three common types of greenhouse foundation: 1) treated 4×4 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. (See price list for
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.
foundation by carefully using a contractor’s level.
Treated 4×4 Wood Foundation (figure 1)
Fasten the treated
4×4’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 4×4’s.
Concrete Wall or Concrete Slab Foundation (figures 2
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.
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 2×4 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
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 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.