How to Cut a Bird's Mouth Notch in a Deck Stair Stringer

Cutting a bird's mouth notch in a deck stair stringer is a fundamental carpentry skill that gives your stair stringer a flat, stable bearing point where it sits on the deck frame or concrete pad below. Done correctly, it produces a strong, level connection that meets the requirements of the NZ Building Code — done poorly, it can weaken the stringer and create a safety hazard.

What Is a Bird's Mouth Notch?

A bird's mouth notch (sometimes called a seat cut) is an L-shaped cut made at the base of a stair stringer. It creates a horizontal seat cut (which bears on the supporting structure) and a vertical plumb cut (which sits flush against the face of the bearer or rim joist). The notch allows the stringer to sit squarely and securely without rocking or twisting.

NZ Building Code Considerations

Before you pick up a saw, check your local council requirements. Deck stairs in New Zealand must comply with NZS 3604:2011 (Timber-framed buildings) and the NZ Building Code, particularly Clause F4 (Safety from falling). Stair geometry — rise, going, and headroom — must fall within prescribed limits. Depending on the height of your deck, a building consent may be required. Always confirm with your local council whether your project needs consent; don't assume it's exempt under Schedule 1. You can find guidance at MBIE Building Performance.

Tools and Materials You'll Need

• Speed square or combination square
• Framing square (essential for accurate layout)
• Pencil or marking knife
• Circular saw
• Hand saw or jigsaw (for completing corners)
• Chisel and mallet (for cleaning up)
• Tape measure
• Treated timber stringer (H3.2 minimum for above-ground exterior use in NZ)

Step 1 — Calculate Your Rise and Going

Before marking anything, work out your total rise (the vertical height from ground or landing to deck surface) and divide it evenly by the number of steps to get your unit rise. Your unit going (horizontal tread depth) must also comply with NZS 3604. A common rule of thumb is: 2 × rise + going = 600–700 mm, but always verify against the standard or your consent documents.

Step 2 — Set Up Your Framing Square

Place your framing square on the stringer timber with the unit rise measurement on the tongue (short arm) and the unit going on the blade (long arm). Many carpenters use stair buttons (small clamps that clip onto the square) to keep these measurements consistent as they step the square along the timber. Mark each tread and riser line along the full length of the stringer.

Step 3 — Mark the Bird's Mouth

At the bottom of the stringer, the bird's mouth is formed by the intersection of:

• The plumb cut line — a vertical line parallel to all your riser lines
• The seat cut line — a horizontal line parallel to all your tread lines

The depth of the seat cut should leave a minimum remaining timber depth in the stringer — typically no less than two-thirds of the stringer's total depth remaining (i.e. the notch itself should be no deeper than about one-third of the stringer's total depth). Cutting too deep significantly weakens the stringer. For a 290 × 45 mm stringer, for example, most carpenters aim for a seat cut no deeper than about 30–40 mm. Check your timber supplier's or engineer's recommendations if in doubt.

Step 4 — Make the Cuts

1. Set your circular saw to the correct depth — just enough to reach the corner of the notch without overcutting. Overcutting past the corner point weakens the timber and is a common mistake.
2. Cut the plumb line first, stopping exactly at the corner point.
3. Cut the seat line, again stopping at the corner.
4. Use a hand saw, jigsaw, or sharp chisel to finish the corner cleanly. The two cuts should meet perfectly to form a crisp right-angle notch.
5. Test the fit against your bearer or rim joist — the stringer should sit flat with no rocking.

Step 5 — Cut the Top of the Stringer

At the top, the stringer typically requires a plumb cut to sit flush against the deck rim joist or header, and the top tread nosing may need to be adjusted so the finished deck surface acts as the top tread. Mark and cut carefully, then test the fit before fixing.

Step 6 — Fix the Stringer Securely

Once you're happy with the fit, fix the stringer using joist hanger hardware or framing anchors rated for the load, along with appropriate structural screws or bolts. Stringers should be fixed at both top and bottom. Use fixings compatible with H3.2 treated timber (stainless steel or hot-dipped galvanised in exposed locations).

Tips for a Clean Result

• Always cut on the waste side of your pencil line.
• Use a sharp blade — a dull circular saw blade tears the grain and makes accurate stopping points harder.
• Cut a test stringer from scrap timber first if this is your first time.
• Check the stringer for crown (bow) before marking — install it crown-up so any natural curve works in your favour.

When to Call a Professional

If your deck is more than 1 metre above ground level, if the stair design is complex (curved, winding, or very wide), or if you're unsure about structural sizing, it's well worth engaging a qualified Licensed Building Practitioner (LBP). Structural carpentry on decks can be restricted building work depending on the situation, meaning it must be carried out or supervised by an LBP. Check the rules at Licensed Building Practitioners. You can find experienced local deck builders on tradedirectory.co.nz.

Safety note: Always wear eye protection and hearing protection when using power tools, and keep hands clear of blade paths. Ensure the timber is firmly supported before cutting.

Sources & Further Reading

• MBIE Building Performance — Building Code and consents
• Licensed Building Practitioners — Restricted building work
• BRANZ — Build magazine and technical guidance
• Standards NZ — NZS 3604 Timber-framed buildings
• WorkSafe NZ — Safe use of power tools
• Bunnings — Timber and stair hardware