Installing a Basement Window - Cutting Concrete Block Foundation

Basement Egress Window - Cutting Concrete Block To Install A Window

Typically we would hire a concrete company to cut foundations for us, but that can get costly at rate of $1600 a day. Using a gas cut off saw was also an option, but this house is occupied and we thought it best to avoid the exhaust from the engine.

We finally settled on using a Husqvarna 9-inch cordless cut off saw. This saw runs on batteries and does not pollute the house with fumes. Click here to read the tool review on the Husqvarna 9-inch Cut Off Saw.

Because this saw has a 3-inch depth of cut we knew we would need to make these cuts from both-sides of the wall.



Measuring The Windows - Layout

We were going to install the windows side by side with a 2x4 between the two for a nailer and to accommodate a small amount of trim. The windows would be installed within a frame of 2x8 pressure treated framing lumber.

To determine our framing rough opening, we placed the windows on the floor, with a 1-1/2” space between the two and measured the width and length, adding 1" to the width and ½” to the height measurement, for plumbing and shimming. Once we had this measurement the next steps was to figure out the framing measurements.

We added 6 inches on the width of the opening to accommodate two 2x8 jack studs on each side.  We decided to cut the bottom of the window on a mortar joint that would allow us to install a  7-1/4 inch header on top of the window. Adding these measurements to our rough opening measurement gave us the total concrete block to cut.



Preparing - Cutting Concrete Block To Install Window

With the total measurement figured out we transferred this measurement to the concrete block wall, and leveled and plumbed out marks on the wall with a permanent marker. The marker does not wash off like chalk does, and is darker and easier to see over pencil.

Once the window was marked out on the inside we used a Makita hammer drill and drilled through holes in the concrete block at all corners. These holes were then used on the exterior of the wall to locate our cut lines. We repeated the step above on the outside.

We then set up a water hose to our saw for a concrete block wet-cutting operation and had a wet vacuum ready to collect the slurry.



Cutting the Concrete

When cutting an opening in an existing block wall, try to align it with existing mortar joints.

We used the saw to score the line and then to make a 3-inch deep cut on both sides. Once complete we used a sledge hammer to break the concrete blocks at the score line.



Once you've opened up the wall, you'll have a number of partial concrete blocks that will need to be filled in with mortar or replaced.

We brought the window opening lower than needed to be able to break the opening at a mortar joint, and to allow for a 7-1/4” header above.



Repair the opening as required, adding wire mesh to the cavities to give your mortar something to hold on to, or install a pressure treated ”wood buck” and secure it in place with Tapcon masonry screws.

NOTE - we installed a 2x6 pressure treated "wood buck" into one cavity and secured it in place with concrete screws.  We were then able to nail our framing to this buck.



Fastening The Framing To The Concrete

We used pressure treated lumber in all areas that contact the concrete. 2x8 pressure treated lumber was used, and the first piece installed was the rough framing sill.  We chiseled as needed, leveled and installed a 2x8 sill plate in the opening. The sill was then fastened to the concrete block with 3-inch Tapcon concrete screws and a hammer drill.