Cantilevered Joist Calculator

Diagram — what is being calculated

Cantilevered joist diagram — back span and cantilever length (NZS 3604:2011 §7.1.8)

01 / Inputs

Joist Size (depth × thickness) — mm

Joist Spacing — mm

Timber Grade

Application

Key rules (NZS 3604:2011):
• Min. joist depth for cantilevered barrier: 190 mm
• Min. backspan for deck: 1.5 × cantilever
• Min. joist total length for balcony: 2.25 × cantilever
• Balcony deck must be min. 50 mm below internal floor level
• Boundary joist required (min. 25 mm, same depth)

02 / Result

Enter inputs to calculate cantilever length

All joist sizes — maximum cantilever (mm) for selected application

Size (mm)	Spacing	Max Cantilever	Min Total Length	Min Backspan

NZS 3604:2011 Table 7.2 — scope and limitations. Applies to cantilevered joists in timber-framed buildings within NZS 3604 scope. For balconies with a cantilevered barrier, minimum joist depth is 190 mm. For decks, joists must be continuous over the outer support. NZS 3604 does not cover cantilevered joists supporting external braced walls — these require specific engineering design. Decking weight must not exceed 40 kg/m² and balustrade weight must not exceed 26 kg/m². All results must be verified by a Chartered Professional Engineer or Licensed Building Practitioner.

NZS 3604:2011 Table 7.2 — Maximum Cantilever Length (mm), SG8

Source: NZS 3604:2011 Table 7.2. SG10 values approximately 10–15% higher. For 2 kPa wet-in-service, spans derived using green stresses per NZS 3603.

Size	Spacing	Wall 1.5kPa Light Roof	Wall 1.5kPa Heavy Roof	Wall 2.0kPa	Balcony / Deck
90×45	600	100	50	100	100
	450	150	100	150	150
	400	150	100	150	150
140×45	600	300	150	300	300
	450	400	200	400	400
	400	500	300	500	500
190×45	600	500	300	500	500
	450	700	400	700	700
	400	900	500	900	900
240×45	600	700	400	700	700
	450	900	500	900	900
	400	1200	700	1200	1200

SG10 approx: multiply SG8 values by 1.10–1.15. Heavy roof reduces cantilever by ~200 mm vs light roof.