How to Cut a Doorway in a Concrete or CMU Foundation Wall

You’re cutting a doorway through a concrete or CMU foundation wall — to add a basement walkout, connect rooms in a finished basement, or open a garage to a new addition. This is structural work in load-bearing concrete. Done right, it takes 1-2 days and lasts 50+ years. Done wrong, the wall above the cut drops, you’re at the ER, and the insurance call gets uncomfortable.

This guide covers the project: permit and code, lintel sizing, saw and blade selection, cut sequence, dust and slurry, and finishing. The egress window guide (project-basement-egress-window-cutout) covers many of the same fundamentals — read that one too if you’re new to cutting foundation walls.

Before You Cut: Permit, Engineer, Lintel

A foundation doorway is structural alteration. You need three things on paper before the saw turns on.

Permit. Most US jurisdictions require a building permit for any modification to a load-bearing foundation wall. The permit application typically requires a stamped structural drawing — drawn by a licensed engineer or architect — showing the proposed opening, the lintel (header), and any reinforcement around the opening. Some jurisdictions accept a contractor-prepared drawing with a pre-engineered steel lintel from a manufacturer (like a Wolmanized steel lintel sized to span); others require engineer review regardless. Call your building department first.

Engineer. A standard interior doorway in a wood-framed wall takes a header sized from IRC tables and you’re done. A foundation wall is different. The load on a foundation header is the wall above the opening plus whatever the foundation is carrying — first floor framing, second floor, roof, snow load, the works. Cutting a 3-foot-wide opening in an 8-inch poured wall removes ~10 sq ft of structural mass; the load above transfers entirely to whatever remains at the corners of the opening. Get the lintel sized by an engineer unless the building department explicitly accepts a pre-engineered alternative.

Lintel. Three common options:

1. Steel angle lintel — typically L4×4×3/8 or L6×4×3/8 for residential foundation doorways, bearing 4-6 inches on each side of the opening. Cheap, fast to install, widely accepted by inspectors. The angle’s vertical leg bears the wall above; the horizontal leg bolts to a steel plate or rebar for tension.
2. Steel I-beam lintel — for wider openings (6’+) or higher loads. W6×9 or W8×10 sections are common. Heavier, more expensive, needs more fabrication. Often required for double-door openings.
3. Reinforced concrete lintel cast in place — for masonry blocks, sometimes preferred because it integrates with the wall. Requires forming and a cure time that can add 3-7 days to the project. Better for finished interior openings where the steel lintel would be visible.

The engineer picks. You install.

Map the Wall Before You Cut

• Probe thickness. Drill a 1/4” pilot hole at the proposed opening corner. Residential foundation walls are typically 8-10 inches thick (poured) or 8-12 inches nominal (CMU). Walkout-grade walls can be thicker. Verify before you set the cut depth.
• Locate utilities. Look for plumbing, electrical, gas, low-voltage, sump discharge, and water service lines running through or against the wall. Trace each conductor or pipe to its source. Cutting through a buried gas line is a catastrophe; cutting through a sump discharge means rerouting drainage. Call 811 (US) before any excavation outside the wall.
• Find rebar (poured walls). Most residential poured foundations have rebar on a 12-18” grid — usually horizontal #4 or #5 at 12-18” vertical spacing, with vertical #4 at 24-48” horizontal spacing. A rebar locator ($50 tool) maps it in 10 minutes. Knowing the layout tells you what kind of resistance the cut will hit and where.
• Find grouted cells (CMU walls). CMU block walls have hollow cells. Some are filled (grouted) with concrete and contain rebar — typically at corners, at openings, and on a 4-8 foot horizontal grid for shear. Grouted cells cut like poured concrete. Hollow cells cut fast. A rebar locator finds grouted cells by detecting the embedded steel.
• Identify the bond beam (CMU walls). Most CMU foundation walls have a horizontal grouted course at the top of the wall — the bond beam. It acts as a continuous beam tying the top of the wall together and carrying load to the corners. Never cut through a bond beam without confirming what’s above it. If the bond beam sits above where your lintel will install, you’re fine — your lintel takes the load and the bond beam continues over it. If the bond beam sits below your planned lintel, you’d cut the structural tie and the wall above the opening becomes unsupported. Stop and re-engineer.

Pick the Tools

Task	Tool	Notes
Score and full-depth cuts	14” handheld cut-off saw	Stihl TS 420, Husqvarna K 770/K 970, Hilti DSH 700-X, Milwaukee MX FUEL COS350
Break out the section	60-90 lb breaker	Bosch Brute, Makita HM1810. Rent if you don’t own.
Cut exposed rebar	4.5” angle grinder + metal cut-off wheel	Or reciprocating saw with metal blade
Dress kerf edges	4.5” angle grinder + diamond cup wheel	Frame seats true on a clean opening
Lift wall section	2 workers + sledge OR skid-steer with grapple	A 36×80 section in 8” concrete weighs ~600-800 lbs intact — break first, lift in pieces

Wrong tool calls (same logic as the egress window article — see that guide for the full discussion):

• A 4.5” grinder for primary cuts: 1.25” depth, wrong tool.
• A Sawzall with carbide blade: works for small cleanup, fails for primary cutting.
• A rotary hammer alone: good for breakout, not for kerf control.
• A gas saw in a basement with the door shut: carbon monoxide. Use battery indoors or ventilate aggressively.

Pick the Blade

The wall material drives the blade choice.

• Poured concrete, light rebar (#3-#4): VA 14” Ultra Value. Medium bond, segmented, ~180-220 LF wet on 4,500 PSI cured concrete per our field logs. A 36×80 doorway is ~16 LF of cut — one blade has margin.
• Poured concrete, heavy rebar (#5-#6): VA 14” Premium Sparkie. Softer bond, 12mm segments, self-dresses on steel.
• CMU with mostly hollow cells: Ultra Value handles block fine. Faster cut speed than poured because hollow cells offer no resistance.
• CMU with grouted cells throughout: treat as poured concrete with rebar — Sparkie or Ultra Value depending on rebar size.
• Older fieldstone foundation (rare in newer construction): specialty work. Most fieldstone foundations need score-and-chisel or air-tool removal rather than diamond saw cuts. Consult a mason.

Verify blade max RPM exceeds saw spindle speed before mounting. K 770: 5,400 RPM. K 970: 4,700. Ultra Value: 5,500 max. The K 770 margin is tight — confirm the blade rating against the saw nameplate.

The Cut Sequence

Cut the lintel pocket first. Install the lintel. Then cut the doorway below. Inverting that order puts the wall above the opening on whatever stub of unsupported wall is left at the corners. Don’t do it.

1. Layout and chalk

Mark the rough opening — typically rough is the doorframe size plus 2-3 inches each side and 1-2 inches above for shimming. Add the lintel pocket above the rough opening — typically 4-6 inches tall (depending on lintel section) and extending 4-8 inches past the opening on each side for bearing. Chalk both rectangles on the inside face of the wall.

2. Score the perimeter

Set the saw to 1” depth. Score both the lintel pocket and the rough opening perimeter. The score cuts give you clean visual layout and create kerf guides for the deeper cuts.

3. Cut the lintel pocket to depth

Full-depth cuts on the lintel pocket — top, sides, bottom. Pop the section out with sledge and pry bar. On poured walls with rebar, the pocket section will hang on intact rebar — cut it with a 4.5” grinder + metal blade after the perimeter cut is through.

Clean the pocket so the lintel bears flat. The lintel must contact the wall at both bearing ends or it doesn’t transfer load.

4. Seat the lintel

Drop the steel angle or beam into the pocket. Bed each bearing end on non-shrink grout (Sika 212, Quikrete Non-Shrink Precision Grout) per the engineer’s spec. Some specifications call for a steel bearing plate under the lintel ends — install that first, then bed the lintel on it. Let grout cure to the spec’s release strength before loading.

5. Cut the doorway below

With the lintel seated and carrying load, cut the rough opening below. Top horizontal cut first (now bearing on the lintel above), then both verticals, then bottom horizontal last. The bottom cut should align with the finished floor — typically a 2×6 PT sill plate sits on the cut concrete.

6. Break and remove the section

The wall section between the cuts will not just fall out. Strike it with the breaker from inside. The section breaks into pieces; lift each piece into the dumpster or wheelbarrow. A 36×80 × 8” section weighs ~600-800 lbs intact — break before lifting, never lift the whole section.

7. Cut exposed rebar

Any rebar projecting into the opening needs to be cut flush or recessed so the doorframe sits flat. 4.5” grinder + metal cut-off wheel handles it in seconds.

8. Dress the opening

The cut faces from a 14” saw are rough. Dress with a 4.5” grinder + diamond cup wheel until the opening is true. Out-of-square or rough openings cause door binding, broken weatherstripping, and water intrusion at the threshold.

Dust and Slurry in a Basement

(This section parallels the egress window guide — same physics, same compliance.)

A 36×80 cut in 8” concrete is ~16 LF × 8” deep = ~10 cubic feet of material plus 6-12 gallons of slurry (wet) or several pounds of respirable silica (dry).

Wet cutting in a basement is the right call for silica control but the slurry has to go somewhere:

• Tarp the slab below the cut with 6-mil plastic, taped to the wall.
• Wet vac at the ready (Pulse-Bac, Ermator, or shop wet-vac for small jobs).
• Slurry into a bucket, solidify with SlurryShield or quicklime, dispose at C&D landfill.
• Do not wash slurry into the basement floor drain — high-pH (12-13), corrodes drainage, prohibited under EPA’s NPDES Construction General Permit. Full disposal details: Wet vs Dry Cutting Concrete.

Dry cutting in a basement requires HEPA-rated dust extraction on the saw, a vacuum shroud, and a fit-tested respirator. A workshop vac is not adequate. Class H (EU) or HEPA (US) certified extractor only.

Carbon monoxide. Gas saws produce CO that doesn’t dissipate in enclosed spaces. Use a battery saw indoors (Milwaukee MX FUEL, Husqvarna K 1 PACE) or ventilate aggressively with the exterior door open and a fan moving air out.

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

Finishing

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

1. Install a sill plate — pressure-treated 2×6 or 2×8 anchored to the concrete with epoxy-set anchor bolts or wedge anchors. The sill provides nailable framing for the doorframe.
2. Frame the rough opening — 2×4 or 2×6 jambs on each side, ripped to the wall thickness so they sit flush with both faces.
3. Caulk the rough opening with high-quality polyurethane sealant before installing the door. This is the primary water barrier on exterior openings.
4. Install the door per manufacturer instructions — shim level, plumb, square. Out-of-square frames break seals.
5. Flash the head and threshold on exterior installations. Metal drip cap above the door, membrane flashing at the threshold. Water finds the cheapest way in.
6. For interior openings, finish the kerf face with drywall over furring or a stone/tile reveal. Bare concrete around an interior doorframe looks unfinished.

What Will Trip You Up

• Cutting before the permit. Inspectors will make you redo work cut without inspection. Worse, they may make you restore the wall.
• Skipping the engineer. The lintel might be fine. It also might not be. A failure 5 years later is your liability if you cut without engineering.
• Cutting through a bond beam in CMU. Bond beams tie wall corners together. Cutting through one without analysis can compromise the wall.
• Cutting through unknown utilities. Trace before you cut. Hidden gas, water, electrical inside or against the wall.
• Forgetting threshold drainage on exterior walkouts. Water collects at the threshold; without a drainage plan it migrates into the building. Membrane flashing + drainage mat under the sill.
• Gas saw in a closed basement. CO. Battery, ventilation, or both.
• Loading the lintel before the grout sets. Non-shrink grout has specific release-strength curing times — 4 to 24 hours depending on product. Loading early cracks the grout bed and compromises bearing.

Related Guides

• How to Cut a Basement Egress Window — same fundamentals, smaller opening, code-driven sizing
• Concrete Saw Buying Guide — gas vs battery for indoor work
• Best Diamond Blades for Concrete — blade picks with field-logged cut data
• Silica Dust Safety Guide — OSHA Table 1 + respirator selection
• Wet vs Dry Cutting Concrete — slurry disposal options