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1. Outline your proposed window size (frame) with masking tape on the wall to get a feel for the placement and size of your window. Add in space for the header, if you place the window perpendicular to the floor joists (like the one here).
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2. Erect a temporary 2x4 support wall if the joists are perpendicular to the wall you’ll be cutting. Place it 3 ft. back from the concrete wall. Screw the top plate to the joists and align the studs directly under the floor joists. Measure and cut each stud for a tight fit.
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3. Tent the area where you’ll be cutting with 6-mil plastic sheeting to confine the dust. Use sheeting at least 8 ft. 6 in. to 9 ft. wide. Make slits in the sheeting to seal the area between the joists and staple it in place.
Tip: The simplest way to avoid the header size issue is to make an existing basement window taller. Typical small basement windows measure 30 in.wide by 15 in. high. If you extend this opening down and install a 29-in. wide by 47-in. high casement window, you’ll satisfy minimum egress window requirements.
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4. Measure down from the joists to locate the height of the bottom cut. Mark the center of the bottom cutting line and drill a level pilot hole with a hammer drill and a bit long enough to go through the wall.
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5. Level from the pilot hole on the exterior in both directions from the center and measure and mark the corners of the cutout. The mortar lines usually serve as a level reference line too. Drill a level hole through the wall at each corner.
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6. Cut a 1/2-in. deep groove in the concrete block with an electric 14-in. concrete saw equipped with a diamond blade. Then complete the cut on a second pass. Wetting the blade as it cuts reduces dust. Caution: Plug the saw into a GFCI outlet.
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7. Finish cutting through the wall by making the same cuts on the outside wall. If you have a brick ledge like we did, some blocks will be 12 in. thick and some will be 8 in. Mark the lines for the outside cut with tape and cover the well area with a tarp to keep it clean.
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8. Break out the block with a 4-lb. hammer, starting at the top center. Work carefully around the edges so you don’t loosen the remaining blocks. For stubborn blocks, first break out the core in the middle of the block, then break the block.
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9. Chip the sides of the opening smooth with a brick chisel.Check to be sure the rough opening is large enough for the rough frame and window.
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10. Fill the open cores under the sill with concrete. Stuff newspaper into the cores to keep the concrete from falling down through them.
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11. Cover the wet concrete ith plastic sheeting so the water from the concrete won’t warp the frame sill. Drive 3-in. deck screws partway into the treated wood sill and push them down into the wet concrete. The screws will anchor the sill and prevent the frame from shifting.
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12. Build the header according to your plan (we installed two), hoist it into place and screw it to the floor joists. Cut the frame sides to length for a very tight fit and bang them into place to support the header. Plumb the sides with a level and toe-screw them to the sill and header.
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13. Anchor each side of the frame to the block wall with two 3/16 x 3-1/4 in. concrete screws. Predrill for the screws with a masonry bit. Countersink the hole for the screwhead with a 3/8-in. drill bit.
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14. Seal gaps between the wood frame and the concrete with polyurethane or other exterior caulk. For gaps more than 1/4 in. wide, push a foam backer rod into the gap before caulking. With the frame securely in place, you can remove the temporary support wall.
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15. Center the window in the opening and level it with cedar shims. Get help here to steady the window while you shim it. Temporarily tack the shims in place and remove the window.
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16. Apply a generous bead of caulk around the treated wood frame to create a seal behind the nailing fin of the window. Set the window in place, check it for level, and drive screws or nails through the nailing fin to secure it to the frame. Some manufacturers specify that the fin must be nailed, not screwed.
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17. Bevel the ledge under the window with mortar so it will shed water. Use a straightedge as a guide because your saw cut won’t be perfectly straight. Wet the ledge, add the mortar and shape the bevel with a small finishing trowel.
Copyright ©2005 Home Service Publications, Inc.
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