Return to main tree page 

AVTreeFarm  

Tips on Building Your Greenhouse 

             

 

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. (See price list for foundation dimensions.)
  • 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 (figure 1)

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  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 (figures 2 & 3)

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 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.

Reglazing With TwinWall or TripleWall
Upgrade your greenhouse

 

 TwinWall is a significant improvement for a single-glazed greenhouse.
  • Remove old glazing
  • Cut TwinWall to size using a box knife or a very fine tooth saw blade
  • Install new glazing gasket if necessary
  • Note panels have one side which must face the sun
  • Remove protective film from inside face
  • Close exposed ends of panels with TwinWall capping
  • Set TwinWall panels in place and secure with the same method as the old glazing.
  • Now immediately remove exterior protective film
  • Never store TwinWall in direct sun or film will not release
  • Apply clear sealant where TwinWall tucks into frame or capping.

 

Adding TwinWall or TripleWall to your wood framework is easier when using these special pieces to hold the panels in place. The Caps are made of a tough, sunlight resistant polycarbonate, and are weatherproof and maintenance-free.

Use End Caps on the top and bottom of TwinWall or TripleWall to close the channels. This keeps out dirt and insects. Also use End Caps on the outside edge of corner panels (Fig. 1).

The Joiner Cap is used to connect TwinWall or TripleWall along vertical edges (Fig. 2).

Builder's notes: Drill Caps every 12" for screws. Design framework 24-1/2" O.C., or plan to rip TwinWall to 23-3/4" wide. Cut capping with hacksaw.

 
Building With TwinWall
 

 

  1. Apply 1/16" Glazing Tape (#1521) to framework. Install TwinWall panels on tape. Seal all roof joints between panels. Also seal between panels and framework with Foil tape.
  2. Apply 1/16" Glazing Tape to underside of Bar Caps.
  3. Apply Sealant #1520 to underside of Roof Bar Caps at screw holes. Install Bar Caps.
  4. Seal screw heads.

How To Build With Fiberglass



1. Use any good quality wood for your greenhouse framework. Be sure to treat the pieces with a wood preservative before assembly, even cedar or redwood. Copper or zinc based preservatives are safest.

2. 2 x 4 lumber is adequate for framing, but use heavier lumber in snow zones or where rafter length is more than 8 ft. (Check your local building code.)

3. Space rafters and studs 2 ft. apart and install blocking every 24" to 36" in walls and between rafters.

4. Flat fiberglass is rolled out horizontally on roof, but can be installed either horizontally or vertically on walls. (Corrugated fiberglass should always be installed vertically.)

5. Caulk all fiberglass seams.

6. Secure flat fiberglass with wood lath on rafters and studs (no horizontal lath). Nail fiberglass to blocking with aluminum washer-head nails.

7. Provide for ventilation with an exhaust fan system, or large vents in walls.

Framing Notes for Fans & Shutters



For the greenhouse:

Frame wall openings square and 1/2" larger than fan blade and shutter sizes.

For finished sunroom:

Frame rough openings 3-1/2" larger than fan and shutter size. Line opening with 2x4, offset enough toward outside to project past siding. Install fan or shutter. Finish inside by lining rough opening with 1x3.

 



 Solar-Powered Vent Controls

These "top of the line" controls will open and close greenhouse vents automatically without electricity.  vent openers convert energy from the warmth of the sun into mechanical energy to open vents. The hotter it gets, the wider the vents are opened.

Materials: openers are made from the best corrosion resistant materials: brass, stainless steel and satin-anodized aluminum. They are designed to give many years of service, are covered by a 2-year guarantee, and can be repaired should the need arise.

Capacity: The opener can exert a force of 14 lbs. at the point where it is attached to the vent. A threaded adjuster bolt can set the opener to start opening the vent at temperatures between 55º F and 75º F. Maximum opening of 12" is reached at approximately 20º F above the starting temperature.

Openers can be installed in a series on a long vent to replace worn out electric vent motors. The adjuster bolt can equalize the loading
so the lifters work in unison.

Installation: Easy to install with screws: top arm to vent, bottom arm to greenhouse frame. Illustrated directions included. Openers are about 12" long and fit neatly up close to the glass.