Greenhouse Foundation Design

Most greenhouses are small structures you assemble on a level patch of ground similar to a shed. So why are we talking about a foundation for this greenhouse?

Part of the fun of our northern climate is that your foundation needs to extend at least 4 feet below grade to prevent frost creep and freeze/thaw damage.

We had a few things to consider when we were thinking about how to execute on the foundation:

• Cost
• Durability
• Stability
• Drainage
• Site access

And we had two choices for a foundation:

• Standard concrete slab/footers
• Permanent wood foundation

From a high level, both of these methods are pretty close in price if you do all the work yourself. Now, if you have to hire a professional to build footing forms and finish the concrete, now a concrete solution is quite a bit more pricey. Here’s our breakdown for each method:

Concrete

• Since we have little to no experience working with concrete, other than securing a mailbox, my estimates came in over $2200 for a slab, footings, and finishing work.
• Durability, stability and drainage are pretty standard across the board if installed correctly
• Site access is a problem. The building site is on a relatively steep hill and it would be tough to get a concrete truck in there. We could use a pumper truck, but that would add almost $1000 to the price.

Wood foundation

• A wood foundation uses standard carpentry techniques with the only difference being the type of wood used and an exterior moisture barrier. Our cost will be the excavation, crushed stone, and pressure treated lumber. This comes in around $1300.
• Durability is pretty good. In most instances, foundation grade lumber is good for 100 years against rot and insects.
• Stability can be just as good as concrete. The foundation needs to be installed correctly with supports and adjoining walls in place to brace against the pressure from backfilling around the walls.
• Drainage is generally fine as well.

Note! With all the above points, correct installation is crucial. A wood foundation will wobble and leak like a sieve if specs aren’t followed closely.

We went with…

We went with a wood foundation. We felt that shape of the building along with my sandy/rocky soil would be plenty stable and drainage would not be an issue. Not having to work with concrete is a big plus if we really don’t require it. My house also has an addition that was built on a wood foundation decades ago and it’s still rock solid, so I’m confident in the durability over the years.

A Wood Foundation, Really??

The basic concept of a wood foundation is that you level your site and add at least 6 inches of crushed stone (not gravel). Crushed stone is more squarish than round and it provides a very stable substrate for our base plates.

There are some specifics on how to build the baseplate and foundation walls, but it really isn’t a whole lot different that framing an exterior wall. Foundation grade pressure treated lumber is required. This is different than the normal stuff you buy at home depot and you will most likely need to special order it from a lumber yard. Our build closely follows the specification from Southern Pine who originally pioneered wood foundations.

Special Considerations

With a wood foundation you need to be very aware of how you build your walls and base plates. Where a block wall easily holds back the weight of the earth around it, wood foundation walls need to be properly built and braced to distribute the forces evenly so the building doesn’t get pushed over.

We can also utilize dead man anchors and use the weight of the backfill to stabilize our walls similar to how retaining walls are built.

Another important detail is locking your walls together. This is really straightforward for a standard rectangle building. You just build the walls into each other at the corners. However we’re building a 15 sided building. Our plan is to build a double base plate/top plate and overlap the them to create a strong connection for our walls. You can see the cut pattern below. We’ll build two layers of these with the second being a reverse of the first and screw them to each other.

We’ll get into the internal bracing of the structure in a later post when we discuss the full design for the greenhouse.

Even though we have great drainage with our rocky, sandy soil; we still need to take precautions for our heavy spring snow melts and occasional torrential downpour. We’ll be installing a 4″ sock drain pipe around the exterior wall and down the middle under the structure. The sock pipe will be connected to a sump, essentially a buried trash can, that will allow a spot for excess water to gather while it drains away into the ground.

The final consideration is to not take this step lightly. It is arguably the most important part of the build and if not done right, you’ll fight it the whole way to completion. Make sure your excavated site is dead level; not close or eyeballed.

For example, our structure will be 20 feet tall from the base of the foundation when complete. If you’re even 5 degrees out of level from the get go, you’re going to be almost 2 inches off by the time you’re installing the roof. Go rent a surveying laser and make sure everything is where it should be.