Graphical Construction Glossary >> Roofs and roofing. >> Roof Framing >> birdsmouth
The purpose of the birds mouth is to allow the rafter to sit easily in the correct position while fixing. If the rafter was fixed without a birdsmouth the carpenters fixing the rafters would have a hard time stopping them sliding downhill while nailing them into place.
Using metal framing anchors or bolted connections there is no reason to have the notch cut out provided that there was some way of holding the rafters accurately in position while fixing them.
In normal framed roofs it is a general rule that the notch should be no more than 1/3rd the depth of the rafter.
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