When walls are thick, you need wide extention jambs and a deep stool.

When trimming windows in thick walls (basement walls, for instance), you need wide extension jambs and thick stools. Montana builder and remodeling contractor Chris Whalen displays his method for building out jambs to accommodate windows in thick walls. Chris starts by building a deep stool extension, then cuts and attaches the stool. Then, using biscuit joints, he attaches the top and side jambs with an offset. Finally, Chris completes the work by attaching the head casing.

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