ud83dudcda Wood Deck Build Log u2014 26 of 45
u2190 Detail #10: Rafter Strength Calc u00b7 Series Index u00b7 Design #12: Column Buckling Calcs u2192
2026 Update from DIY Dad: Final Planning Phase u2014 Preparing for Construction
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Pressure-Treated Deck Lumber
Pressure-treated lumber is the baseline u2014 skip it and you’ll be rebuilding in 5 years.
This is the latter half of the planning phase. The plan continued through 2018, but by then the final design was confirmed and construction preparations were in order. The biggest design change was switching the structural skeleton to steel scaffold pipe. ^ ^
Why I Chose Steel Scaffold Pipe (u5358u7ba1) for the Skeleton
I originally planned to use aluminum square tube for the skeleton, but switched to steel scaffold pipe (u03a648.6 mm). The reasons were cost savings and ease of availability. Scaffold pipe is cheap at any home center and can be freely assembled with couplers/clamps.
The downside is rust, but that can be handled with anti-rust paint before installation. Structural strength is more than adequate u2014 scaffold pipe has a proven track record as a wood deck skeleton. m(_ _)m
Final Design Specifications
DIY Dad’s confirmed wood deck specifications:
- Decking: Bangkirai hardwood, 20 mm thick u00d7 120 mm wide
- Skeleton: Steel scaffold pipe + couplers
- Deck surface: 4.5 m wide u00d7 2.4 m deep
- Roof: Corrugated polycarbonate panels (pergola structure)
- Railing / fence: Bangkirai hardwood
- Stairs: 2 steps
The long planning period meant far fewer mistakes during construction. “Rush it and regret it” vs. “plan it thoroughly and build it right” u2014 that’s the DIY Dad style. ^ ^
Timing Your Material Orders
Hardwood is purchased from specialist online lumber shops, but lead times can be 1u20132 weeks. The best strategy is to order before construction starts, so the wood arrives just as the skeleton is going up. m(_ _)m
Purlin (u6bcdu5c4b) Strength Calculation
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Stain every 2u20133 years. It’s the single cheapest way to double deck lifespan.
Hi, I’m DIY Dad. ^ ^
Today’s topic: the purlins that support the roof sheeting. The member is SPF 1u00d74 (19 u00d7 89 u00d7 6 ft).
Normally you’d use a 30 mm square timber for this role, but I want to maximize the rafter span, so I’m going with 1u00d74 lumber.
u2191 Purlin layout
Strength Calculation
SPF material properties (assuming Grade 1 framing lumber u2014 bending):
- Long-term allowable stress: 8.14 N/mmu00b2
- Short-term allowable stress: 14.8 N/mmu00b2
- Modulus of elasticity: 10,000 N/mmu00b2
Load conditions are the same as for the beam. Snow load: 2,569 N.
Purlins are spaced at 600 mm pitch, limited by the fixing pitch of the corrugated roof sheeting u2014 that gives 6 purlins.
Treating this as a uniformly distributed load:
u03c9 = 2,569 / (5 u00d7 994) = 0.517 N/mm
Clear span = purlin-to-purlin distance = 993.3 mm u2248 994 mm
Section modulus: Z = bhu00b2 / 6 = 18 u00d7 89u00b2 / 6 = 23,763 mmu00b3
Second moment of area: I = bhu00b3 / 12 = 18 u00d7 89u00b3 / 12 = 1,057,454 mmu2074
Bending moment: M = u03c9Lu00b2 / 8 = 0.517 u00d7 994u00b2 / 8 = 63,852 Nu00b7mm
Bending stress: u03c3 = M / Z = 63,852 / 23,763 = 2.69 N/mmu00b2 < 8.14 N/mmu00b2 u2192 OK
Deflection: u03b4 = 5u03c9Lu2074 / (384EI) = 5 u00d7 0.517 u00d7 994u2074 / (384 u00d7 10u2075 u00d7 1,057,454) = 0.07 mm < 3.3 mm (L/300) u2192 OK
Both stress and deflection are within allowable limits. ^ ^
Next up: the columns.
That’s all for today! (^^)/~~~
ud83dudcda Wood Deck Build Log u2014 26 of 45
u2190 Detail #10: Rafter Strength Calc u00b7 Series Index u00b7 Design #12: Column Buckling Calcs u2192
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How to read, compare, and negotiate contractor quotes. This manual saved me u00a5800,000 on a single renovation project.
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15 years of landlord experience u00b7 3 apartment buildings u00b7 DIY renovations that saved millions of yen. Browse all articles at diytosan.com
