Birdsmouth Cut | Rafter Notching Code Requirements

Author : Building Code Trainer

In residential construction, wood framed roof systems are commonly used across the United States. And while you might size these roof members properly for all the given loads that are applied to it, an incorrect notch or cut of a member can compromise its structural integrity leading to potential damage.

Fun Tip: To learn how to easily size roof rafter, be sure to check out this POST.

One common cut is known as a “Birdsmouth Cut” and in this article we will explore Section R802.7 of the 2021 International Residential Code which sets forth the requirements related to this type of a cut. Understanding these requirements is essential for architects, builders, and inspectors to ensure that the construction meets safety standards while maximizing the structural capabilities of the wood used in roofing.

The IRC establish limits of this type of cut which typically occurs at the cantilevered portion of rafters. In this article we will break down this code requirement and explain its importance in maintaining this code section which ensures safety and durability in residential roof construction.

What is a Birdsmouth Cut?

A birdsmouth cut is a triangular-shaped notch cut into a rafter, allowing it to sit properly on a wall plate or supporting beam. The birdsmouth cut ensures that the rafters are stable and securely rest on the supporting structure, distributing the roof’s weight evenly.

Now when we look at the code requirements in the International Residential Code regarding this cut, you will notice that no such term exists. Which is why it is important to understand building code terminology.

Although the term is widely used in the industry, it is more of an informal term. The code does not have a formal term however when referring to this type of a cut, the code refers to “Notches on Cantilevered Portions of Rafters”. Since this is what the code refers to, moving forward in this article we will be using the code term.

Notches on Cantilevered Portions of Rafters

In Section R802.7, the code says structural roof members are not to be cut, bored or notched beyond the limitations set forth in the code section. While cuts, notches and holes in solid lumber joists, rafters, blocking and beams are to comply with Section R502.8.1, cantilevered portions of rafters must comply with Section R802.7.1.1. This section is specific to this type of cut.

Here is what Section R802.7.1.1 says:

Notches on cantilevered portions of rafters are permitted provided the dimension of the remaining portion of the rafter is not less than 3-1/2 inches and the length of the cantilever does not exceed 24 inches in accordance with Figure R802.7.1.1.

Based on this section you can see that the cantilevered rafter, which is notched at the top plate, must have a minimum depth of 3-1/2 inches at the notch.

Here is the Figure to help see this visually.

Note: One important thing to note here is that these requirements are prescriptive requirements meaning if you follow the IRC without an engineered design, then these requirements apply. If you want to exceed the eave length or increase the depth of the cut, then it will be considered an engineered method and a licensed design professional will need to provide calculations to see if exceeding the limitations are allow per an alternative engineered method.

Can a Birdsmouth Cut be Used on All Roof Types?

Yes however it is important to note that these requirements are applied to Sawn Lumber and not to Engineered Lumber. For engineered wood products the cutting and notching limitation for sawn lumber do not apply. Instead Section R802.7.2 would apply.

Section R802.7.2 says that cuts, notches, and holes bored in engineered wood projects are prohibited unless where permitted by the manufacturer’s recommendations or where the effects of these cuts are considered in the design by a registered design professional.

Final Thoughts

Correct understanding of rafter notching, particularly in cantilevered sections, is essential for maintaining the structural integrity and safety of residential roofs. This article has explored the code requirements to obtain a correct Birdsmouth cut as prescribed by the IRC so that you can gain a better understanding of the codes applied and how they affect us as construction professionals.

This sums up the basic code requirements for notches on cantilevered potions of rafters that are subject to the International Residential Code (IRC). For more information on rafter notching subject to the 2021 International Residential Code, be sure to check out Section R802.7 of the IRC.