How to Lay a Concrete Base for a Greenhouse

Building the Form

• 1

  Mark the area for the concrete base or pad with marking paint following the planned dimensions for the greenhouse carefully. Extend the marks out by 3 inches to accommodate the space necessary for the form.

• 2

  Dig out the marked area to a depth of 6 inches. Compact the ground with a tamper, which is a long metal pole with a flat base and two handles on the side. You can also use a mechanical tamper available for rent from most hardware stores.

• 3

  Spread landscape fabric over the base of the hole including the sides of the hole and overlapping edges by 3 inches.

• 4

  Arrange 2-inch by 6-inch planks around the perimeter of the hole with the 6-inch edges standing vertically. Drive 3-inch nails through the sides of corner planks into the ends of adjoining planks to make the 90-degree corners. Drive nails through the top edges of the planks at an angle into the ends of adjoining planks to create a straight line with multiple boards.

• 5

  Hammer ground stakes with a mallet spaced every 2 feet along the external edge of the form with an additional stake placed at each junction between boards. The top of the stakes must sit flush with or slightly below the top of the form.

• 6

  Place a level on top of the form. Raise or lower the form as needed to obtain a level structure. Hammer 3-inch nails through the ground stakes into the form to lock in the position. Leave the head of the nail extruding slightly from the ground stake for easier removal.

Pouring the Concrete

• 7

  Pour a 1-inch layer of crushed gravel into the hole. Compact the gravel with a tamper. Verify the depth of the hole is at 5-inches with the gravel added. Add or remove gravel as necessary.

• 8

  Mix concrete in a cement mixer and pour the mixed concrete into the hole. Fill the form to half-full. Lay out strips of steel rebar in a grid fashion creating 2-foot square grids over the surface of the concrete.

• 9

  Fill the remainder of the form with concrete while agitating the concrete to remove air pockets. Drag a spare wood plank across the top of the form to remove excess material and to leave the surface relatively even. Smooth the surface completely with a float.

• 10

  Finish the concrete with a trowel for a smooth finish once the surface water evaporates and the concrete has a dull-gray appearance. Drag the bristles of a push broom across the surface to create grooves for a skid-resistant finish. Drag the point of the trowel across the surface of the concrete from side to side to create expansion grooves in the concrete. Space the grooves by 10-feet from the front to the back of the structure.

• 11

  Cover the concrete base with plastic for seven days to allow the cement to cure properly. Remove the concrete form and back fill the area around the form with dirt or rock.