Build a cold frame box as four walls on level, well-drained ground: a back wall 40-50 cm tall, a front wall 20-30 cm tall, and two side walls cut to taper between them, all enclosing a 120 by 180 cm bed. Use rot-resistant boards, logs, stacked stone or earthbags, because the walls sit directly on damp soil for years.
The box of a cold frame is a bottomless, tapered enclosure whose walls hold the lid at a slope and keep the warmed air and soil in.
Tapered Walls Give The Lid Its Slope
The only structural idea in a cold frame box is the taper. Because the back wall is taller than the front, a lid laid across the top slopes down toward the sun, sheds rain and catches low winter light. Everything else is ordinary carpentry or masonry, so the work goes quickly and the materials can be almost anything that does not rot.
What You Need
- Wall material — boards, logs, stacked stone or earthbags. For timber, rot-resistant species such as cedar, locust or oak last longest; treating plain boards with linseed oil also helps
- Corner fastening — nails, pegs or lashing, plus a stake at each corner to hold the walls square
- A level and a string line to keep the top edges true
- Soil or straw to bank around the outside in cold weather
- A lid and hinge, covered in Lid Materials & Hinges
Avoid salvaged lumber that was pressure-treated or soaked in preservative if the soil inside will grow food, since preservative can leach into the soil.
Method: Build The Box
Step 1: Level and mark the footprint. Clear the sod, level the ground, and mark a rectangle 120 cm front to back and 180 cm wide. Check that the two diagonals are equal; if they are, the box is square.
Step 2: Set the back wall first. Build it 40-50 cm tall along the north edge. A wall of stacked stone or earthbags needs a firm base; boards need stakes on the outside.
Step 3: Build the front wall. Build it 20-30 cm tall along the south edge. Check with a level that both walls are level along their length.
Step 4: Cut the side walls to taper. Each side wall is a board (or stack) whose top edge runs from the back height down to the front height. Mark the slope with the string line and cut along it. In the example below, each side wall is 120 cm long, 45 cm tall at the back and 25 cm tall at the front.
Step 5: Fasten the corners and backfill. Pin the four walls together. Press soil against the outside base so wind and drafts cannot get under. In cold weather bank soil or straw higher up the walls.
Step 6: Prepare the bed. Loosen the soil inside, work in compost, and rake it level. See Raised Beds & Ground Beds for bed preparation.
Worked Example
Lid slope from the wall heights: back 45 cm, front 25 cm, depth 120 cm. Rise = 45 − 25 = 20 cm. Angle = arctan(20 / 120) = 9.5°. Lid area = 1.2 m × 1.8 m = 2.16 m², so two window sashes each 90 cm wide set side by side cover the 180 cm length (2 × 90 = 180 cm).
To steepen the lid, shorten the depth or raise the back. A shallower 60 cm box with a 30 cm difference between back and front gives arctan(30 / 60) = 26.6°. The price is a bed half as deep, so you can plant less of it.
Common Mistakes
- Untreated, soft boards on bare soil — they rot within a few seasons; use a rot-resistant species, stone, or a gravel base.
- Walls out of level — the lid rocks and gaps let cold air in.
- No bank of soil outside — wind slides under the walls and the frame loses its heat.
Safety
Lifting and hand tools
Stone and earthbag walls are heavy; lift with your legs and with a partner. Wear gloves when working with salvaged boards, which may hold nails.
What’s Next
- Learn next: Lid Materials & Hinges — what to put on top and how to open it
- Combine with: Hotbed Variation (Manure-Heated) — the same box over a manure layer