While most windows can be installed or replaced with ready-made or custom order options, some single-pane windows, such as those used in playhouses and sheds, are easily constructed from scratch. A simple wooden window frame can be made from stock timber with common tools in just a few hours.
Sash and sill
Measure your window opening top to bottom, side to side and inside to outside. Assume a window 60 cm (24 inches) high, 60 cm (24 inches) wide and 13.7 cm (5 1/2 inches) deep inside to outside. Adjust the measurements to the size of your window.
Cut your sill and sash pieces from 2.5 by 15 cm (1 by 6 inch) and 2.5 by 10 cm (1 by 4 inch) timber, using a circular saw. Cut two pieces at 60 cm (24 inches) and two pieces at 28.7 cm (22 1/2 inches) cut from 2.5 by 15 cm (1 by 6 inch) timber. These are your sill. Cut two at 28.7 cm (22 1/2 inches) and two at 52.5 cm (21 inches) from 2.5 by 10 cm (1 by 4 inch) timber. These are your sash.
Build the sill frame from the 2.5 by 15 cm (1 by 6 inch) timber. Create a square 60 by 60 cm (24 by 24 inches). Stand the lumber on edge and use glue and nails to fasten the corners. The two longer pieces will run horizontally along the top and bottom, overlapping the shorter vertical pieces at each side by 1.8 cm (1/2 inch) at each corner.
Repeat the process outlined in step 3 to build the sash frame from the 2.5 by 10 cm (1 by 4 inch). The finished dimension should be 28.7 x 28.7 cm (22 1/2 x 22 1/2 inches).
Installing the glass
Cut eight pieces of 2.5 by 5 cm (1 by 2 inch) timber for glass stop. Cut four at 52.5 cm (21 inches) and four at 48.7 cm (19 1/2 inches).
Lay the sash frame flat on a sturdy work surface. Set two of the 52.5 cm (21 inch) stop pieces in the frame -- one against the top and one against the bottom, flush on the table so that they match up to the back edge of the frame. Glue and nail in place.
Set two of the 48.7 cm (19 1/2 inches) pieces of stop inside the frame -- one against the right side and one against the left. Their ends should touch the top and bottom pieces of stop to form an interior frame with an opening 45 by 45 cm (18 by 18 inches).
Run a bead of siliconized glazer's caulk along the face of the inside 2.5 cm by 5 cm (1 by 2 inch) frame to stick the glass to.
Lay the glass on top of this caulk. The glass should be cut 3 mm (1/8 inch) narrower and shorter than your frame.
Finishing the window
Add the remaining 2.5 by 5 cm (1 by 2 inch) stop to the inside of the glass, running a bead of caulk along the face of each piece that will be touching the glass. Glue and nail in place.
Set the sill frame into the opening and nail or screw into place, and set the sash frame into the sill frame so that the front edge is 2.5 cm (1 inch) back from the lip of the sill on all four edges -- top, bottom and two sides. Glue and nail in place.
Caulk all of the seams in both frames before adding casing and exterior trim to finish the window.