Graphical Construction Glossary >> Roofs and roofing. >> Roof Framing >> birdsmouth

A timber joint,a notch cut out of an angled piece to let it sit on a cross member. Typically in a rafter at the wall plate or at a underpurlin.

a birdsmouth cut

birdsmouth in a common rafter

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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Please Note! The information on this site is offered as a guide only!  When we are talking about areas where building regulations or safety regulations could exist,the information here could be wrong for your area.  It could be out of date!  Regulations breed faster than rabbits!
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