Build a Post and Beam Greenhouse


Greenhouse Foundation Considerations

We live on the west coast and our winters are generally pretty mild so I can get away with pouring this foundation at a shallow depth.

Checking the concrete form walls with a spirit level
Checking the concrete form walls with a spirit level

Other existing buildings and workshops on our property were built this way (with no sign of heaving) so I decided to follow that.

Setting the Concrete Forms in Place

So I can position, level, and square the inner and outer walls of the forms. And then stake and brace them in place.

Staking and supporting the wood forms before pouring concrete
Staking and supporting the wood forms with braces before pouring concrete

I bent some rebar and suspended it from wire. And added some 1×2’s across the top of the forms to hold the anchor bolts in place.

Laying rebar into concrete foundation forms
Laying the bent rebar into the foundation forms

The form work is labour intensive but the materials and concrete is not particularly expensive for this pour. I want my projects to last so I’m happy to put in the extra effort if that’s what will make the difference. Rebar too is not very expensive so I put two rows of it in.

Tying rebar together in the concrete foundation forms
Tying rebar together in the concrete foundation forms with specialized tool

The wood for the form walls I was lucky enough to buy a few years ago so that wasn’t too bad either. And I will use all those 2×4’s and 1×8’s again in some other project.

Pouring the Greenhouse Foundation

I have a small concrete mixer that can handle two bags at a time.

A small concrete mixer can handle two bags
Our small concrete mixer can handle two bags, and that’s fine

So it was a full day for Marilyn and myself to do this foundation pour. I did the mixing and finishing and she filled the forms and settled the concrete.

Pouring concrete into forms with a mixer and wheelbarrow
Mixing and pouring the concrete into the foundation forms

Galvanized 6×6 post saddles were set into the wet concrete at each corner. And 4×4 saddles on each side of the doorway.

setting a 4x4 galvanized post saddle into wet concrete
Setting a 4×4 galvanized post saddle into wet concrete for the door posts
setting a 4x4 galvanized post saddle into wet concrete
Setting a 4×4 galvanized post saddle into wet concrete for the door posts

A few days later I stripped the forms and everything looked good. Nothing moved or bowed out during the pour.

Stripping forms from a concrete foundation
Stripping the forms a few days after pouring the concrete foundation
A poured concrete foundation for a post and beam greenhouse
The poured concrete foundation is finished

The Greenhouse Post and Beam Framing

All the posts, beams, ties, and rafters I had milled full dimension from trees felled on our property over the past five years or so. I then stickered and covered stacks of various sizes to air dry for a few years. The in fill walls I was planning to build from construction grade 2×4 cedar but I was also able to get those from some of the milled material I had. In the plans I did spec nominal 2x4s there.

Pencil marks around a 6x6 post
Making pencil marks around a 6×6 post

The posts and cross ties will be made from Grand Fir. This type of fir is not very weather resistant so I’ll add a few extra coats of stain to them. I was short of cedar 6×6’s so I needed to substitute.

With all my timbers and boards selected and brought into the garden, I can start cutting and notching and finishing each of the pieces for this build. This is not timber framing and the joinery I’ll be using is fairly simple and straightforward.

I’ll start with the four 6×6 corner posts. I cut them to length and trim down the bottom ends so they will fit in the saddles. You can get saddles that will accommodate a full dimension 6×6 but they are special order in my area. So I use the ones designed for a nominal 6×6. That being one that is 5-1/2 inches square.

Measuring an embedded post saddle bracket
These post saddles were made for a nominal 6×6

It’s no big deal to shave these down with a series of wafter cuts. I then break those off with a hammer and clean up the rough edges with a chisel.

Cutting the ends of a 6x6 square with a circular saw
Cutting the ends of a 6×6 with a circular saw and speed square

Finishing the Posts Beams and Rafters

For all the posts, beams, ties, and rafters I chamfered the edges with a power plane and a sanding disc on my angle grinder.

A power plane chamfers the corners of a greenhouse fir post
A power plane chamfers the corners of a post

Then I rolled three coats of stain on these fir posts and let them dry overnight.

Rolling on stain for greenhouse corner posts
Three coats of stain were rolled on these finished fir corner posts

I can now stand each post up and secure them with screws through the saddles and into temporary braces staked to the ground.

Setting a corner post into a galvanized saddle for a greenhouse
Setting the first of 4 corner posts into a galvanized post saddle
Plumbing a 6x6 fir corner post for a greenhouse build
Plumbing a corner post before attaching temporary braces

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