How to Build a Cold Frame

The window lid will be hinged along its length and sloped to let in maximum sunlight. Measure the window's long edge to get the length for the front piece and the two back pieces. Sandwiched between, the sides taper back to front, which means they'll be shorter than the window is wide. To determine their length without resorting to geometry, stage one end of the frame two-dimensionally: Butt two boards end-to-end, then pair up a third parallel with one of them to mimic the height of the back. Slide the single board away from the pair and open a gap approximately as wide as the window. Then stand the window on end, angling it from the "back" to the "front," and adjust the gap until the window just overlaps the boards at either side, as shown. Now measure the gap to determine the length of each side piece.