If you’re thinking of building a greenhouse, one major decision is deciding what type of wood to use and where to get it from. I’ve got four good options for you: pressure treated lumber, naturally rot-resistant lumber, cull wood, and pallet wood. Let’s look at each of these.
Pressure Treated Lumber
This type of lumber is very common and is typically found anywhere that lumber is sold. It’s commonly used for decks and fences, and is usually southern yellow pine. During pressure treatment, air is removed and water and chemicals are added. This preserves the wood so that it rots much less quickly and lasts a lot longer. Untreated pine will start to rot within 1-2 years. Pressure treated pine will last 10-40 years.
If you’ve heard that pressure treated wood isn’t safe for use in vegetable gardens, it’s because that was true for a long time. In the United States from the 1940’s to 2003, arsenic (and other chemicals) were used to treat lumber, and there was concern that it would leach into the soil and into the plants growing in that soil. In 2003, regulation was passed that discontinued its use in residential construction, playgrounds, landscaping, fences, and decks.
It’s a good choice for a greenhouse, but often needs to dry for 3-4 months before it can be painted. Since you’ll want to paint your greenhouse before you add the greenhouse plastic, you’ll want to build with lumber that’s already dry. You can tell if it’s dry if it’s absorbing water. If it’s still wet, water droplets will form beads on the surface.
Naturally Rot-Resistant Lumber
Cedar and redwood are two of the more common types of rot-resistant lumber. Cedar can last for 15-20+ years and redwood can last for about 30 years. It’s a good idea to seal the lumber periodically to minimize cracking and keep the surface looking nice.
These are great choices for a greenhouse, but they’re more expensive than pressure treated lumber. However, if you like the idea of a cedar greenhouse, I think it’s worthwhile to go ahead and calculate the cost. I used cedar and my entire greenhouse cost (plastic, doors, and all other materials) was only around $700. I think this is still a bargain price to be able to have a lovely greenhouse.
Cull Wood
Cull wood refers to pieces of lumber that have defects that make them unsuitable to sell at the regular price. They might have knots, splits, or warping. Or they might be short pieces leftover from a larger cut. However, these defects don’t mean that the wood is worthless. It just means you might have to cut off/work around the bad pieces or work with shorter lengths.
This lumber is heavily discounted, often at 70% off or even free. Many big box stores and lumberyards sell cull wood and you’ll just need to find it. Some stores mark the pieces with spray paint and place them on a cart near the back of the store. Some stores place the pieces in a scrap bin. If you don’t see either of these, then ask. And if that store doesn’t have a cull pile, check around at some other stores!
This is a great option for finding lumber to use for a greenhouse. The inventory changes daily and weekly, so check back often to see what’s been added to the pile. You may need to collect pieces over a period of time before you have everything for the project.
Pallet Wood
Using pallet wood for DIY projects is really popular and many folks use pallets to build their greenhouse. The pallets can either be broken down so the wood can be reused to build walls, or the pallets can be used as-is and screwed together to create a wall.
This is also a good option and there are two things to consider when using pallet wood. First, consider the wood quality of the pallets you find. Some wood is very thin or is cracked. These may not hold up long and may need replacing sooner than you’d like. Other pallets are of better quality and the wood holds up for years.
Second, be sure to research the markings on pallets to understand which ones are safe to use. They may be chemically treated and/or fumigated to prevent insect damage. In general, avoid pallets marked MB, EUR, or that are colored.
I hope this gives you lots of good information to help you decide which is best for your build!
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