How to Cut a Basement Egress Window Opening in Concrete or Block

You’re cutting a hole in a basement foundation wall to install an egress window — a code requirement for any below-grade bedroom and a value-add for converting basements to living space. This guide covers the project end-to-end: code sizing, structural lintel, blade and saw selection, dust and slurry control, and the order of operations that prevents the wall above the cut from dropping on you.

Before You Cut Anything: Code, Lintel, Locate

Code sizing (IRC R310). The International Residential Code requires every basement bedroom to have an emergency escape and rescue opening. Net clear opening: minimum 5.7 sq ft (5.0 sq ft for openings at grade level), minimum 24” high, minimum 20” wide, sill no more than 44” above the finished floor. The net clear opening is what’s left when the window is fully open — not the rough opening. Pick the window first, then size the rough opening to match the manufacturer’s spec, then add the lintel height and the bearing space above.

Lintel sizing — get it engineered. A 36×48 rough opening in an 8-inch poured concrete foundation wall removes roughly 12 square feet of structural wall. The load above that opening — first-floor framing, second floor if present, roof — transfers to whatever sits at the corners of the cut. A steel angle lintel (commonly L4×4×3/8 or L6×4×3/8 depending on span and load) bears on each side of the opening and carries the wall above. Sizing depends on the load above and the wall thickness — talk to a structural engineer or the lintel supplier before you cut. In most jurisdictions, the building department wants a stamped drawing for the lintel before they’ll issue the permit.

Locate utilities and rebar. Before any saw touches the wall, find what’s in it. A $50 rebar locator (Bosch D-tect 150, Zircon HD900) scans for steel reinforcement. Most poured concrete foundations have rebar on a 12-18” grid. CMU block walls usually have rebar in grouted cells at corners and every 4-6 feet. Map the rebar before you cut — knowing where the steel is changes your blade choice and your cut sequence. While you’re at it, check for plumbing, electrical, and gas inside the wall cavity if there’s furring or insulation.

Permit and inspection. Pull the permit before you cut. Egress windows in below-grade bedrooms are inspected for opening size, sill height, well dimensions (if window well is required), and ladder requirements. Cutting first and inspecting later means you might be redoing the cut.

Pick the Saw

Wall type	Saw	Why
Poured concrete, 8-10” thick	14” handheld cut-off saw (gas or battery)	Need 4-5” cut depth per side; gas runs all day, battery does the cuts in 1-2 battery cycles
CMU block, 8” with grouted cells	14” handheld cut-off saw	Same — the grouted cells contain rebar and act like poured concrete
CMU block, 8” ungrouted hollow	14” handheld cut-off saw OR demo saw chain	Hollow block cuts faster; you can use a smaller saw, but if any cell is grouted you need the 14”
Stone foundation (rare)	14” with multi-purpose blade + air chisel	Stone foundations are unpredictable — assume hand work after the score cut

The Stihl TS 420, Husqvarna K 770, Hilti DSH 700-X, and Milwaukee MX FUEL COS350 are all correct picks. Rent for $80-120/day if you don’t own one. See the Concrete Saw Buying Guide for the gas vs. battery tradeoffs that matter for indoor work — a battery saw is usually the better call in a basement because the gas exhaust has nowhere to go.

Wrong tool for this job: a 4.5-inch angle grinder (1.25” depth — you’d be cutting from both sides and still not meeting in the middle), a Sawzall with a carbide blade (will cut concrete eventually but takes hours and chews through blades at $25 each), or a rotary hammer alone (good for the breakout, not for the perimeter cut).

Pick the Blade

The wall material dictates the blade bond.

• Poured concrete with light rebar (#4) — VA 14” Ultra Value (medium bond, segmented). Roughly 180-220 linear feet wet on 4,500 PSI concrete per our field logs. A 36×48 opening is ~14 linear feet of cut — one blade does the job with margin.
• Poured concrete with heavy rebar (#5-#6) — VA 14” Premium Sparkie (softer bond, 12mm segments). The Ultra Value will cut #5+ rebar but glazes faster — you’ll feel the blade quit and have to dress it. The Sparkie self-dresses on steel and holds up.
• Green concrete (pour less than 7 days old) — VA 14” BD Asphalt/Green Concrete. Don’t cut green concrete with a medium-bond blade — it’ll glaze in the first 2 minutes.
• CMU block, mostly hollow — Ultra Value handles block fine. Don’t pay for the Sparkie if you know the wall is ungrouted.

Always verify the blade’s max RPM exceeds the saw’s spindle speed. K 770: 5,400 RPM; Ultra Value: 5,500 RPM max. That’s a 100 RPM margin — fine for the rated saw. Mounting an Ultra Value on a 6,000 RPM industrial saw is not.

The Cut Sequence (Don’t Skip Steps)

The wall is load-bearing. The order you cut matters because partial cuts release load in ways the final geometry doesn’t. Cut the lintel pocket first, install the lintel, then cut the opening below. Cutting the opening first and trying to drop a lintel into an already-removed wall is how guys get crushed.

1. Score the perimeter

Mark the rough opening on the inside face with chalk. Add the lintel pocket above — typically 3-4” tall and extending 4-6” past the opening on each side for bearing. Set the saw to a shallow first cut (1”) and score the entire perimeter and the lintel pocket. This gives you a clean kerf for the deeper cuts to track and lets you verify your layout before committing to depth.

2. Cut the lintel pocket

Cut the lintel pocket to full depth (through-cut on the wall). Pop the section out — usually with a sledge and pry bar, sometimes with a rotary hammer at the corners. Clean the pocket so the lintel bears flat.

3. Drop the lintel in

Set the steel angle (or pre-cast concrete lintel) into the pocket. Bed it on non-shrink grout or hydraulic cement so it bears uniformly. Let it set per the grout manufacturer’s spec — usually 4-24 hours depending on product.

4. Cut the opening below

Now cut the rough opening below the seated lintel. The lintel is carrying the load above; you can remove the wall section under it without dropping the floor. Cut top horizontal, then both verticals, then bottom horizontal last.

5. Break out the section

The wall section will not just fall out — it’s typically held by rebar even after the perimeter is cut. Hit it with a sledge from inside, pry from outside, cut any remaining rebar with a reciprocating saw and metal blade or a 4.5” grinder with a metal cut-off wheel. Be ready for the section to weigh 200-400 lbs depending on thickness and material.

6. Dress the opening

The kerf from a 14” saw leaves a rough edge. Dress with a hammer-and-chisel or a 4.5” grinder with a diamond cup wheel. The window frame needs to seat true — out-of-square or rough openings cause leaks and broken seals.

Dust and Slurry — The Part Nobody Plans For

A 36×48 cut in 8” concrete is roughly 14 linear feet × 8” deep = ~9 cubic feet of material to remove plus another 5-10 gallons of cement slurry (wet) or several pounds of respirable silica dust (dry).

Wet cutting in a basement

Wet cutting is the right call for silica control. The slurry problem is real, though: water hits the saw, slurry runs down the inside face of the wall, pools on the basement slab. Plan for it:

• Tarp the slab inside the cut area with 6-mil plastic, taped to the wall above the cut. Slurry runs onto the tarp.
• Wet vac at the ready (Pulse-Bac, Ermator, or any wet/dry shop vac if budget is tight — but for production work use a slurry-rated unit).
• Bucket and slurry solidifier. Pour slurry into a 5-gallon bucket, add SlurryShield or quicklime per the package. Solid in 5-10 minutes. Dispose at C&D landfill.
• Do NOT wash slurry into the basement floor drain. It’s high-pH (12-13), it’ll corrode cast iron, and it’s a prohibited non-stormwater discharge under EPA’s NPDES Construction General Permit.

Full slurry disposal options in Wet vs Dry Cutting Concrete.

Dry cutting in a basement

Dry cutting is faster and avoids the slurry mess. The dust problem is worse than you think:

• HEPA-rated dust extractor, not a workshop vac. Class H (EU) or HEPA (US) certification. A Festool CT or Bosch GAS unit works; a $80 shop vac with a “HEPA filter” sticker does not.
• Vacuum shroud on the saw — Husqvarna and Stihl both make handheld saw dust-shroud attachments. If your saw doesn’t have one, dry cutting in an interior space is not OSHA Table 1 compliant.
• Respirator — at minimum a fit-tested N95 (APF 10), better is a half-face P100 (also APF 10 but better seal). Beard or stubble breaks the seal entirely.
• Negative-pressure enclosure if the basement is finished or contains HVAC returns. Silica gets into ductwork and you’ll be cleaning carpet and drywall for months.

See the Silica Dust Safety Guide for the full PPE and exposure-control discussion.

Cleanup and Finish

Once the section is out, the lintel is set, and the opening is dressed:

1. Brush and vacuum the cut faces so the window frame seats clean.
2. Caulk the rough opening with a high-quality polyurethane sealant before installing the window — this is your primary water barrier.
3. Install the window per manufacturer instructions. Shim level, plumb, square. Out-of-square frames break seals within a year.
4. Flash the head with a metal drip cap or membrane flashing. Basement window wells flood — water always finds the cheapest way in.
5. Install the window well if grade requires it. Verify well dimensions meet code: minimum 9 sq ft floor area, minimum 36” projection from the wall, and ladder if depth exceeds 44”.

What Will Trip You Up

• Cutting before the permit. Inspectors will make you redo work cut without inspection.
• Skipping the lintel. “I’ll add a lintel after I cut the opening” — no, you’ll add a lintel after the wall above the opening fails.
• Wrong window for code. Most “small” basement windows do not meet R310. Verify the window’s net clear opening on the spec sheet before you buy.
• Hitting electrical conduit in CMU cells. Older homes routed wires through block cells. Always kill power to circuits running near the cut.
• Forgetting the window well drain. Code requires drainage at the bottom of the well — to a sump, a foundation drain, or daylight. No drain = bathtub.
• Running a gas saw with the basement door shut. Carbon monoxide kills people every year doing exactly this. Battery saw, ventilation, or an open exterior door.

Related Guides

• Concrete Saw Buying Guide — gas vs battery for indoor work
• Diamond Blade Buying Guide — bond, segment, RPM fundamentals
• Best Diamond Blades for Concrete — full blade picks with field-logged cut data
• Silica Dust Safety Guide — OSHA Table 1, respirator selection
• Wet vs Dry Cutting Concrete — slurry disposal options