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Pressure-treated timber is best, right?

Well, actually, it’s complicated.

We’ve been manufacturing and installing garden buildings in Scotland for more than 30 years, so we know a thing or two about timber. And once upon a time, we made all our buildings from pressure-treated timber. 

But these days, we don’t do that anymore. 

Why not? Keep reading!

What is pressure-treated timber?

A side-by-side comparison of two lengths of timber. One is labelled pressure-treated and is slightly greenish, and the other is labelled not pressure-treated and is the usual colour.

Pressure-treated timber is stronger and more resistant to rot and insects than untreated timber. 

To create this, you immerse the wood in a chemical solution under high pressure. The pressure forces the solution deep into the fibres of the timber. Tanalisation is another name for this process.

Pressure-treated timber has a slight greenish tinge to it, caused by the chemicals used in the process. 

Stronger and rot-resistant are good things, yes? So why did we stop using pressure-treated weatherboard cladding on our sheds and summerhouses?

What is water ingress?

Water ingress is when water gets into a building. It can happen because of leaks, structural issues, and because water comes through the walls of a building. 

Timber is a natural, porous material. When it’s exposed to water, it will absorb some of that water. If this timber forms the wall of a shed or summerhouse, its continuous exposure to water can mean that water starts to seep through the walls of the building. This can lead to problems with damp, mould and damage to the contents of the building, as well as the building itself.

Water ingress happens to some degree in all single-skinned timber buildings. However, you can minimise this issue in several different ways. 

How can I stop water ingress?

To minimise water ingress, we ensure that:

  • We use high-quality, slow-grown 19mm Scandinavian redwood weatherboard cladding.
  • We use Sadolin on all our buildings and recommend that you do too.
  • We install all our buildings so that they’re square, solid and sealed.
  • We use steel box profile roofing as standard.

But we don’t use pressure-treated weatherboard. Not any more. 

Our specification has evolved over the years for one very simple reason: our aftercare service. If something goes wrong with one of our buildings, we make sure we sort it out. We have a good overview of any recurring problems that need addressing. 

What’s the problem with pressure-treated weatherboard?

When we started using pressure-treated weatherboard, something interesting happened. 

Instead of our buildings staying well-protected from the weather, we found that our customers had more issues with water ingress. 

This didn’t make any sense, until our shed gurus Grant, Cara, and Gordon put their heads together to figure out what was going on.

Here’s what they realised:

Think of a sponge. When you first buy a sponge, and it’s completely bone-dry, it takes a while for water to soak in. To start with it almost seems to be waterproof. Sometimes you have to squeeze the sponge a bit to let the water in. After you squeeze it you can really soak it with water. 

In the same way, during pressure treatment, you have to apply pressure to let the chemical solution soak deep into the fibres of the wood. 

After you’ve soaked a sponge for the first time, filling it with water again is easy. 

We found that the same thing happens to pressure-treated timber. It’s as though the pressure treatment stretches the pores in the wood. As a result, the wood takes moisture in more easily, just like the sponge. 

If you need a single water-resistant layer of timber, don’t use pressure-treated weatherboard. 

When should I use pressure-treated timber?

Hang on…You can use pressure-treated timber for docks and decks, and literally put in it rivers and expect it to stay strong and rot-free! It has a 25-year guarantee!

This is absolutely correct. But none of those applications require a water-resistant layer. It doesn’t matter if a timber bridge or deck absorbs water because they’re not designed to keep water out. 

Pressure-treated timber is great in any application involving exposure to water or where there’s a risk of rot or insect infestation. The extra strength of pressure-treated timber makes it fantastic for fences and all sorts of outdoor and indoor uses. 

This is why we make the framing of all our buildings from pressure-treated timber. This makes it strong, and resistant to rot. Framing doesn’t come in contact with water, so there’s no problems with pressure-treatment.

However, in a single-skinned timber building, the benefits of pressure-treated cladding are outweighed by the increased potential for water ingress. 

Is pressure-treated timber a sign of a quality building?

Sheddies, beware! Makers of cheap sheds and summerhouses often use the 10-, 15- or even 25-year anti-rot guarantee of pressure treatment to imply that their buildings will last the same amount of time. This is seriously bad form as far as I’m concerned. 

Because rot is only one of the reasons for a short-lived, shoddy shed. Just because the timber won’t rot, doesn’t mean that it will keep water out of your shed. A pressure-treated shed is just as susceptible to water ingress and the problems it causes as an untreated one.

If you’re looking for a top-quality single-skinned building, check out our Shed Buyer’s Checklist/Summerhouse Buyer’s Checklist. You can use these to make sure that your building specifications are up to scratch, wherever you buy it from. 

Do Gillies & Mackay ever use pressure-treated weatherboard?

A small timber garden room with an apex roof, painted pale blue with white trim.

This is a very astute question. Well done for paying attention. 

We DO use pressure-treated weatherboard for our garages and garden rooms. These are not single-skinned buildings. Instead, they have layered walls that are compliant with Building Standards. The outer cladding of the building is still made of weatherboard, but underneath this weatherboard, there is a cavity gap and a damp-proof membrane. Under the damp-proof membrane is a layer of OSB, and in garden rooms, there’s also insulation and lining.

A diagram of the G&M garden room wall specification, showing each of the five layers.

If the outer cladding of a garage or garden room soaks up some water, the very worst that can happen is that the water will condense on the damp-proof membrane. However, the cavity gap provides enough insulation to make this very unlikely. 

We’ve specifically designed our layered walls to prevent water ingress of any sort, so you can enjoy the benefits of pressure-treated cladding. 

Do we use pressure-treated weatherboard?

Gilles & Mackay don’t use pressure-treated weatherboard for single-skinned buildings. We do use pressure-treated weatherboard for 3-tier garages and 5-tier garden rooms.
And if you don’t know your sheds from your garages? Head to our Learning Centre where you can find out all about the big issues in the world of timber buildings. If you can’t find an answer to your question there, please let us know, and we’ll sort that out!

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