Technical Booklet D Structure·Page 36·4B.13
Notching and drilling restrictions
Notching or drilling of rafters or purlins requires specialist calculation, except birdsmouthing of a rafter to a depth not greater than one third of the rafter depth.
Notching or drilling of a rafter or purlin should not be carried out unless justified by a specialist calculation. This does not apply to the birdsmouthing of a rafter to a depth not greater than one third of the depth of the rafter.
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