Replacement Windows

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Window flashing must be slid under the siding that extends across the top of the enlarged opening to make a drip cap to lead rain water out over the window—so it won't get behind to rot the wood. (See illustration: "Window flashing ....") Measure the thickness of the top trim boards to know what width of preformed (L-bent) window flashing to buy. Or, you can bend aluminum flashing to make your own—necessary for board and batten and other unconventional siding.

I strongly recommend painting/staining both sides of unclad wood sash before installation. The vinyl gasket holding the insulated glass in place and the jamb liners in which the sash slides should not be painted. Keep paint off the weather stripping as well. Bare wood should be primed with an oil-base primer to avoid raising the wood grain, whether you're using oil or latex finish coats.

Lift the new window into place from the outside. Open double-hung sash to provide a better grip if you wish, but be sure to shut it tight before fastening or you can bow the frame. The top trim board should fit in under the drip cap. Center the unit in the opening so the space around it is equal on both sides. A helper can hold the window in place while you square and fasten it from outside. Use long finish nails to fasten through wood; long flat-headed roofing nails are used to fasten the plastic or metal nailing flange that protrudes around the margin of many clad cases.

Use a carpenter's level to determine which end of the window sill is lowest. Tack the higher side with a galvanized finish nail through the lower corner of the side trim and about an inch from the outside edge of a flat casing so it will reach into the surrounding studs of the rough opening. Using wedges or wood blocks, block the sill up till it is level and tack the corner on the free side.

Plumb one side of the window (so it is perfectly vertical side-to-side and front-to-back) with the level. Tack nails through the casing sides about halfway up and in the upper corners. Confirm that your diagonal bracing held the window square and the sashes look to be evenly spaced, plumb and level in the frame. Complete fastening the case with nails every foot or so around the frame, including across the header trim. Drive the nails home with a nail set, plug nail holes and finish.

Use a long-lasting caulk all around the outside of the window. If you find more than a 1/4-inch gap between the trim and the siding, whittle spacers from scrap, tack them in, and caulk over.

You'll need to splice in new siding if the window is smaller than original; many an old window modification is revealed by a frame of short pieces of siding. In any event, a new section of clapboard or other siding should be spliced in beneath the sill to extend up into a manufacturer-routed channel, and the joint generously caulked.

Finishing Up Inside

Inside, support the center of the sill with a wedge block or shims. The side jambs must be supported with wedges or shim shingles. Tap wedges in the open space around jambs at top, center and bottom. Move the sashes up and down and use a straightedge to be sure you're not bowing out the center of the jamb. When jambs are snug all along, nail through frame and shims and cut off shims behind the plane of the wall surface. Only then remove the diagonal.

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