, moisture will get in but won’t get out.

Make sure that your expectations match the performance of the building. If you’re looking for a building that will stay bone dry in even the wettest weather, a single-skinned building won’t live up to those expectations.

If you’re looking for a 100% watertight building, you’re going to need some extra layers – three to be exact.

How are 3-Tier walls constructed?

We can cure all of the issues we see in single-skinned buildings with one simple remedy: a cavity.

This creates an outer wall and an inner wall. The outside wall deals with all the ingress or penetration of water and the cavity stops it from travelling to the inner 9mm OSB boards which is the internal wall you see. We use this 3-Tier specification on our Garages and our first-stage BGRs.

The interior of a large timber garage, showing the concrete floor, walls and roof timbers. There are several rectangular windows in the walls and a double access door.

We also recommend that any shed larger than 14′ x 8′ (4.2m x 2.4m) uses a 3-tier specification. This prevents water ingress from causing any structural issues in these large buildings.

How are 4-Layer walls constructed?

The 3-Tier construction isn’t just a pretty finish – I mean, it is, if you like your buildings structurally sound, Building Control-compliant and looking like a very handsome Garage. But for those of us who prefer a clean-dressed timber finish with the added efficiency of insulation, for any building under 12m2, the 4-Layer construction is ideal.

A diagram of the Gillies & Mackay 4-tier Radiata ThermoWood Garden Room Scotland specification.

This again, uses the same design as the 3-Tier with a cavity but constructed slightly differently to accommodate insulation. The foil back of the rigid insulation board also acts as a moisture barrier. The V’d Lining forms the finished internal wall which looks much prettier than OSB and is 16mm thick giving an overall wall thickness of 105mm. This is our Under 12m2 Radiata ThermoWood Garden Room Specification. 

How are 5-Layer walls constructed?

A diagram showing the 5-layered construction of a Blackstone Garden Room Scotland.


These buildings are all pretty damn solid. But what if you’re looking for even more? Maybe you want a garden building you can live in.

We’ve got you. By combining our 3-Tier and 4-Layer specifications we’ve created Building Control-compliant walls that have an additional 100mm thick insulation and 16mm V’d Lining. This is our 5-Layer Blackstone Garden Room specification for buildings over 12m2 that need the additional structural integrity of 3-Tier walls and the functionality of being an extension of your home.

Why do timber buildings need layers?

A Gillies & Mackay timber garage painted pale blue, with anthracite grey garage door and personnel door.

Why all this fuss? Are we just crazy about layers?

We are Gillies & Mackay, and we’ve been designing, manufacturing and installing timber buildings for more than 30 years. Those years brought storms, subsidence, and serious amounts of water – and we’ve poured all our expertise and experience into creating timber buildings that withstand the Scottish weather for decades.

We’ve pondered over, compared, tested and perfected every single layer to ensure that each element of our specifications does its job exactly the way you need it to.

So if you’re looking for a ridiculously long-lasting timber building, we should talk. Come over and see us at our Show Area in Errol, where you can see our beautiful buildings for yourself. Or if you have questions you can book a Consultation to learn even more about how our buildings can be your buildings, however many layers you need.

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