What's inside
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A small cedar cold frame is a simple way to protect seedlings from wind and light frost while giving them a head start outdoors. This plan makes a box about 3 feet wide by 4 feet long, sized to sit over a garden bed or hold nursery pots. It uses common lumber, a hinged clear lid, and basic tools. Expect to spend an afternoon building it, plus time for the finish to dry if you use one.
Contents
Choose a size and site
Set the frame where it will get at least six hours of sun, ideally with the back edge facing north and the lid sloping toward the south. That orientation admits low spring sun and sheds rain. Leave room to open the lid fully and reach inside without stepping on the bed.
The dimensions below give a 36-by-48-inch footprint. The back wall is 16 inches high and the front is 10 inches high, creating a slope for the lid. These heights fit many seed trays and small pots. If you plan to grow taller starts, raise both walls, but keep the lid’s slope and check that you can still reach the back row.
Materials and tools
Use cedar boards that are untreated and reasonably straight. Cedar resists rot, but it is not rot-proof: soil contact, standing water, and trapped moisture will still shorten its life. You’ll need two 1-by-8 cedar boards, 8 feet long, for the sides; one 1-by-10 and one 1-by-6, each 8 feet long, for the front and back; and four cedar corner cleats about 1½ inches square and 10–16 inches long. Actual board widths vary, so measure yours before cutting.
For the lid, use a 36-by-48-inch piece of UV-resistant twinwall polycarbonate, or a suitable clear window panel. Polycarbonate is light and less likely to shatter; glass is clearer and usually resists scratching better, but it is heavier and can break dangerously. A twinwall polycarbonate panel is a practical choice for a small hinged lid. Also get two exterior-rated hinges, exterior screws, and a lid prop or chain. A drill, saw, tape measure, square, and eye protection are the basic tools. A box of exterior-rated screws helps avoid rust stains and loose joints.
Cut the boards
Cut the two long side boards to 48 inches. Cut the front and back boards to 36 inches. Set the front board to 10 inches tall and the back to 16 inches; if your lumber is sold in those nominal widths, trim to the desired height with a saw. The sloped top edges of the side boards need to rise 6 inches from front to back over 36 inches. Mark the line with a straightedge and cut both sides to match.
Dry-fit the four walls on a flat surface. The side boards should sit between the front and back boards, giving a final outside length close to 48 inches. Adjust the cuts if your chosen layout differs. The lid should overlap the outside of the box by roughly ½ inch on each edge to shed water.
Assemble the box
Place a corner cleat inside each corner and screw through the walls into the cleats. Drill pilot holes near board ends to reduce splitting, especially in dry cedar. Use two or three screws at each joint. Check the box for square by measuring both diagonals; they should be within about ⅛ inch of each other. If they differ, push the longer diagonal inward before tightening the last screws.
Leave the bottom open so water can drain and roots can reach the soil if the frame sits directly on a bed. If it will sit on paving, add a removable floor made from slats with ¼-inch gaps rather than a sealed panel. A solid bottom holds water and can rot seedlings as well as wood.
Build and hinge the lid
For a simple lid, make a cedar frame from 1-by-2 stock to fit around the clear panel. Assemble the four sides with exterior screws, then attach the panel as directed by its manufacturer. Twinwall panels have channels; orient them so the channels run downhill and can drain. Seal or cap the upper ends as recommended, but do not block drainage at the lower ends.
Lay the lid on the frame with its back edge aligned to the taller wall. Attach two hinges to the back, spacing them about 8–12 inches from the ends. Open and close the lid before loading the frame. Fit a prop, chain, or purpose-made cold-frame lid prop so a gust cannot slam it shut. Never rely on a loose stick if children or pets can reach the frame.
Compare lid materials
| Material | Advantages | Trade-offs |
|---|---|---|
| Twinwall polycarbonate | Light, insulating, and less prone to shattering | Costs more than basic plastic; needs correctly oriented channels |
| Clear glass | Good light transmission and a durable surface | Heavy and breakable; use only if the frame and hinges support the weight |
| Clear plastic film | Cheapest and easy to replace | Sags, tears, and insulates less; needs a taut frame and more frequent repairs |
For a small frame that will be opened every day, polycarbonate is a sensible balance. Film is fine for a low-cost seasonal build if you accept replacing it. Avoid ordinary glass unless it is safely supported and you are comfortable handling it.
Protect the wood and manage heat
Sand sharp edges and brush off dust. Leaving cedar unfinished is acceptable, especially if you want to avoid coatings near edible plants; expect weathering and eventual replacement. If you apply a finish, use one labeled for exterior use and follow its cure time before putting seedlings inside. Keep the bottom edges out of standing water and set the box on gravel or firm soil that drains.
A closed cold frame can overheat on a sunny day even when the air feels cool. Open the lid a few inches when the interior climbs above about 65–70°F, and close it before temperatures drop in the evening. Use a thermometer at plant height rather than guessing. Water sparingly: the lid reduces rain, but poor ventilation and wet soil encourage mold and damping-off. In a hard freeze, the frame offers only limited protection; add row cover over the plants or move tender starts indoors.
Before sowing, check that the lid opens freely, the panel drains, and the prop holds securely. These small checks prevent the common failures: water pooling in the lid, a warped box that no longer closes, and seedlings cooked by a sunny afternoon.