How to build a cold frame

Inspired by a helpful YouTube guide. We based this walk-through on the popular UK video "How to Make A Cold Frame Step-by-Step" from the GrowVeg channel, which covers sizes, materials, and glazing options clearly. The bit on using old window frames as the lid is worth watching — it is a brilliant use of something most people throw away.

1. Decide on size and materials

A good starting size is around 1.2 metres wide by 0.6 metres deep. That is comfortable to reach across and fits neatly over a standard seed tray. Make it longer if you have the space — the lid can be split into sections so you can open part of it at a time.

Pressure-treated timber is the practical choice. 150mm × 25mm boards are plenty sturdy enough and available at any timber merchant. You can also use exterior-grade plywood or, if you have them, old railway sleepers.

For the lid, options include twin-wall polycarbonate sheet, old single-glazed window frames, or even bubble wrap stapled to a timber frame in a pinch. Polycarbonate is lighter and safer than glass; old window frames are free but heavy.

2. Cut the side boards

Cut two side pieces to your chosen depth — say, 600mm long. These will be the left and right sides of the frame. Cut the front board to your chosen width. Cut the back board to the same width.

The front should be shorter than the back so the lid sits at an angle and rainwater runs off. A front height of around 200mm and a back height of 300mm works well. That 100mm difference across 600mm of depth gives a gentle but effective slope.

3. Shape the side boards for the slope

Mark a diagonal line on each side board from the back height down to the front height. Cut along this line with a hand saw or jigsaw. Both sides need to be mirror images of each other, so take your time with the marking. A straight cut here makes everything else easier.

4. Build the frame

Assemble the four boards into a rectangle. Fix corners with 75mm exterior screws — two screws per corner is enough. For extra rigidity, nail or screw a short off-cut of timber into each internal corner as a cleat.

That is essentially the box done. Simple. It sits directly on the soil or on a level hard surface. You do not need a base.

5. Make and glaze the lid

Build a lightweight frame from 50mm × 25mm timber to match the top dimensions of the box. Fix the corners with halving joints or simple butt joints and screws. The frame needs to be rigid enough to carry the glazing without flexing.

Cut twin-wall polycarbonate sheet to fit the frame and fix it with polycarbonate screws and washers. Seal the open channels at each end with purpose-made end caps or a strip of waterproof tape — otherwise algae gets in and it looks murky within a season.

If you are using an old window frame, just check the glass is in good condition and that the frame is not so heavy it is awkward to lift. Hinges are optional — many people simply prop the lid open with a stick.

6. Position the cold frame

South or south-west facing is ideal. The sloped lid should face the sun. A sheltered spot near a fence or wall is better than an exposed one — you want protection from north and east winds.

On cold nights, drape an old piece of fleece or bubble wrap over the lid for extra insulation. Ventilate on mild days by propping the lid open — seedlings cook quickly if it heats up inside. Mind you, even in March a cold frame can reach 30°C on a clear day.

When to call a handyman

If you want something more permanent — a brick or block cold frame built against a south-facing wall, or a lean-to greenhouse style structure — that is a job for Richard. Likewise if you need raised beds built at the same time and want the woodwork done properly in a single visit.