ud83dudcda Wood Deck Build Log u2014 9 of 45
u2190 Material Comparison Pt.1 u00b7 Series Index u00b7 Roof Material Pt.1 u2192
→ See the complete Wood Deck DIY Guide
2026 Note from DIY Dad: Choosing Deck Materials
ud83duded2 RECOMMENDED PRODUCTS
Pressure-Treated Deck Lumber
Pressure-treated lumber is the baseline u2014 skip it and you’ll be rebuilding in 5 years.
The most important decision in wood deck planning is material selection — natural wood vs composite — because it determines cost, durability, and maintenance effort.
Material Selection Overview
ud83duded2 RECOMMENDED PRODUCTS
Deck Stains & Waterproof Sealers
Stain every 2u20133 years. It’s the single cheapest way to double deck lifespan.
I chose natural hardwood (Bangkirai) for decking and Super Light 700 steel pipe for the structural frame.
Originally published: 2017-04-15. In the previous post I compared the surface treatments and basic strength of SPF wood versus Super Light 700 steel pipe. Now I will compare beam strength using formulas. Considering a simply-supported beam with a concentrated load at mid-span:
Notation
- δ: deflection [mm]
- σ: stress [N/mm²]
- σm: allowable bending stress [N/mm²]
- S: safety factor
- M: bending moment [N·mm]
- L: span between supports [mm]
- W: load [N]
- Z: section modulus [mm³]
- I: second moment of area [mm⁴]
- E: Young’s modulus [N/mm²]
Strength Formulas
M = WL/4 (1)
σ = M/Z (2)
S = σm/σ (3)
δ = WL³/(48EI) (4)
From equations (1)–(3): with the same loading conditions, the higher the allowable stress, the higher the safety factor (stronger). Safety factor is proportional to allowable stress.
From equation (4): with the same loading conditions, the higher the Young’s modulus, the smaller the deflection. Deflection is inversely proportional to Young’s modulus.
This confirms that Super Light 700 has far superior strength:
- Long-term allowable stress: 19.3× higher
- Short-term allowable stress: 15.9× higher
- Young’s modulus: 20.5× higher
Supplementary: Allowable Stress for SS400 Steel
Tensile strength: 400 N/mm². Above 235 N/mm² the steel yields — deformation becomes permanent. Therefore structural design limits stress to 235 N/mm² (short-term allowable stress). Long-term allowable stress accounts for creep fatigue with a safety factor of 1.5, giving 157 N/mm².
What is Strain?
When a body is pulled, it elongates. The ratio of elongation to original length is strain: ε = λ/L. Longitudinal strain is in the direction of loading; lateral strain is perpendicular (where the bar contracts): ε = δ/D.
What is Deflection?
Deflection is bending deformation of a loaded structural member — deformation perpendicular to the member axis (as opposed to axial elongation or compression).
In conclusion, steel clearly outperforms wood in both strength and corrosion resistance by a wide margin.
ud83dudcda Wood Deck Build Log u2014 9 of 45
u2190 Material Comparison Pt.1 u00b7 Series Index u00b7 Roof Material Pt.1 u2192
DIY Renovation Quote Review Manual
How to read, compare, and negotiate contractor quotes. This manual saved me u00a5800,000 on a single renovation project.
ud83cudfe0 More from DIY Father
15 years of landlord experience u00b7 3 apartment buildings u00b7 DIY renovations that saved millions of yen. Browse all articles at diytosan.com
