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By Amy Hanlon on 29 Sep 2023

The Best Timber Garage Specification

 Even the best timber garage specification can still result in a soggy building, right?

If you think that timber garages are leaky, damp wrecks that rain and groundwater destroy, have we got news for you!

Because in many cases, you’re absolutely right.

A single-skinned timber garage draws moisture from the air and passes it through its walls to the inside.

But what if you don’t want it to do that? What if you want the inside of your garage to be nice and dry? Can you stop it from passing moisture through to the internal part of the structure?

Well, as you know, in Scotland, we get a whole lot of rain. In fact, it feels like it’s been raining for 4-5 billion years or so. While this may be a slight exaggeration, the past 5 years have been relentless.

Garage walls constructed from a single skin of timber don’t do well in all that rain.

And we don’t build shonky, damp garages. That’s not who we are AT ALL.

But before you write off timber garages entirely, take a look at our absolutely foolproof method of stopping water ingress in a timber garage.

What is water ingress?

Water ingress is when water gets into a building through the walls, roof or floor.

Water ingress doesn’t happen because there’s a hole in the building that’s letting water in. This term refers to water that comes through walls, roofs and floors.

But how does that happen?

Here’s how it happens in a timber building:

Timber is a natural, breathable material that was once part of a tree.

Trees transport water throughout their entire structure in much the same way that we transport blood around our bodies.

When the tree is cut down, all of the channels and pores in the wood that used to transport water are still there, and they’re still able to take in water from the air around the timber.

Once moisture has soaked into a timber board, it will pass through the board using the same transport system that the tree used to use.

While this may seem a bit alarming, the pores and channels in timber make it breathable. This means that any rain and ground water that gets inside a timber building is able to pass out through these pores, which prevents condensation. But this process also works in the other direction.

While wood is porous, it’s also stable. Water shouldn’t pour through a garage wall at a rate of knots.

But when you have a single-skinned timber building, water ingress will always happen to some extent.

What problems does water ingress cause?


Water ingress isn’t the same as a leak, which would cause water to dramatically pour into your shed or garage.

For the most part, water ingress is minimised by airing your building regularly. We’re not talking about huge puddles or a streaming garage wall here.

But over time, a build-up of water in a timber building leads to problems, particularly in climates that see a lot of rain. Sound familiar?

Can water ingress cause mould?

The interior of a single-skinned timber building showing mould around the cladding boards as a result of water ingress.

It certainly can. Mould spores in the air settle, grow and multiply in damp and warm areas.

If your timber building is regularly experiencing water ingress, it provides a perfect environment for mould to grow and rot to happen. Since mould and wood rot both damage wooden structures, this is bad news for your building in the long term.

And let’s not forget about the contents of your building. If you’re storing anything inside your garage that is made of organic material – paper, cardboard, fabric, wood etc. – then mould will also grow on these items.

Single-skinned timber buildings are susceptible to water ingress, which can lead to mould and rot.

What is a single-skinned building? Garage or big shed?

A screenshot from the Gillies & Mackay YouTube channel showing Cara and the caption "Buyers' Prep. Big Shed vs Garage: What's The Difference?

There’s that term again: single-skinned.

This means that the garage walls are made of a single layer of building material. In this case, it’s timber cladding fixed to a frame. On the external walls of a single-skinned building, you see one side of this “skin”. On the internal walls of the building, you see the other side.

If you’re shopping for a timber garage you need to be aware that most garages on the market are single-skinned. In other words, they’re big sheds.

There’s absolutely no difference in construction between most timber garages and most sheds.

Can I prevent water ingress in a single-skinned garage?

A Gillies & Mackay timber shed with a steel box profile roof, painted pale blue.

Don’t get me wrong: we love sheds. We’ve got nothing against a good shed. But a shed isn’t the same as a garage.

For one thing, you usually store far more expensive and precious items in a garage than you would in a shed. You want those expensive, precious items to stay safe, dry, and mould-free, totally protected from rain and ground water.

Our top tips to prevent water ingress in a single-skinned timber garage include painting the garage walls with water-based exterior paint, like Sadolin.

But there is a better way to stop water ingress completely…

How do you stop water ingress in a timber garage?

It’s nigh on impossible to stop water ingress completely in a single-skinned building. So to solve this problem, we stopped building single-skinned garages. It was the only way to stop water ingress once and for all.

But we didn’t stop building garages. Instead, we developed a specification that delivers a watertight, solid building, suitable for storing even the rarest classic cars or precious heirlooms.

What’s the best construction for timber garage walls?

The external walls of our garages are still made of timber cladding. But under the timber cladding, there’s a whole lot more going on. Let’s take a look:

A diagram of Gillies & Mackay's timber garage specification, showing the layers that make up the timber frame garage walls.

19mm thick tongue and groove pressure-treated weatherboard – Pressure-treating the cladding protects it against rot for over 15 years but we always recommend painting your garage to see the wood lasting many years more. Other cladding options are available, as long as they keep out the water!

22mm pressure-treated batten – This is what creates the cavity. Water can do a lot of things but jumping 22mm with no propelled effort is beyond conceivable – however, just to be sure…

Breather membrane – This damp-proof membrane acts as a barrier from the outside to the inside and stops any moisture from the external cladding making its way into the internals.

9mm OSB sheeting -OSB or oriented strand board is made of wood fibres bonded together with adhesives. It creates strength and excellent load-bearing properties.

95mm x 45mm pressure-treated framing – Our framing is C24 graded  – meaning it’s really strong and can withstand the weight it must carry plus any additional loads – weather or human!

What’s the best construction for a timber garage roof?

A Gillies & Mackay garden room with a steel box profile roof and guttering. It has three double glazed windows.

Minimum 95mm x 45mm pressure-treated framing – bigger buildings need a sturdier timber frame – and our C24 framing is more than up to the task.

16mm tongue and groove sarking treated with anti-fungal wash – We like to use the tongue and groove 115mm wide boards because of their bracing qualities. The timber is treated to protect it from mould, just in case!

22mm pressure-treated batten – there’s that cavity again! As well as keeping water out, an air cavity provides insulation, which helps prevent condensation.

Proctor Roofshield membrane – This provides a secondary barrier to the ingress of rain, wind and snow. It has a low vapour resistance and is air-permeable; additionally, it eliminates the incidence of condensation in pitched roofs.

0.5mm Polyester coated steel box profile sheeting [20-year guarantee] – Steel Box Profile is brilliant, reliable and durable. It’s so good we have a whole blog on it.

We make our Garages in sections in the workshop and then assemble them like a massive jigsaw on site. We introduced this new garage specification in 2016, but we’ve been doing it for years with our garden rooms so we know it works.

What’s the difference between a garage and a garden room?

Our Blackstone Garden Rooms are pretty special. They’re fully compliant with Building Standards, meaning that they are just as structurally solid and waterproof as your home is.

While garages as usually built for storage, garden rooms can be used for anything that a room in your home could be used for.

But there’s actually only one structural difference between our garage walls and garden room walls.

A diagram and photo of Gillies & Mackay's timber garden room wall specification, showing the layers of material used.

A garden room is a lined and insulated garage. The lining and insulation help keep the warm air in during cold weather, and the cool air in during warm weather. They also contribute to the finish of the internal walls, which can be timber or plasterboard depending on your taste.

Garden rooms don’t allow water ingress either. The key to a truly waterproof timber building is layers.

What is the best specification for a garage base?

You can’t go putting a top-quality building on a substandard base! I mean, you can, but it does undermine all the hard work that went into making the building watertight!

Just as moisture in the air can seep into garage walls, rain and ground water can permeate through a garage floor.

That’s why our timber garage specification also includes very strict base specifications for the concrete plinth that the garage stands on.

An autocad drawing of the concrete base specification needed for Gillies & Mackay timber garages. The garage wall and base are shown

The foundation of the building keeps it square, level and solid. It’s absolutely vital to the success of your garage project. That’s why we recommend accredited contractors to build your garage base.

As the diagram shows, building a base involves excavating any existing ground to make sure the site is absolutely level.

Then a layer of Type 1 hardcore is laid and well-compacted.

On top of this, a damp-proof membrane is laid to prevent rising ground moisture.

The concrete comes next, with a reinforcing mesh for structural integrity.

We can’t emphasise enough how important your garage base is. Our garages are large, heavy buildings, meant to house large, heavy vehicles and whatever else you want to put in there.

Getting the base right is another way to extend the life of your garage.

What’s the best garage door for a timber garage?

We work with highly qualified companies like Tayside Garage Doors, who have 30 years of experience supplying and fitting garage doors.

Or if you’re further north, SD Garage Doors has been supplying and fitting garage doors all across Scotland for over 25 years.

There are so many options for garage doors. You might want a manual roller door, an up-and-overhead door, or an automatic fancy affair with a remote, an x-ray machine and a retinal scanner! (OK. I might have made that last one up…)

But as long as you choose a reputable company and an established brand, your garage door will do its job keeping the wind and rain out and securing your garage.

What are the benefits of a well-designed timber garage?

Interior of a Gillies & Mackay timber garage. Each garage wall is visible, and an open white up-and-over garage door can be seen.

Investing in a well-designed timber garage means that the building will stay solid and reliable for decades to come. Here’s what you can expect from a Gillies & Mackay garage:

Watertight and weatherproof

Our three-tiered wall construction and roof specification ensure a garage that can withstand the Scottish weather.

High-quality materials

We use pressure-treated timber, durable cladding, and plastic-coated steel to ensure longevity and resistance to rot.

Expert craftsmanship

Our expert joiners meticulously construct each garage to guarantee a reliable and well-built structure.

Customizable options

We offer various sizes and designs, allowing you to choose a timber garage that suits your specific needs.

Long-lasting investment

A timber garage from Gillies & Mackay is built to stand the test of time, providing a lifetime of use and enjoyment.

Where can I buy a long-lasting timber garage?

A Gillies & Mackay timber garage with adjoining carport. Garage walls constructed of timber cladding with a layered structure, stained dark brown. There is a classic VW Campervan under the carport.

Wherever you buy your timber garage, remember that single-skinned buildings are susceptible to water ingress. Around these parts, we have no shortage of rain and groundwater. So ask your timber garage manufacturer how their buildings are constructed.

If you buy a big shed rather than a garage, you’ll have more work to keep moisture out of the building. The lifespan of the building will be shorter, and the contents of the garage will be at risk of mould or water damage.

But if you want a reliable, dry, solid garage that lasts a lifetime, we can help! Book a consultation with our Sales Team or come on down to our Show Area to check out our gorgeous show garage.

More reading:

How much does a small timber garage cost?

How much does a medium timber garage cost?

How much does a large timber garage cost?

How much does a garage base cost?

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