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Your ultimate guide to decoding timber grades

From hardwood to softwood, timber is the comforting foundation for your home’s flooring, fencing and decking. Timber is fondly enjoyed by your closest friends and family, but ready to wear away at your hard work are freeloading termites. Your timber’s resistance to insects and harsh environmental conditions is determined by a range of ratings to grade strength, durability and hazard levels.

When embarking on a new DIY challenge at home, quickly decoding the numerous timber grades and classes is your head start in building your upcoming timber project. Knowing the different types of timber ratings gives you long term security when it’s time to choose the right materials for your project.


  1. Hazard levels and where to apply them
  2. Timber durability classes
  3. Timber strength ratings

Hazard levels and where to apply them

Hazard levels determine the degree of biological hazard timber will be exposed to in specific applications. The varying levels represent chemical treatment used to increase the timbers’ resistance to termites, decay and fungal growth. The hazard level is rated on a scale from H6, the highest level, down to H1, the lowest level.

H6 Timber

  • Ideal for marine use
  • Protects against marine wood borers and decay
  • Suitable for being submerged in saltwater

H5 Timber

  • Ideal for full contact to the ground and fresh water
  • Protects against borers, termites, insects and extreme decay
  • Suitable for critical use and areas exposed to extreme leaching and wetting

H4 Timber

  • Ideal for outdoor use
  • Protects against borers, termites and severe decay
  • Suitable for in ground areas with severe leaching and wetting

H3 Timber

  • Ideal for outdoor use
  • Protects against borers, termites and decay
  • Suitable for above ground areas that allow moderate leaching and wetting

H2 Timber

  • Ideal for indoor use
  • Protects against borers and termites
  • Suitable for above ground areas protected from leaching and wetting

H1 Timber

  • Ideal for indoor use
  • Protects against borers
  • Suitable for well-ventilated above ground areas

Timber Durability Classes

The class of a durability rating relates to the natural ability of timber to resist decay and insects. For timber decking, pergolas and handrails, we recommend the use of class 1 or 2 hardwood. If you’re looking to use a lower class with sapwood, the timber should be treated with H3 or removed.

Class 1

  • Long-lasting and extremely durable
  • Protected indoor areas: 50+ years
  • Outdoor above ground areas: 50+ years
  • In-ground areas: 25+ years

Class 2

  • Long-lasting and durable
  • Protected indoor areas: 50+ years
  • Outdoor above ground areas: 30+ years
  • In-ground areas: 15-25 years

Class 3

  • Protected indoor areas: 50 years
  • Outdoor above ground areas: 15 years
  • In-ground areas: 8-15 years

Class 4

  • Protected indoor areas: 50 years
  • Outdoor above ground areas: 5-8 years
  • In-ground areas: less than 5 years

Timber Strength Ratings

Strength ratings identify how your timber will hold when it is under stress. It considers the application of the timber and how it will be used to determine what the suitable rating for your project will be. For the strongest level of protection, we recommend higher ratings for timber to give you long-lasting security.

Hardwood and Softwood

F ratings indicate the strength of hardwood and softwood flooring. The level of ratings start from F1, the lowest grade, and exceed past F40. For decking, joists and bears, hardwood timber rated F14 or higher is recommended.

Timber Species Strength Durability
Ironbark F22 – F42 Class 1
Merbau F22 – F27 Class 1 (above ground only)
Blackbutt F17 – F34 Class 1
Tallowwood F17 – F34 Class 1
Spotted Gum F17 – F34 Class 1 (above ground unless treated to H4)
Blue Gum F14 – F24 Class 2 (above ground unless sapwood treated to H4)
Brushbox F14 – F24 Class 3

Discover our recommended timber types for your decking project.

MGP Timber – MGP10 vs MGP12 vs MGP15

Suitable to judge the strength of joists and bearers, MGP (machine graded pine) assesses compressive and structural strength of the pine timber, as well as the stiffness. The more stress the pine timber can withstand, the more MGP it’ll have.

There are three ratings available for MGP:

MGP10 = F7 MGP12 = F8 MGP15 = F11
Highly versatile all-round timber
Minimum requirement for building
Higher strength than MGP10
Suitable for frames, walls, floors joists and supports
Higher strength than MGP12
Suitable for frames, walls, floor joists and supports

While MGP can be substituted for F graded material, F graded material shouldn’t be applied where MGP material has been specified.

Contact Narangba Timbers

As Brisbane’s premium timber decking supplier, we have an extensive range of timber, hardware and accessories ready to be delivered to your door.

Give us a call on 1300 477 024 or contact us online today.