Roof Pitch and Birdsmouth Calculator
As a design–build professional (still not sure what the proper title is 😅), calculating birdsmouth cuts for rafters in different sizes has always been a serious headache. It takes time, focus, and way too much double-checking.
So instead of repeating the same manual calculations, I decided to create a tool to solve the problem. That tool is EpiphyX, a practical toolkit built specifically for builders and contractors.
It’s designed to help you quickly figure out the numbers you need for your house, building, shed, or whatever you’re putting a roof on, without burning your brain cells on repetitive calculations. Less head-scratching. More building.
Or use the web app here: https://app.epiphyllum.studio/roof.
Beyond basic pitch or rise calculations, this tool also provides additional outputs such as birdsmouth cuts, tail length, and the required post height for structural support. It also includes visual references to help you understand how the rafter will be cut for different sub-fascia and soffit configurations.
At the moment, it supports fundamental roof types like gable and lean-to. More complex roof configurations are currently in development (Mar 2026).
In real-world framing, two pieces of information are usually known from the start:
- Run, based on the building’s wall layout or rafter span
- Pitch, which comes from the roof design (for example, an 8/12 slope that achieves the desired look and drainage)
From there, this tool provides more of what builders actually need, the derived dimensions that make real-world work easier. It can calculate rise, rafter length, birdsmouth cuts, tail length, and more, helping you move from design intent to accurate cuts without switching between formulas or charts.
It was built out of the same small frustrations that come up on real jobs, the kind that slow you down when all you really need are the right numbers to keep building.
How to Use
In Tab 1 – Diagram, this is where you design your roof and choose between the available options.
- Choose what you want to find
In most cases, builders want to calculate the rise, rafter length, or birdsmouth cut. Select the mode that fits your need. - Enter your known measurements
You can input them in feet + inches or as total inches. The default is feet + inches. Set your rafter size and top plate thickness
These values directly affect the birdsmouth geometry, the seat cut sits on the top plate, and its length determines how deeply the heel cut goes. A longer seat cut increases bearing but reduces rafter depth above the plate.Add the overhang
This extends the rafter length beyond the wall. While it doesn’t change the core geometry, it’s factored into the total rafter length.- Ridge board thickness
By default, the ridge board thickness is 1.5″. Its width (5.5″ or 7.25″) doesn’t affect the math unless you’re using double ridges or engineered members like LVL or I-joists.
Example: Calculating Rafter Length for a 10×12 Gable Roof Shed
Let’s look at a quick example using this calculator. We’ll build a 10 ft × 12 ft gable roof shed with a 7/12 slope. The total span is 10 ft, so the ridge board sits at the midpoint.
The run is measured from the outside edge of the exterior wall to the base of the ridge support. Since the ridge board has a thickness of 1.5″, we subtract half of that from the total half-span:
Run = 10 ft ÷ 2 − 1.5″ = 4′ 11 1/4″
Now go back to the calculator and select your mode.
Enter the following values:
Run: 4′ 11 1/4″ (or 59.25″ if using Total Inches mode)
Slope: 7/12 (the design pitch of this shed)
Overhang: 12″
Don’t forget to switch back and forth between Sub-Fascia Type and Size to dial in the exact design you’re aiming for.
It can be a little tricky to visualize what the returned numbers mean, so here’s how to interpret them. You’ll see two different rafter lengths, for example: 6′ 10 1/2″ and 5′ 8 5/8″
The shorter number (5′ 8 5/8″) represents the rafter length up to the wall line, meaning the plumb cut stops flush with the exterior wall, with no overhang. This design approach is common in modern or minimalist roofs where the rafter line finishes cleanly at the wall. The longer number (6′ 10 1/2″) includes the 12″ overhang, representing the total rafter length from ridge to tail end.
A diagram below can help visualize how these measurements relate, showing where the plumb cut meets the ridge and how the overhang extends beyond the wall.
In Tab 2: Rafter Detail, this is where you get the final output, the complete rafter details ready to use.
After getting your cutting diagram, the next step is figuring out how to transfer those measurements accurately onto a real piece of lumber.
For that, check out this guide: How to Cut a Birdsmouth – No Rocket Science Involved. It walks you through the practical steps of laying out and cutting the birdsmouth correctly on an actual rafter.
