Frame a greenhouse in rot-resistant wood (10×10 cm posts at 120-180 cm spacing with 5×10 cm rafters), in bamboo lashed with cordage or wire, or in salvaged conduit, rebar or pipe bent into hoops. Whatever the material, check the rafter against the snow it must carry, and remember that snow load is a force measured in newtons, not kilograms.
A greenhouse frame is the skeleton that holds the glazing against wind and snow while casting as little shadow as it can.
The Frame Must Be Light Enough To Admit Sun And Strong Enough To Hold Snow
Shadow from the frame subtracts light the plants need, so a good frame is slender. Slender members are also close to their limit when snow lands on them, so the real design question is how much snow the roof will see, and whether the rafters can take it. A typical wood frame uses 10×10 cm posts at 120-180 cm spacing and lighter 5×10 cm rafters supporting the glazing.
Wood is the most accessible. Use rot-resistant species (cedar, locust, oak) or treat the timber with linseed oil. Bamboo is excellent where it grows: lash the joints with cordage or wire, and expect to replace it every 3-5 years. Salvaged metal — conduit, rebar or pipe — bent into hoops makes a tunnel greenhouse, fast and cheap, and covered with plastic film. Timber and Building Materials covers sourcing wood; Structural Engineering explains bending.
What You Need
- Posts and rafters, bamboo poles, or lengths of pipe
- Cordage, wire, nails or pegs to join them
- A snow estimate for your site: the depth and wetness of the heaviest snow you have seen
- A tape and a notebook, to do the sum below before you cut anything
Method: Check A Rafter
Step 1: Decide the design snow. Choose a depth and density from local experience. The sum below assumes 0.30 m of wet snow at 250 kg/m³; both are assumed inputs, not measurements, and wet snow is far heavier than fresh powder.
Step 2: Convert to a force per area. Mass per area times g gives force per area. Do not stop at kilograms.
Step 3: Find the load on one rafter. Multiply by the width of roof the rafter carries (its spacing).
Step 4: Find the bending stress. Treat the rafter as a simply supported beam: maximum bending moment M = w × L² / 8, and stress = M / Z, where Z = b × d² / 6 for a rectangle.
Step 5: Compare with an allowable stress for your timber. A reasonable allowable bending stress for sound, dry softwood is on the order of 5-10 MPa; this is a recalled textbook range, not a measurement of your timber. Use a lower figure for knotty or weathered wood.
Worked Example
Inputs (assumed): snow depth 0.30 m, snow density 250 kg/m³, rafter span L = 3.0 m (horizontal), rafter spacing 0.6 m, rafter section 5 cm wide × 10 cm deep, glazing is 3 mm glass at 2,500 kg/m³, post bay 1.5 m × 3.0 m.
- Snow mass per area = 0.30 m × 250 kg/m³ = 75 kg/m².
- Snow force per area = 75 × 9.81 = 735.75 N/m² (about 0.74 kPa).
- Glass force per area = 0.003 × 2,500 × 9.81 = 73.6 N/m².
- Total vertical load on the roof = 735.75 + 73.6 = 809.3 N/m².
- Load on one rafter per metre of its length = 809.3 × 0.6 = 485.6 N/m.
- Bending moment = 485.6 × 3.0² / 8 = 546.3 N·m.
- Section modulus Z = 0.05 × 0.10² / 6 = 8.33 × 10⁻⁵ m³.
- Bending stress = 546.3 / 8.33 × 10⁻⁵ = 6.56 × 10⁶ Pa = 6.56 MPa.
That is inside the 5-10 MPa range but not far inside it, so margin is thin if the timber is poor.
The force on one 1.5 m × 3.0 m post bay is the snow force per area times the area: 735.75 N/m² × 4.5 m² = 3,311 N, about 3.3 kN. In kilograms-force that is 3,311 / 9.81 = 337.5 kg, the weight of about 340 kg. Writing “3,300 kg” would overstate the force by a factor of 9.81.
Common Mistakes
- Reporting force as kilograms — 3,311 N is the weight of 337.5 kg, not 3,311 kg.
- Using powder-snow density for wet snow — wet snow is much heavier and falls just as often.
- Ignoring drifts and ponded water — a hoop covered in sagging film collects snow and rain in the dip.
- Salvaged treated lumber over food soil — creosote and arsenical preservatives leach into the bed.
Safety
Roof failure and sudden snow slides
A glazed roof that fails under snow drops glass on anyone beneath it. Clear heavy snow from the outside with a soft pole, working from a stable position and never from inside the greenhouse, and keep out from under the eaves after a heavy fall.
What’s Next
- Learn next: Wind Loads — the other load on a thin glazed structure
- Combine with: Glazing Options — covering the frame