A bird's mouth notch is the triangular cut near the base of a rafter that allows it to sit squarely and securely on the top plate of a wall. Getting it right is critical — a poorly cut bird's mouth can weaken the rafter, cause the roof to sit unevenly, or fail to comply with NZS 3604, New Zealand's timber-framed building standard. With careful layout and a steady hand, most experienced DIYers can cut a clean bird's mouth using common tools.
What Is a Bird's Mouth and Why Does It Matter?
The bird's mouth is made up of two cuts:
- A seat cut (horizontal) — the flat portion that bears on the top plate
- A plumb cut (vertical) — the face that sits against the outside edge of the wall plate
Together they form a notch that hooks the rafter over the wall plate, transferring the roof load down into the wall framing. Under NZS 3604, the depth of the seat cut is limited — typically no more than one-third of the rafter depth — to preserve the structural integrity of the timber. Always check the specific requirements for your roof pitch and rafter size with your local council or a Licensed Building Practitioner.
Tools and Materials You'll Need
- Framing square (speed square or combination square)
- Tape measure
- Pencil or marking knife
- Circular saw or handsaw
- Timber rafter (correctly sized for your span and pitch per NZS 3604)
- Safety glasses and hearing protection
Step 1 — Determine Your Roof Pitch
The roof pitch controls the angle of both cuts. In New Zealand, pitch is commonly expressed as a ratio (e.g., 25°, 30°) or as rise over run. Confirm your design pitch from your plans or consult your council-approved drawings. If you're working on a consented project, the pitch will already be specified.
Step 2 — Set Up Your Framing Square
Using a framing square, set the rise measurement on the tongue (short arm) and the run measurement on the blade (long arm) to match your roof pitch. For example, a 30° pitch equates to roughly 115 mm rise per 200 mm run — but always calculate or verify this from your specific plans rather than relying on rule-of-thumb figures.
A speed square with degree markings is often easier for NZ tradies on site — simply set it to your pitch angle and scribe directly onto the rafter.
Step 3 — Mark the Plumb Cut at the Ridge
Start at the top (ridge) end of the rafter. Hold your speed square against the top edge of the timber and mark a plumb line at the correct angle. This is your ridge plumb cut. Cut this line first so you have a clean reference end to measure from.
Step 4 — Measure the Run and Mark the Bird's Mouth
From the ridge plumb cut, measure along the top edge of the rafter to the point where the bird's mouth will sit over the wall plate. This distance is your rafter's horizontal run — taken from your roof plan.
At that point, use your framing square or speed square to:
- Mark a plumb line (vertical cut) — this is the vertical face of the bird's mouth
- From the bottom of that plumb line, mark a seat cut (horizontal line) — this runs toward the tail of the rafter, perpendicular to the plumb line
The seat cut depth should equal the width of your wall top plate (commonly 90 mm for standard framing) so the rafter bears fully on the plate. Crucially, the notch depth must not exceed one-third of the rafter's total depth — check NZS 3604 or your plans for the exact limit.
Step 5 — Cut the Bird's Mouth
- Set your circular saw to the plumb cut angle and carefully cut down the plumb line, stopping exactly at the intersection point. Do not overcut — overshooting weakens the rafter.
- Adjust the saw to 0° (square) and cut along the seat cut line, again stopping at the intersection.
- Use a handsaw or chisel to finish the corner cleanly if the circular saw can't reach it without overcutting.
- Test the fit over a scrap piece of top plate timber before cutting all your rafters.
Step 6 — Cut the Tail (Eave) End
Mark and cut the tail plumb cut at the eave end of the rafter to give you your fascia line. The overhang length will be specified on your plans.
Checking Your Work
- The rafter should sit flat on the top plate with no rocking
- The plumb face should be vertical when the rafter is in position
- There should be no gaps between the seat cut and the plate
- The notch depth must stay within the one-third rule
Cut a pattern rafter first, check it thoroughly, then use it as a template for all remaining rafters.
NZ Building Code Considerations
Roof framing in New Zealand is governed by NZS 3604 for light timber framing. Most domestic roof work requires a building consent — check with your local council before starting. Some minor repair work may fall under Schedule 1 exemptions, but confirm this with your council first. For consented work, a Licensed Building Practitioner (LBP) must carry out or supervise restricted building work. BRANZ also publishes excellent guidance on roof framing best practice.
When to Call a Professional
If you're framing a new roof, replacing multiple rafters, or working at height, it's strongly advisable to engage a qualified carpenter. Roof framing is classified as restricted building work under the Building Act when it forms part of the primary structure, meaning it must be carried out or supervised by an LBP. Working at height also carries serious safety risks — review WorkSafe NZ guidelines before going up on any roof. You can find experienced local carpenters on tradedirectory.co.nz.
