Cold Frame

Cut list

Board layout. Each row is one board, cut left to right, with the remainder hatched. Dark lines are saw cuts, about ⅛″ wide.

Build it

1. 
   01

   Cut the pieces

   Check your panel first. This frame is sized for one twin-wall polycarbonate panel 48″ × 24″. Turn it so its flutes, the hollow channels, run along the 24″ side: they should run down the slope so water and condensation drain out.

   Cut every piece from cedar:

   • A Back and front boards, five at 48″
   • B Side boards, four at 21″
   • C Taper boards, two at 21″
   • D Back posts, two at 16″
   • E Front posts, two at 11″
   • F Side cleats, two at 13½″
   • G Lid rails, two at 48″
   • H Lid stiles, three at 21″
   • I Prop stick, one at 18″
2. 
   02

   Cut the taper boards

   On each taper board (C), mark the full 5½″ width at one end and ¼″ up from the bottom edge at the other end. Join the marks with a straightedge and cut on the line with a jigsaw or circular saw. The two boards are the same: one goes on each side, the wide end at the back.

3. 
   03

   Build the two sides

   Lay a back post (D), a side cleat (F) and a front post (E) on the bench, bottoms lined up: the back post at one end, the front post at the other and the cleat with its center 10½″ from the back end of the side boards. Screw two side boards (B) across them, one above the other, flush with the bottoms and with the outside faces of the posts. Use two 1⅝″ exterior screws through each board into each post and the cleat.

   Set a taper board (C) on top, wide end at the back post. Screw it to the back post and the cleat. Build the other side as a mirror image.

4. 
   04

   Add the back and front

   Stand the two sides up, 48″ apart at the outside. Screw three back and front boards (A) across the back posts, stacked flush with the bottoms, and two across the front posts. The ends are flush with the outside faces of the sides. Check the box is square by measuring its diagonals.

   TipBuild it where it will sit, or on a flat floor. A frame that's out of square won't let the lid sit tight.
5. 
   05

   Build the lid frame

   Lay the two lid rails (G) on a flat surface with the three lid stiles (H) between them: one at each end and one in the middle. The frame is 48″ × 24″, exactly the size of the panel. Pre-drill and drive two 3″ exterior screws through each rail into the end of each stile. Check the diagonals match.

6. 
   06

   Screw on the panel

   Check your panel's own instructions for which face goes up: most have a UV-protected side, marked on the film. Seal the top edge with solid tape and the bottom edge with vented tape, then peel the film.

   Lay the panel on the frame (G, H), flutes running from the back rail to the front rail, edges flush all round. Drill a ¼″ hole through the panel at each screw spot, a little bigger than the screw so the plastic can grow and shrink. Drive the #10 × 1¼″ sealing-washer screws snug, not tight: the washers should just squash.

7. 
   07

   Hinge the lid

   Set the lid on the box, back rail (G) flush with the back of the box, panel up. It rests on the taper boards (C) and the top edges of the back and front, and overhangs the front a little so rain runs off.

   Screw two 3″ stainless butt hinges across the back, 6″ in from each end: one leaf on the back of the lid's back rail, the other on the top back board (A).

8. 
   08

   Add the prop stick

   Drill a ¼″ hole ¾″ from one end of the prop stick (I). Screw it to the inside of the front wall (A), the hole centered side to side and 5″ up from the bottom, with a 1¼″ exterior screw and a washer on each side. Leave the screw a little loose so the stick swings.

   To vent the frame, lift the lid and swing the stick up under the middle stile (H). The lid opens about 11¾″ at the front. Lay the stick down flat along the front wall to close it.

   TipFor a smaller gap on cool days, prop the lid on a scrap block instead. Or fit an automatic vent opener: it lifts the lid by itself as the frame warms up. Check the opener's own instructions for the lid weight, about 7 lb.
9. 
   09

   Set it out

   Set the frame on level ground in full sun, the low side facing south. Rake the soil inside level, or set it over a bed you've already planted. Leave the cedar bare, or oil the outside only.

   Watch outA closed cold frame can cook plants on a sunny day, even in winter. Open it whenever the sun is on it and the air is above about 45 °F, and close it before evening. Don't leave it propped open in strong wind: a gust can catch the lid.