Quick Answer
Framing windows in timber structures requires precise cutting and fitting to ensure a watertight seal and structural integrity. A popular method involves using a "rabbet" joint, where the window frame is recessed into the surrounding timber frame. This joint is typically cut with a 1/2-inch (1.3 cm) rabbet, allowing for a snug fit.
Cutting Rabbet Joints
To cut a rabbet joint, use a circular saw or a router with a 1/2-inch (1.3 cm) rabbeting bit. For precise cuts, set the saw or router to 1/2 inch and make multiple passes, checking the depth with a caliper after each pass. For a more accurate cut, use a story stick to mark the location of the rabbet on the surrounding timber frame.
Fitting the Window Frame
When fitting the window frame into the rabbet joint, use a rubber gasket or weatherstripping to ensure a watertight seal. The gasket should be 1/4 inch (6 mm) thick and cut to fit the rabbet joint. To secure the window frame, use 2-inch (5 cm) screws with a rubber grommet to protect the surrounding timber from damage.
Additional Tips and Considerations
When framing windows in timber structures, consider using a “stop” to prevent the window from being pushed further into the rabbet joint. The stop can be a 1/4-inch (6 mm) thick piece of timber, attached to the surrounding frame with 1-inch (2.5 cm) screws. This will also help to prevent water from entering the joint. Use a weather-resistant sealant, such as silicone or polyurethane, to seal any gaps between the window frame and the surrounding timber frame.
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