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Building a Greenhouse
     1. Getting Started
 -->2. Foundations <-- You're here!
     3. Framing
     4. Glazing / Covering
     5. Heating, Venting & Cooling (HVAC)
     6. Finishing Touches: (A) Spires (more soon)

Chapter 2 -- Foundations:
  The type of structure influences the foundation design.   Big glass houses will need a robust concrete base that will not shift.  Smaller plastic panel or film house can handle some settling or drift.  Types of Foundations

  1. Raised Slab
  2. Concrete block
  3. Concrete wall (soon)
  4. Posts in ground or cement
  5. Low-Tech foundations

The following steps (and pictures) are based on the raised slab design used the Victorian Conservatory (Plan#001) featured on this site.


Start with the 16' back wall and plant two stakes equal distance from the property line or fence. Allow 1 " for the form board if using 2x12s for the frame. The setback in my back yard had to be at least 5 feet so the stakes had to be a minimum of 4'10 " from the property line. Some areas require 10 feet or more. Be sure and check your local building codes.

Main Room Form

  • Position a straight16' treated 2x12 between the stakes and make sure it's level. Before attaching it to the stakes, make sure the top of the stakes will be below the top edge of the board so they do not interfere with the screed.
  • To lay out the main room, 16' x 16', use batter boards and string lines. Measure diagonals to square-up. When diagonals are of equal length the room is square.
Side room Form
  • Cut the rest of the boards:
    (4) boards at 4'-1 " long
    (2) boards at 4' long
    (6) boards at 7'-10 "
    (3) boards at 8'-3"
    and position them aligned with the string as shown.

  • Place stakes with kick braces every 2' to 4'. Level forms starting in the back corner and working around the parameter. Nail boards to stakes.
  • Leave one of the 8'-3" long boards off so it's easy to get the wheelbarrow in and out of the greenhouse. Remove all sod and dig a trench around the parameter (inside of the forms) at least 8" deep. See cross section view (next page).

Forms: Framing The Foundation

Click for larger view
Cross Section of concrete / trench.

Roughing in the Plumbing & Electrical

  • Drain Piping: Put at least one drain per wing and slope pipe " per foot. Run under framing. You may want to add a manufactured concrete catch basin at the end of the pipe and pipe overflow to a storm drain, pond or ditch.
  • Water, Gas and Electric: Underground or overhead? Up the outside or inside. I ran all utilities underground and came in through the interior wall in the back corner. I dropped a 4 " pipe into the dirt inside the form that I poured concrete around. Eventually the pipes will be covered with cabinets. I recommend a 2" PVC conduit for the electrical and a separate pipe for the gas and water. Make sure pipes are at least 4-5.5" from the inside of the form to allow for the brick wall and at least 8" from corners to allow for corner post timbers.
  • The Pour
  • Fill dirt: use clay dirt or crushed rock to fill the center of form to 4"-5" below the level of the concrete. Use a compactor to compress dirt. Clean up trenches around the parameter.
    To calculate cubic yards take the area in square feet times the depth in feet and divide by 27. [(3 rooms * 7ft x 7ft x 1/3 ft )+ (15 ft x 15 ft x 1/3 ft)]/27cubic ft/ cubic yard = 4.59 cubic yards
  • Treat soil with termite chemical if desired or required.
  • Plastic sheeting: Cover soil and trenches with 6 mil polyethylene plastic sheeting.
  • Re-bar: Place 2 pieces of #4 re-bar in the trench 2-3" from bottom around the parameter of the form. Use wire to tie together and support on rocks.
  • Wire Mesh: lay welded wire mesh over the entire surface curving edges into trench.
  • Calculate the amount of concrete: Check ditch dimensions and floor depth. Here's the math:
    Floor: (3 rooms x 8' x 8' + 16' x 16') x 4"/12"/ft thick / 27 cu.ft/cu.yd
    = 5.53 cubic yards.
    Integral Beam: 16" deep x 10" wide x (16' + 6*8' + 6*4') / 144 sq.in/sq.ft. / 27
    = 3.62 cubic yards.
    Total: 5.53 + 3.62 = 9.1 cubic yards
    Adjust numbers to actual dimensions and order a little extra concrete, just in case.
  • Invite lots of friends over to help spread concrete. Pour in sections separating the wings with a 2x4 and dividing the main room into quadrants with 2 x 4. Pour wings first. Remove 2x4 dividers as the concrete as the concrete sets up being sure to fill in gaps..
  • Don't Forget to Put the Anchor bolts in before the cement gets too hard. One in each corner and two on back wall 2-3/4 inches from the edge.


[Getting Started] [Foundations] [Framing] [Spires]


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