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Pressure treated wood is rot-resistant, but it’s not waterproof.
The treatment forces preservative chemicals deep into the timber’s cellular structure to prevent fungal decay and insect attacks. Even so, the wood remains on porous, natural material. It will still absorb and release moisture depending on the weather. This guide explains why.
How Does Pressure Treatment Work?
Pressure-treating requires resources, expertise, and a lot of work. The process starts with cutting and drying the wood, then sealing it in an airtight cylinder. A vacuum pulls out the air, which causes the chemical preservatives (e.g., ACQ) to seep deep into the grain inside and out.
The result gives the wood a greenish tint along the grain and around knots caused by copper in the additives. This is a great indicator that a garden building is pressure treated.
What pressure treatment does is protect the wood from deterioration, making it resistant to fungal decay and insect attack. What it doesn’t do is seal the timber. It can get wet, but it resists the damage such wetness can cause.
This is why we recommend paying a little more for a pressure treated unit. If you’re looking for storage solutions, you can browse our selection of sheds, all backed by a 15-year guarantee against common issues.
Does Pressure Treatment Stop Water Absorption?
No, it does not. The pores remain open, so a pressure treated garden room will still absorb and release moisture. That’s natural for wood. The difference is that this moisture won’t lead to rot or the same level of damage you’d get with untreated timber.
Because the wood still breathes and takes on water, you should expect to see the following natural changes over time:
- Physical movement: The wood can still warp, expand and contract over time with changes in temperature and humidity. Learn more about log cabin expansion and contraction.
- Standing water: Even a pressure treated garden building shouldn’t sit in a puddle. While the chemicals prevent fungal decay, prolonged contact with moisture can still lead to structural damage.
- The wetting cycle: While pressure treated wood can still harbour some easily-cleaned surface mould, the treatment prevents the wood from becoming a food source for rot, but the physical structure of the timber can still degrade from constant wetting and drying cycles if not maintained with a water-repellent finish.
How to Make Pressure Treated Garden Room Water-Resistant
You need a secondary treatment to stop water absorption and prevent the wood from moving. Follow these steps for any pressure treated garden building:
- Let it season: New pressure treated timber is often wet from the process. Wait for at least 2 to 6 months for the wood to dry out completely before applying anything else.
- Test with water: Splash water on the surface. If it beads, wait longer. If it soaks in, the wood is ready to be sealed.
- Seal it with a sealer or paint: Use a high-quality water repellent, wood preserver, or exterior paint. This adds a layer that repels liquid, while the internal pressure treatment stands guard against rot.
Sealing or painting is effective, especially if you’re using the garden room as a year-round retreat. If you’re looking for options, you’ll find pressure treated log cabins below that can be used throughout all seasons.
Shop Pressure Treated Log Cabins
Read our guide on painting pressure treated wood to do it right.
Round-up
Pressure treatment is the most effective way to stop your garden building from rotting or being destroyed by insects. It doesn’t make the wood waterproof, but it does mean that when the timber gets wet, it won’t decay.
A simple maintenance routine is all you need. Once the timber has seasoned, adding a sealer will tackle the wet-dry cycle and help prevent the wood from warping or splitting over time.
Explore more styles in our summer house collection. You get the same pressure-treated protection against decay, with different layouts to fit your garden.
Shop Pressure Treated Summerhouses
FAQ
Do I need to treat the underside of a pressure treated shed?
Yes. While the floorboards are rot-resistant, the underside is vulnerable to trapped moisture. Make sure your base is well-drained, and apply a water-repellent treatment to the floor joists before assembly to prevent the wood from soaking up ground water.
How often should I apply a water-repellent to pressure treated wood?
For the best results, apply a fresh coat of water-repellent sealer or exterior paint every 12 to 24 months.
Can I use pressure treated wood for a vegetable garden?
Modern pressure treatment (like ACQ) is generally considered safe for garden use, but consider lining the inside of planters with a heavy-duty membrane. Doing so prevents direct soil contact and ensures the preservatives don’t interfere.
Is it normal for my pressure treated building to have small cracks?
Yes, these are called checks. They occur as the wood dries out and the fibers shrink. Since the treatment doesn’t seal the pores, the wood moves as it loses moisture. Don’t worry, as these surface cracks don’t affect the structural integrity or the rot-resistance of the building.
