Do It Yourself : Step-by-Step Guide to Building A Cold Frame

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Of the many relics from the Victorian era that are worth re-establishing in our gardens, one of the most useful is the cold frame. Estate houses and market gardens in Europe, America and even Australia used large banks of these mini greenhouses to start seedlings in winter and to grow crops in the adjustable microclimate within.

The simple genius of the cold frame is the clear lid, which when tilted towards the low winter sun, allows light to penetrate the box and warm the interior, while maintaining humidity. In spring, when days are warm and nights are still cold, the lid can be opened to allow hot air to vent during the day, then closed again at night to protect tender plants from frost. This makes a cold frame the perfect environment for raising summer seedlings early in the season, and is useful for striking cuttings during the warmer months of the year.

TOOLS REQUIRED

- A tape measure, builder’s square and pencil.
- A circular saw and hand saw.
- A cordless or electric drill for drilling holes and fixing screws.
- Tin snips for cutting clear roof sheeting.
- Saw horses, a workbench or flat, solid ground for constructing the various pieces.
- Safety gear, including eye and ear protection.

CUTTING LIST

- 3 x 1100 x 150 x 25mm back/front pieces.
- 2 x 500 x 150 x 25mm side pieces.
- 1 x 500 x 150 x 25mm side piece, cut on the diagonal to form two identical pieces.
- 2 x 150 x 25 x 25mm corner posts.
- 2 x 300 x 25 x 25mm corner posts.
- 2 x 1150 x 50 x 25mm for front/back of lid frame.
- 3 x 470 x 50 x 25mm for sides and centre rail of lid frame.
- 2 x 600 x 600mm clear mini orb polycarbonate roof sheeting.

MATERIALS

- 60mm 6-gauge galvanised screws.
- 20mm self-drilling metal fixing screws.
- 2 x butt hinges.
- Natural timber preservative (optional).

Step-by-Step Guide

Step 1 : My cold frame design is a basic box without a floor and with a pitched roof to allow low sunlight to penetrate the interior during winter. I used recycled Oregon (Douglas fir) to make the box, but you could also use untreated hardwood or painted pine. Start by using a circular saw to cut the main box pieces to size, according to the cutting list above.

Step 2 - 3 : Cut internal corner posts to help give the box rigidity. Lay the two pieces that will form the back of the box on a flat surface and attach the corner posts using 60mm 6-gauge galvanised screws for each post. Do the same with the piece that forms the front of the box.

Step 4 : Assemble your box. Attach a side piece by screwing it to the corner post from the inside. Do the same for the other side. Once the two sides are attached to the front piece, fix them to the back pieces by again screwing to the corner post from the inside.

Step 5 : Make a frame for the lid by butt joining four pieces of timber cut according to the list above. Fit a third piece to the centre of the frame to keep it rigid. Cut two pieces of clear mini-orb polycarbonate sheeting using tin snips, and fix them to the frame with self-drilling metal screws.

Step 6 : Attach your lid to the box using butt hinges. To make the lid easy to lift, I attached a cup hook to the front of the frame as a handle. NB: You could also use a recycled window as the lid, but be aware that older windows are unlikely to contain safety glass and might pose a danger to children and pets. Polycarbonate sheeting is lightweight, safe and durable.

Step 7 : Your cold frame is basically complete and can be placed on a gravel or concrete surface ready to use. To help my cold frame resist decay I finished the timber with a natural preservative. If using something prone to rot, such as pine, apply an exterior paint.

Step 8 : On cold winter days keep the lid closed day and night to trap warmth inside. To allow better ventilation on warm spring/autumn days, prop the lid open with a short piece of timber. Close the lid when the temperature drops at night to keep the plants inside warm and toasty

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