Pressure
Treated Wood
What is pressure treated wood?
The timber used for our greenhouses is all pressure treated timber, also known as tanalised wood.
However, pressure treated wood has been through a process that makes it more long lasting. The wood itself is placed in wood preservative, and then put into a vacuum. The vacuum forces all of that preservative deep inside the wood, meaning it gets completely saturated and is treated even on the inside.
This kind of treatment does two things. Firstly, it protects the timber against the damp, which in our climate is extremely important. This means that you can safely leave pressure treated timber outside, and even placing it in contact with wet soil is fine (though it’s advisable to avoid this if possible). Secondly, the chemicals that are in the preservative will also help to protect the wood against insects, so you won’t need to worry about critters eating your new greenhouse.
Then Why Do I Need to Treat the Wood Further?
Given that you’ve already paid a little extra for pressure treated wood that’s supposed to be long lasting and durable, you might wonder why you’d need to do anything else to the timber. What you need to remember here is that although pressure treating does stop damp from developing into rot, it doesn’t protect the wood itself against weathering (which doesn’t always mean damp)!
Just as an example, the next time you go to the garden centre take a look at the stacks of pressure treated lumber they keep there. You’ll see that some pieces are cracked or warped. This is because pressure treated wood is bundled whilst still damp, and the pieces that were on the outside of the bundle dry too quickly in the sun and therefore get damaged. What you need to do is prevent this from happening, which is why the correct treatment is so important.
The bottom line is that even pressure treated timber will crack or warp if not taken care of properly, which means that little bit extra you paid to get wood that was treated to last longer will be a complete waste. When our timber has come out of the pressure treatment tank it then goes through a drying process where each piece is latted to allow air flow to pass through in a controlled process until it has a moisture content reading of between 9-12%.
It’s key to any tanalised building that treated saturated timber is allowed to dry in this way to avoid shrinking. Once it’s reached the required moisture content it’s important not to let the water to get back into the timber, therefore should apply a topcoat treatment.
What are my options here?
You have three basic options for care and maintenance of pressure treated wood: stains, sealants, and paints. You may also choose to oil the wood, which is fine and will give wood a glossy glow, but that doesn’t mean you can skip the preservative stage. Any preservative, whether that’s stain, sealant, or paint, will slow down the drying process and help keep your wood supple and straight.
Here’s a breakdown of those three choices and all you need to know about them:
Sealant
Basically, sealant is a kind of oil that acts to stop the treatment process. Wood that comes out of the factory is already treated, yet those chemicals used to treat it will continue to act on the wood until it has completely dried out (something that can take months). A sealant will give you the greatest level of protection and should be applied as soon as possible and then reapplied once a year. Look out for a sealant with UV protection, since these will help protect your furniture against the sun.
Stain
Unlike a sealant, stain will actually colour the wood that you use it on. There are two kinds of stain; latex and oil, and you can really use either. You don’t have to apply a stain if you don’t want to, but as well as making your greenhouse look a little nicer, a good stain should also include a water repellent and more UV protection, increasing the life of your furniture even more. Stain can only be put on dry wood, so you should wait a month or so after purchase before staining to ensure that your pressure treated timber is truly dry.
Paint
Finally, you may also paint your wood if you like. If you want to paint, it’s important that you first use a sealant. Check the instructions on the sealant you choose, and it will tell you how long you need to wait before painting (at least several days, probably longer). Use a good oil-based primer first, and then a couple of coats of latex topcoat.
Pro Tip: Look for paint oils or stain that says it is moisture vapour permeable or microporous. This kind of preservative stops moisture going into the wood, but lets additional moisture escape from the wood, allowing the timber to breathe.
At Coppice all of our buildings are treated with an oil treatment sealant that allows the timber to still breathe but doesn’t allow water penetration. This needs applying once or twice a year, we suggest for the best results this is done before winter and before summer, please follow instructions on the product and apply as it states.
