Building a cold frame from old windows

Old windows can become the glazed roof of a compact seedling shelter, giving discarded timber and glass a practical second life. The basic build consists of a timber base, sloping lid, and screened ventilation. It extends the growing season by trapping warmth, sheltering tender plants from frost and wind, and reducing damage from heavy rain. A salvaged structure suits a backyard plot, community garden, allotment, or even a sunny side passage.

Australian conditions determine how the frame should work. Melbourne’s cool, damp winters reward good drainage, while Sydney’s humid summers can turn an unventilated box into an oven. Adelaide often experiences sharp spring warmth followed by hot, dry weather, making shade and adjustable airflow as important as insulation. Melbourne salvage yards, Sydney architectural-reuse shops, and Adelaide secondhand building dealers are useful places to look for mismatched windows, but every opening should be measured before purchase.

Option	Best suited for	Main strengths	Main limitation
Portable cold frame	Seed trays and small seedlings	Cheap, movable, simple to build	Less stable in strong wind
Ground-level permanent frame	Vegetables and mixed crops	Stable, roomy, good soil depth	Requires careful drainage
Raised bed with a glazed lid	Larger garden beds	Combines planting space with crop protection	More materials and a larger roof
Small window cloche	Herbs, lettuce and vulnerable plants	Low cost, quick to assemble	Limited headroom and ventilation

Choose and inspect salvaged windows

Choose windows with sound timber joints and panes that do not rattle in their frames. A lid around 900 millimetres wide and 600 millimetres deep is large enough for several trays, while a smaller unit can cover one square metre of bed. Old sashes are particularly useful because their opening sections can be incorporated into the side walls, although the height must still allow room for ventilation.

Measure the visible glass area rather than only the outer frame. Check that each pane is flat, free of deep scratches, and held securely by putty or glazing beads. Cracked glass should be replaced with safety glazing or rigid polycarbonate rather than left overhead. Avoid sanding old paint until you know what it contains, and wear eye protection, gloves, and a suitable dust mask while cleaning.

Weathered timber can remain visible if it is structurally sound. For a clean visual result, pair it with simple fittings and plain finishes; the restrained forms in modern decorating ideas illustrate how uncluttered details can make reclaimed materials look intentional. Seal any gaps around the panes with a flexible, paint-compatible sealant after checking that the old putty is not crumbling.

Set the dimensions and layout

A portable frame works well over a group of propagation trays, while a fixed version can straddle a narrow bed. Choose the site before construction. It needs several hours of direct winter light, protection from prevailing wind, and enough space behind the frame for a person to open the lid and access the soil. Avoid placing it where roof runoff from a larger structure will collect around the base.

The back wall should be roughly 450–600 millimetres high, with a front wall around 200–300 millimetres. This creates a slope that directs water toward the front drainage gap rather than into the back corner. Mark the base as a rectangle, then add about 50 millimetres to each side for the timber frame. Draw the lid opening on the ground before cutting any parts.

Use exterior-grade timber, corrosion-resistant screws, and sealed joints. If you need additional framing, base the order on actual measurements rather than the dimensions of a sale item. For online purchases, check the payment payout information before assuming that a quoted discount has reached your account.

Build a stable, insulated base

Place the frame on level, well-drained ground. Paver stones, brick pads, or short treated posts can create an elevated base and prevent timber from sitting directly in wet soil. In areas where termites are active, use treated structural timber, metal brackets, or a physical barrier installed according to local guidance. Do not rely on a shallow timber perimeter alone where floodwater or summer rain regularly reaches the site.

Add a waterproof membrane beneath the soil and a shallow gravel layer where necessary for drainage. Rigid insulation fitted inside the timber perimeter can reduce heat loss, but it must not block drainage outlets. A removable insulation lid is useful during cold nights and can be taken away as temperatures rise. Seal gaps around pipes or service openings so that warm air cannot escape through unintended channels.

Fill the bed with a loose, fertile growing mix suited to the crops. Use mature compost rather than fresh manure, following the site’s home composting guide for proper pile management and a safe curing period. Add leaf mould or well-rotted organic matter to improve structure, and avoid compacting the bed heavily before sowing.

Frame the sides and lift the roof

Construct the rear and side walls from exterior plywood or reclaimed timber, using the dimensions already marked out. Brace the corners with short offcuts and fasten each joint with screws rather than nails alone. A central upright on the back wall provides extra support for a wide lid and reduces flexing when the roof is opened.

Mount the salvaged windows horizontally over the base to form a sloping roof. Hinge the upper edge at the tall rear wall so that the lower edge lifts easily. Add a centre rail if the windows are small or their timber is thin, and reinforce the outer frame with timber battens. The lid should open far enough for access without putting strain on the hinges.

Secure the glazing with removable screws, clips, or flexible glazing tape. Do not overtighten old frames or force cracked panes into position. Leave a small drainage space at the front edge and fit a shallow rain lip to prevent water from running back beneath the roof. The result should feel firm when lifted, yet still be light enough for one person to reposition when the base is portable.

Add ventilation and weather protection

A cold frame needs a low ventilation opening even if the roof can be raised. Leave a 50–100 millimetre gap above the front wall and cover it with fine insect mesh. If possible, fit the same opening along the upper rear wall for cross-flow. A simple prop holds the lid at a chosen angle, while a purpose-made automatic vent opener allows the roof to rise as the internal temperature increases.

During heavy rain, direct the water through the front gap rather than over the rear wall. Check that the lid cannot slide or slam shut in a gust, particularly if the structure stands near a fence. In coastal districts, salt-laden air can corrode unprotected fasteners, so rinse them periodically and use galvanised or stainless components where practical.

Shade cloth is essential in much of Australia. A 30–50 per cent shade over the roof during hot weather limits overheating without blocking winter light. Remove fallen leaves and keep combustible materials away from the frame, particularly during the bushfire season. Fine metal mesh can reduce ember entry, but it does not make the structure fireproof, so the surrounding area should remain clear and the frame should not be operated during dangerous fire conditions.

Prepare soil, water and heat

Create a free-draining root zone roughly 200–300 millimetres deep for most vegetables. Fork the compost through the soil rather than placing it in a heavy layer at the bottom, which can leave roots sitting in damp conditions. Raise seedling trays above the soil if drainage is slow, and position them near the lid so they receive extra warmth without being scorched.

Water in the morning so that excess moisture can evaporate during the day. A narrow drip line beneath the lid can provide regular moisture, while a container with a slow-release wicking outlet is useful where mains irrigation is unavailable. Captured rainwater suits annual crops, although householders in areas of Sydney and other Australian cities should check current tank and water-saving rules before installing substantial storage.

A little heat can come from the soil itself. Mature compost, moist growing medium, and a closed lid on a mild day may raise the internal temperature several degrees. Electrical heating cables require a thermostat, protective earth, and a residual-current device; use certified equipment and have the installation checked if the supply is exposed to damp conditions. Remove electric heating during warm weather so that ventilation and shade remain effective.

Finish, monitor and maintain the build

Apply a low-toxicity exterior sealer to sound timber that will be handled frequently. Water-based paints are easier to repair than brittle oil coatings, although they should not cover glazing putty. Use masking tape and a brush with enough control to avoid marking the glass. A removable finish allows weathered timber to age naturally when that suits the garden design.

Inspect the frame weekly for cracked panes, loose screws, blocked vents, condensation, and pooled water. Remove dead leaves and decaying plant material promptly to discourage slugs, ants, fungal problems, and rodents. Replace damaged mesh, since small insects can enter through gaps that appear negligible. Check termite shields and timber that meets the soil, particularly after prolonged wet weather.

Review the setup with each seasonal change. In Melbourne, improve winter drainage and retain the roof insulation. In Sydney, increase shade and ventilation as humidity rises. In Adelaide, monitor the interior temperature closely during sudden hot spells. Clearing vents, washing glass, checking anchors, and renewing the soil surface keeps the recycled structure productive through repeated seasons.