MGP10 and H3 Explained
1. What is MGP10 treated pine?
MGP10 stands for Machine Graded Pine 10. It is a structural grade of seasoned softwood, generally radiata pine, where the timber is machine graded for its structural properties.
It is important to understand that MGP10 describes the structural grade, not the preservative treatment. For external applications the timber also needs the appropriate hazard-level treatment. H3 is the relevant above-ground external hazard level for applications such as deck framing, pergolas and other weather-exposed structures.
2. What does H3 mean?
H3 is a preservative hazard level, not a structural grade.
H3 treatment is intended for timber used above ground and exposed to weather or periodic wetting, providing protection against decay fungi and insects including termites and borers.
For our external structural timber we only supply H3-treated pine for external above-ground applications. So the two specifications to keep separate are:
- MGP10 — structural strength and grade
- H3 — preservative protection for the intended exposure
Both matter when selecting timber for an external deck or structure.
3. Does H3 treatment change the MGP10 structural grade?
No. Treatment level and structural grade are separate specifications. A piece of pine can be structurally graded MGP10 and then preservative treated to H3 for above-ground external use.
When specifying our external structural pine, the combination that matters is MGP10 + H3 — MGP10 relates to structural performance, H3 relates to durability against biological deterioration in above-ground external exposure.
4. What sizes do you stock?
Our structural pine is supplied as LOSP Treated MGP10 H3 in lengths from 3.6m to 6.0m, in these sections:
- 70×35mm and 70×45mm
- 90×35mm and 90×45mm
- 140×35mm and 140×45mm
- 190×35mm and 190×45mm
- 240×45mm
- 290×45mm
Treatment Types
5. What is the difference between CCA and LOSP treated pine?
CCA stands for Copper Chrome Arsenate, a water-borne preservative treatment using copper, chromium and arsenic compounds to protect against fungal decay and insect attack.
LOSP stands for Light Organic Solvent-Borne Preservative. Rather than a water-based process, preservatives are carried into the timber using an organic solvent. H3 LOSP formulations protect against both fungi and insects.
Side by side:
- Treatment type — CCA is water-borne; LOSP is organic solvent-borne
- Can achieve H3 — both can
- Appearance — CCA generally changes the timber colour; LOSP can retain a more natural timber appearance
- Structural grade — separate from the treatment in both cases
The important point is that CCA and LOSP are treatment methods. Neither one by itself defines the structural grade of the timber.
6. Why don't you sell CCA treated pine?
We have standardised our external structural range around LOSP H3-treated MGP10 pine rather than offering CCA-treated pine. This gives a single clear specification for external above-ground applications.
LOSP is also useful where a cleaner, more natural-looking timber appearance matters, because the treatment process can leave the timber's light colour relatively unchanged.
This does not mean CCA is unsuitable where it is permitted and correctly specified. CCA and LOSP are both recognised preservative treatment systems, and the appropriate treatment depends on the intended hazard class and application.
7. What happens if I cut, drill or notch H3 treated pine?
Any cutting, drilling, notching or machining can expose untreated internal timber. With preservative-treated timber the newly exposed surface may require appropriate resealing or preservative treatment in accordance with the treatment manufacturer's requirements.
Follow the treatment supplier's instructions for resealing cut ends and exposed surfaces.
Using MGP10 H3 on a Deck
8. Why is H3 important for a deck?
A deck frame is normally exposed to outdoor conditions including rain, humidity and periodic wetting. H3 treatment is intended for above-ground external exposure, helping protect the pine against biological deterioration such as fungal decay and insect attack.
That is why our external structural pine specification is H3 treated rather than untreated MGP10.
9. Is MGP10 H3 suitable for deck joists and bearers?
MGP10 H3 is commonly used for structural deck framing including joists and bearers, provided the member size, span, spacing, loads and site conditions are appropriate.
Keep the two specifications separate: MGP10 tells you about structural performance; H3 tells you about preservative protection. The correct member size should then be established from the applicable span tables or engineering design.
Span Tables & Deck Design
10. What are span tables?
Span tables are engineering-based reference tables that help determine how far a particular timber member can span under specified conditions.
The allowable span of an MGP10 joist depends on far more than its length. It can depend on:
- Timber grade — for example MGP10
- Timber dimensions — for example 90×45mm
- Joist spacing
- Whether it is a single or continuous span
- Design loads
- Wind classification
- Deck configuration and support conditions
- Other project-specific requirements
Always use span information applicable to the exact timber grade and application, rather than assuming one MGP10 size has a universal maximum span.
11. How can I check the required span for my deck?
Use our Deck Estimator Pro calculator to work through your deck requirements. Enter your deck dimensions and structural parameters, and it will help you determine the required framing configuration.
This matters because the required timber size is not determined by the deck's overall dimensions alone — joist spacing, bearer arrangement, spans and the structural grade of the timber all affect the design.
We also publish span tables for reference.
12. Can I simply use a larger MGP10 timber if my span is too long?
Not necessarily. Increasing the size of a member can increase its capacity, but structural design involves more than choosing a larger section — deflection, bending, shear, support conditions, connections and load paths can all need to be considered.
The Deck Estimator Pro calculator is useful for preliminary sizing, but unusual structures, large spans, elevated decks, significant loads or situations outside the calculator's assumptions should be checked by a suitably qualified building professional or structural engineer.
13. Why can't I use an MGP10 span table for MGP12 or another grade?
Because different structural grades have different characteristic structural properties. MGP10, MGP12 and MGP15 are different machine-graded classifications, so the applicable span information must correspond to the actual grade being used. Do not use an MGP12 span table simply because MGP12 figures are easier to find.
Always check in this order: Species → Grade → Size → Spacing → Span → Application.
Important: span tables and calculators are design aids, not a substitute for project-specific engineering where engineering is required. The applicable Australian Standards and actual site conditions should always be considered.
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