Page 36 - Pocket Guides

POCKET GUIDE 20

HOLES AND NOTCHES IN JOISTS
Electrical installers erecting electrical systems within premises of traditional
construction often need to cut notches or drill holes in joists for the passage of
cables and/or their enclosures.
No wiring system should penetrate an element of building construction which is
intended to be load bearing unless the integrity of the load-bearing element can be
assured –by a structural engineer – after such penetration (522.8.14).
Clear practical guidance on drilling or notching joists in a manner that does
not detrimentally affect their load bearing capability is given in BS 6891:2015
Specification for the installation and maintenance of low pressure gas installation
pipework of up to 35 mm (R11⁄4) on premises. The advice given in this standard and
used as the basis of this Guide is equally applicable when installing cables and/or
wiring systems as it is to gas pipework.
Where cables or other wiring systems are installed within roof spaces, building
elements such as roof rafters, purlins, trussed rafters, bracing, etc., may only be
notched, drilled or cut away with the approval of a structural engineer.
FIG 1. ACCEPTABLE ALTERNATIVE ARRANGEMENTS FOR NOTCHING
≤0.25L ≤0.2L
≤0.125D ≤0.15
≥0.07L ≥0.1L

D ≤250 0.5D 0.5D

≥100 mm
L

Note 1: Notch dimensions applicable to top or bottom of joist.
Note 2: Notching can be carried out at both ends, on either the top or the bottom
(but not both at the same end) without a design check.

FIG 2. LIMITS FOR DRILLING JOISTS WHERE DRILLING AND NOTCHING OCCUR
≤0.4L ≤0.4L
≥0.25L ≥0.25L

D ≤250 ≤0.25D 0.5D 0.5D

Hole centres at least 3 x their diameter apart
L
Note: Holes must be located at least 100 mm horizontally from notches.

updated to
TH
EDITION
BS 7671:2018
© CERTSURE LLP 2019
updated to
TH
EDITION
BS 7671:2018