Decking Subframe Design Guidance

Decking Beam Span Guidance

We recommend beams are constructed from two 44 x 145mm Q-Deck deck joists and are placed at maximum centres of 1.8m. In this instance the maximum span between deck post centres is 2.07m (C16) or 2.24m (C24).

Cut away diagram of finished raise decking

Annotated decking diagram

Joist and Deck Board Spacing Guidance

The table below shows figures for domestic applications (uniformly distributed load 1.5kN/ m2) and commercial applications (uniformly distributed load 4.0kN/ m2). Dimensions stated represent the maximum length of unsupported joists (ie. between beams or slab etc). Please note: Decking laid at a 45° angle will require a reduction of joist spacing centres of 30%.

Joist Spacing (centres)

Maximum joist support centres for Q-Deck deck boards

Deck Board Size
(mm)
Joist centres (mm) for C16 grade joists - SD Description
1.5kN/m2 - Domestic 4.0kN/m2 - Commercial
20.5 x 95 300 - Cambridge
20 x 138 400 TBC SmartBoard
27 x 144 500 400 Winchester, Canterbury
33 x 120 600 500 York

Maximum support centres for Q-Deck Twinson® deck boards

Deck Board Size
(mm)
Joist centres (mm) for C16 grade joists - SD
1.5kN/m2 - Domestic 4.0kN/m2 - Commercial
28 x 140 500 350

Deck joist spacing centres diagram

PLEASE NOTE: This information has been prepared to assist in the preliminary design of timber decks, eg for species selection and initial member sizing. The detailed structural design for an individual deck will need to be confirmed by an appropriately qualified engineer. Reproduced with permission from TRADA (Timber Research and Development Association).

Decking beam span illustration

Joist and Beam Span Tables

Strength Grading.

For decks 600mm or more above ground level, support posts should be of strength graded material. If you need strength graded components please consult your Q-Deck® retailer who will pass the enquiry onto Hoppings Softwood Products for consideration.

C16

Table A: Maximum joist spans - Strength Class C16

Imposed load 1.5kN/m2
Joist centres (mm)
Imposed load 4.0 kN/m2
Joist centres (mm)
Joist section (mm) 400 500 600 400 500 600
Width Depth Maximum clear joist span SJ (metres)
45/47 97 2.00 1.86 1.75 1.47 1.33 1.22
45/47 122 2.51 2.33 2.20 1.85 1.66 1.51
45/47 147 3.02 2.81 2.64 2.20 1.97 1.80
45/47 170 3.49 3.24 3.05 2.52 2.26 2.07
45/47 195 4.00 3.71 3.50 2.87 2.57 2.35
45/47 220 4.20 4.18 3.94 3.21 2.88 2.63

Table B: Double member beam spans - Strength Class C16

Imposed load 1.5kN/m2
Joist centres (mm)
Imposed load 4.0 kN/m2
Joist centres (mm)
Beam Size (mm) 1.2 1.8 2.4 3.0 1.2 1.8 2.4 3.0
Width Depth Maximum clear beam span SB (metres)
2 x 45/47 122 1.99 1.72 1.55 1.43 1.45 1.22 1.05 -
2 x 45/47 147 2.39 2.07 1.87 1.73 1.74 1.46 1.25 1.02
2 x 45/47 170 2.76 2.40 2.16 1.99 2.02 1.67 1.44 1.18
2 x 45/47 195 3.16 2.75 2.48 2.29 2.31 1.90 1.64 1.35
2 x 45/47 220 3.56 3.10 2.80 2.58 2.61 2.13 1.83 1.53

Imposed loads 1.5kN/ m2 - domestic applications (single occupancy) 4.0kN/m2 - commercial applications (dining areas, cafes, restaurants, balconies)

C24

Table A: Maximum joist spans - Strength Class C24

Imposed load 1.5kN/m2
Joist centres (mm)
Imposed load 4.0 kN/m2
Joist centres (mm)
Joist section (mm) 400 500 600 400 500 600
Width Depth Maximum clear joist span SJ (metres)
45/47 97 2.15 1.99 1.87 1.58 1.47 1.38
45/47 147 3.24 3.01 2.83 2.40 2.22 2.08

Table B: Double member beam spans - Strength Class C24

Imposed load 1.5kN/m2
Beam centres (m)
Imposed load 4.0 kN/m2
Beam centres (m)
Beam Size (mm) 1.2 1.8 2.4 3.0 1.2 1.8 2.4 3.0
Width Depth Maximum clear beam span SB (metres)
2 x 45/47 147 2.57 2.24 2.02 1.86 1.88 1.62 1.37 1.09

Imposed loads 1.5kN/ m2 - domestic applications (single occupancy) 4.0kN/m2 - commercial applications (dining areas, cafes, restaurants, balconies)