Building Your Greenhouse Foundation

There are three common types of greenhouse foundation: 1) treated 4x4 wood, 2) concrete wall, and 3) concrete slab. A poured concrete foundation to frost depth is your best choice. For smaller greenhouses (less than 12 ft. x 20 ft.), a wood foundation is popular due to ease of installation and low cost. The diagrams should give you a good idea of how each foundation type could be built. Be sure to check your local building code requirements.

• Foundations must be square and level.
  
• Remove plants and sod, then level the site.
  
• Square the foundation by measuring diagonally from opposite corners and shifting the frame until the measurements are equal.
  
• Level foundation by carefully using a contractor’s level.

Treated 4x4 Wood Foundation
Fasten the treated 4x4’s together using 3" deck screws, making sure the foundation remains square and level. (Greenhouses over 10 ft. x 12 ft. should have a double layer of 4 x 4’s.) Pound 24" rebar into the ground on the inside of the 4 x 4’s spaced about 4 ft. apart. Attach the rebar to the 4 x 4’s with conduit “J” nails. For windy sites, use anchor stakes (#1405) to hold the 4 x 4’s in place. Use 1" wood screws to attach the greenhouse to the 4x4’s.

Concrete Wall or Concrete Slab Foundation
Cover the pour area with 2" of gravel. For a slab, use rebar and wire mesh to strengthen the floor. On top of the concrete foundation use a treated 2 x 4 wood sill to which your greenhouse will be anchored. Place 1/2" x 9" anchor bolts into the fresh concrete, leaving only 1-1/2" above the concrete surface. Counter-sink nuts into wood sill so that greenhouse base will not rest on the nuts. Set bolts within 1 ft. of each corner, then space additional anchor bolts about 4 ft. apart.

Greenhouse Floor
If you have not poured a concrete slab, you will need a walkway down the middle of your greenhouse. First, lay down landscape fabric (#2380) over the entire floor. For an aisle of bricks: frame the walkway with treated 2x4 lumber, lay down 2" of crushed rock, then 1" of sand, and set the bricks with 3/8" spacing. A final touch is to plant lemon thyme between the bricks. Finish the remainder of the floor with 2" of pea gravel.

Choosing a Greenhouse Site
If possible, locate the greenhouse where it will receive at least 6 hours of direct sunlight during the winter months. A good site would also be sheltered from high winds, close to water and electricity, and easily accessible from your home and garden. Avoid a site that is boggy, soft landfill, near a children’s play area, or shaded by buildings or trees during the winter.

The best orientation is to position the greenhouse with length running east and west. This will provide more heat gain from the sun during the winter. If the southern exposure is restricted, but open to the east, southeast, southwest, or west, turn the greenhouse to the winter sun. Remember that the sun is much lower during winter.