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Do I need “bird’s mouths” in my shed gable roof rafters?

Alan asks:

I’m about to build a shed/storage building in my driveway. It looks pretty simple, but I have a question about the gable roof.

I see that one is supposed to cut a “bird’s mouth” into the rafters where they sit on the top beam of the side walls. Is there an easy way to cut this so that it sits correctly on the top beam at the right place without complicated geometry to determine the angle etc…?

Good news, Alan – our pal Trev advises that with a lightweight structure such as the one you’re planning, it shouldn’t be necessary to cut “bird’s mouths” into the rafters at all.

(Some of you may not know what on earth a “bird’s mouth” is! Fortunately, Alan’s very kindly come up with a picture for us, just to clarify. Here it is:)

Bird's Mouth diagram

The idea is that the “bird’s mouth” – the rebate cut into the rafter at the lower end – sits squarely on the top beam of the wall to transfer the load of the roof more evenly from the rafter onto the top beam, thus reducing stress on both. However, as Trev tells us, the main idea is to stop the roof spreading under the weight of heavy-duty roofing such as tiles or slates. So they’re probably unnecessary for the more lightweight shingle roof you’re planning.