Build a small cold frame with an open-bottom wooden box and a light, removable polythene lid. The finished box in this plan is 900 mm wide and 600 mm deep, with a 240 mm front and 360 mm back. It is intended for a few trays or pots in a sheltered garden, where you can check it daily.
The lift-off lid is secured with catches when closed and stored safely when removed for ventilation. There is no glass pane to handle and no loose stick holding up a raised lid. This is an original dimensioned project, informed by the sources linked below; we have not physically built or wind-tested it. For the routine after construction, see our beginner’s guide to using a cold frame.
Table of Contents
Decide whether this design suits your plants
A cold frame provides an intermediate growing space, not guaranteed frost protection. The RHS cold-frame guide notes that frames can shelter young plants from mild frosts, while their small air volume allows rapid changes in conditions. Check where to position a cold frame before buying materials: light, shelter and comfortable daily access all matter.
This design uses removable film glazing. It suits someone who can take the lid off in suitable daytime weather and replace it later. If you need an automatically opening roof, continuously adjustable ventilation or documented wind performance for an exposed plot, choose a purpose-designed frame instead. Do not add an automatic opener to this loose lid.
Materials, tools and cut list
All dimensions below are in millimetres and refer to actual finished timber sizes. Measure stock before cutting: timber sold under the same nominal size can finish differently. Use sound exterior plywood, straight exterior-suitable timber and corrosion-resistant fixings. Avoid unknown painted or contaminated reclaimed wood.
| Part | Quantity | Cut size and notes |
|---|---|---|
| Plywood sides, 18 mm thick | 2 | 600 long; mark 240 high at the front and 360 at the back, then cut the sloping top |
| Front, 18 mm plywood | 1 | 864 wide × 244 high blank; final outer top edge 240 high |
| Back, 18 mm plywood | 1 | 864 wide × 360 high blank |
| Corner cleats, 44 × 44 timber | 4 | 2 at 200 long for the front; 2 at 320 for the back |
| Lid front and rear rails, 44 × 44 timber | 2 | 940 long |
| Lid sides and centre rail, 44 × 44 timber | 3 | 562 long; fitted between front and rear rails |
| Film-clamping battens, 38 wide × 19 thick | 4 | 2 at 940 long; 2 at 574 long, fitted between the long battens on the lid underside |
| Clear horticultural polythene | 1 | 1,200 × 900; UV-stabilised, about 180 microns thick, with supplier fitting instructions |
You also need exterior wood screws, a suitable wood finish, eight small right-angle reinforcing brackets for the lid, four positive-closing draw latches with matching keepers, a handle, and base anchoring brackets and fixings suitable for your chosen site. Buy hardware before assembly so its mounting holes fit the rails. For the box, 4 × 40 mm screws pass through the 18 mm panels into the cleats; select shorter screws for thin battens and brackets so their tips cannot emerge.
Tools include a tape, square, pencil, clamps, supported cutting bench, saw suitable for the plywood, drill/driver, pilot bits, countersink, hand plane and abrasive paper. Follow the tool instructions, support work fully and use the specified eye, hearing and dust protection. A timber merchant can cut the main blanks if you cannot support sheets safely.
1. Make the sloping box
Mark the two side panels together, then cut and smooth their top edges. Stand them with the front and back panels between them. The outside width is 18 + 864 + 18 = 900; the sides provide the full 600 mm depth. Keep all four bottom edges level. Clamp a cleat inside each corner, pilot-drill and screw each adjoining panel into the cleat, staggering screws so they do not collide.
Measure the two diagonals across the footprint before tightening: equal diagonals confirm a square rectangle. Check the base does not rock. The open floor is intentional, allowing the frame to sit around pots or over soil. Choose the site using the comparison of soil, gravel and paving bases.
The lid needs a continuous sloping seat. Plane the front and back top edges to follow the side-panel slope. Across each 18 mm panel thickness, the height changes by 3.6 mm: the front is 240 at its outside face and 243.6 at its inside face; the back is 360 outside and 356.4 inside. Mark these lines before planing, then check with a straightedge across both sides. Do not leave the front square-topped and force the lid down.
2. Assemble the lid square and flat
Lay the two 940 mm rails parallel. Fit the two 562 mm sides between them: 44 + 562 + 44 gives the 650 mm lid depth. Centre the third 562 mm rail between the sides to support the cover. Check diagonals, pilot-drill the joints and secure them, then reinforce the four corners and both ends of the centre rail with brackets fitted to the inside vertical timber faces. Use one bracket at each corner and two at each centre-rail end. Keep all brackets and screw tips clear of the film and the underside seating surfaces.
The body’s sloping top is approximately 612 mm long, rather than 600 mm. A 650 mm lid therefore provides about 19 mm overhang at each sloping end when centred, plus 20 mm at each side. Mark this centred position on the box before fitting catches. Check that the bare lid sits evenly. The underside battens added next form the final contact surfaces; check the seating again after covering, before locating latch keepers.
Round every timber edge that will touch the film and remove splinters. Apply your exterior finish, including to plywood edges, following its label. Let it cure fully before attaching the cover or placing plants inside. Bosch’s cold-frame project provides another example of a timber-box build, but its dimensions and rigid-sheet glazing are different from this plan.
3. Fit the replaceable cover
Use genuine horticultural film with stated UV protection; 180-micron polytunnel film is one suitable material type. A supplier’s UV warranty is not a guarantee that this DIY lid will survive storm damage. Read the film’s orientation and chemical-compatibility instructions; First Tunnels’ information sheet sets out handling and installation conditions.
Centre the 1,200 × 900 sheet over the lid. This leaves 130 mm at each side and 125 mm at the front and back for wrapping. Pull it evenly across the top without wrinkles, wrap it around the outside edges, and trap it under the underside battens. Clamp first, then pilot-drill and screw the battens in place, working from the middle of opposing edges outwards. Fold corners neatly without cutting into the covered area; trim only the excess beyond the secured battens.
Timber battens distribute the grip over the film rather than relying on isolated staples. This follows the fixing principle described by Northern Polytunnels and First Tunnels’ covering guidance; it does not transfer a polytunnel’s structural rating to this box. The finished cover should shed water down the slope without sagging pockets.
4. Secure the lid and the whole frame
Fit the handle to a solid rail, not through the film opening. Fit one draw latch on each side of the box, with its matching keeper on the lid rail, in the centred position marked earlier. Follow the hardware instructions for screw size and mounting orientation. All four must close positively without distorting the lid. Open every latch before lifting; never pull against a partly fastened lid.
The RHS identifies gusts as a major cause of frame damage and advises securing lightweight frames. Attach the box through suitable brackets to appropriate ground anchors or sound hardstanding, following the fixing manufacturer’s instructions. Check for buried services before driving anchors or drilling. Soil and paving need different fixings; short tent pegs and loose bricks are not a universal solution.
This build has no certified wind or snow rating. On an exposed site use a rated commercial structure; before forecast severe weather, move the plants to suitable shelter and dismantle or secure-store this frame. Do not leave the film lid loose outside, and never use stones on top as makeshift catches.
5. Ventilate, check and maintain
Begin with a thermometer at plant height and observe conditions before filling the frame with valuable plants. For ventilation, unlatch the whole lid, lift it off and store it securely; do not balance it on the box or prop one edge open. Follow the daily ventilation guide and overheating precautions, adjusting protection to the plants and forecast.
Use the frame for gradually hardening off young plants, while moving frost-sensitive plants back to adequate protection when necessary. Inspect the cover, battens, latches and anchors regularly. Repair tears promptly with film-compatible repair tape, replace badly damaged film, and tighten loose hardware. If upgrading to a rigid sheet later, use the glazing comparison and redesign its fastening for that sheet’s expansion and support requirements; do not substitute it into this film-batten arrangement.