Good quality materials are the second critical element in the decking system.  The choice and specification of suitable and quality materials is essential as it is the materials which will be left in place to be subjected to sun and rain after the construction is completed and the performance of the project will largely depend on the quality and suitability of materials for their intended purpose. 

Good quality materials will provide good performance and longevity (together with good construction methods), however, poor quality materials will deliver disappointing performance. 

Materials are often selected at the design stage to allow strength and material characteristics to be incorporated into the design.  Some aspects of material selection which are required for design or for specification on the drawings have been covered in "Design & Detailing" but are addressed in greater detail below. Please click on the following links to gain information on each of these material groups:

Timber Selection

Timber for the Structure

Strength
Quality
Durability

Note on the lack of relationship between stress grade and durability

Note on durability and preservative treatment

Stability/Weathering Characteristics
Suitable Structural Timbers

Timber for the Decking

Note on subframe and decking timbers

Sealants

Fixings

Stainless Steel Fixings

Finishings

Click on the following link for a downloadable Deck Master_® Decking Materials Advisory, as reference before, or when, talking to your decking materials supplier. 

Timber Selection

Timber requirements for building a deck fall into two categories:
- timber for the structure

- decking subframe and roof structure and

- decking timber

In the traditional delivery of decking projects the choice of:

- structural timber is often made on the basis of initial strength
  with price an important factor

eg strength or “F ratings” are often the primary means of specification

- decking timber is often made on the basis of price and initial appearance

eg quality of machining, a colour preference or a rustic appearance - characteristics which have no bearing on long term performance, especially in weather exposed applications.

However, the choice of decking and subframe timber is not as simple as it seems.  To a greater degree than many other building materials the performance and longevity of both subframe and decking timber is dependant on the characteristics of the timber species incorporated into the project - characteristics which vary markedly between species. 

To access subframe and decking timber for your project, which will give satisfying performance over a long life, there are a number of factors which must be considered and appropriately specified.

The requirements for subframe and decking timber are covered in greater detail below:

Please note that some of the information below which is also relevant to design and material specification is also provided on the "Design and Detailing" page.

Timber for the structure

Stress grades are often used as a primary specification for structural timber used externally, above or in ground. However, stress grades are an imperfect guide for overall suitability of timber for fully weather exposed conditions.  Specifying timber requirements according to strength (or stress grade) will only ensure that the timber supplied is of sufficient strength, when milled, to perform a structural purpose.  It will not ensure that the timber will be suitably durable for the application or geographical location or sufficiently stable that excessive shrinkage or distortion will not cause aesthetic or structural problems. 

In addition to strength, other characteristics, such as quality, durability, shrinkage, and weathering properties, are also important and should be specified to ensure satisfactory performance over the life of the deck.  More detailed information on these characteristics is provided below.

As the structural timber is often not directly exposed to the weather and is generally orientated vertically so that the water does not pool in any defects, it can be of a lesser quality and allow a greater range of species than the decking timber.

Suitable structural timber can be sourced in hardwood or softwood, however different hardwoods and softwoods are not equally suitable for every application and the specifier, builder or purchaser must ensure that the subframe and decking timber specified is fit for the purpose and conditions for which it is to be used - in the long-term.

Strength (Stress Grade)

Commonly available stress grades in unseasoned structural hardwood are F14 and F17 with F7 being commonly available in seasoned softwood.

In hardwood, the timber strength specified may depend on availability in your area, however, F14 should be the minimum stress grade specified in, considering the type and extent of natural characteristics allowed under the grading rules for the various stress grades. F17 Hardwood is preferred for weather exposed conditions as it generally includes a lesser number and size of natural characteristics, which can be a source of accelerated deterioration when exposed to the weather.

Note on lower hardwood stress grades: In some areas, hardwood structural timber may only be readily available in lower stress grades and lower durability.

Unseasoned structural timber in F11 stress grade may be suitable, provided that it is:
• minimum durability class 2 (above ground classification);
• of low shrinkage or seasoned;
• well ventilated;
• not used in conditions of consistent high humidity and
• used in combination with other good building practices, particularly the sealing of cut ends and sealing of the tops of joists with a waterproof membrane to shed water away from the joist.

For structural timber of durability class 2, these requirements would be necessary to satisfy the performance requirements of the BCA.

In softwood, seasoned F7 should be the minimum stress grade specified.

Softwood structural timber also should be well ventilated; and used in combination with other good building practices, particularly the sealing of cut ends and sealing of the tops of joists with a waterproof membrane to shed water away from the joist. Check with your supplier whether any limitations on use, or particular construction techniques, are appropriate for conditions of regular high rainfall and humidity.

Quality

Timber supplied to a stress grade will contain natural characteristics, such as knots, to the extent allowed in the grading rules and will not guarantee supply to a visually attractive standard that you may consider appropriate for your situation.

You may prefer that the posts supporting the roof structure and the roof timbers be of a higher standard of appearance, containing few and small characteristics, whereas the decking subframe may only need to be of the specified structural standard, as it is concealed from view.

If you require a high standard of appearance in addition to appropriate strength, specify ‘Structural Appearance Grade’. Structural appearance grade timbers are more expensive than structural grade timbers. If the appearance of your structural timber is an issue which is important for your project, speak to your timber merchant about the visual standard of the timber which would be supplied.

Durability

Natural durability is the ability of the heartwood of a timber species to resist decay and insect attack and is determined by the “extractives” eg oils, waxes and resins which fill the capillaries, changing sapwood into heartwood as the tree grows.

Durability ratings have been determined from the performance of samples of heartwood both in-ground and above-ground at a number of test sites around Australia, from expert opinion and from experience of in-service performance.

Durability is categorized into 4 durability classes – from Class 1 for highly durable species to Class 4 for non-durable species, with each durability class having a probable life expectancy for “outside above ground” and “in ground contact” (AS reference) .

It would be expected that the life expectancy of timber above-ground would be better than in-ground, reflected in the above-ground rating for a species being the same as or better than the in-ground rating.

We recommend that only durability class 1 hardwoods, or appropriately treated seasoned softwoods be considered for use for each of the in-service conditions “outside above ground” and “in ground contact”.

Where the underside of the structure is greater than 400mm above the ground this should be considered an above-ground application and subframe timbers shall be selected durability class 1 (outside above ground classification) structural hardwoods with sapwood treated to H3 (hazard level 3); or structural softwoods treated to H3.

Where the underside of the structure is less than 400mm above the ground, this should be considered an in-ground contact application and subframe timbers, shall be selected termite resistant durability class 1 structural hardwoods (in ground contact classification) with sapwood treated to H5; or structural softwoods treated to H5.

In extreme exposure conditions subject to regular rainfall or high humidity such as in tropical areas, structural timber and decking should be durability class 1 hardwoods (in ground contact classification).  Where regular rainfall is expected, consider protecting your deck with a roof with wide overhangs.     

Posts extending into footings shall be selected termite resistant durability class 1 hardwoods (in ground contact classification) with sapwood H5 treated or H5 treated round softwood posts. H4 treatment may be acceptable where the application is not critical and the posts can be replaced when they deteriorate.

Note that using termite resistant species does not negate the requirement for termite protection in accordance with the Building Code of Australia.

Note on the lack of relationship between stress grade and durability

There is no direct relationship between stress grades and durability- a higher strength timber will not necessarily be more durable – an example is Tasmanian Oak which is a low durability hardwood with a relatively high stress grade.

Strength and durability should be considered and specified separately or by proxy by specifying a species to achieve the required durability characteristics eg F14 or F17 Spotted Gum - Spotted Gum is a durability 1 species (outside above ground classification) which is also very stable.

Note on durability and preservative treatment

• Durability classes only refer to the heartwood (diagram).
• Sapwood is considered to be non-durable and must be preservative treated to improve its durability.
• When timber is treated, the preservative chemical only penetrates and protects the sapwood.
• Treatment of sapwood does not improve the durability of heartwood.
• The sapwood of some timbers, such as cypress, cannot be preservative treated and hence can not be made durable.  Decking from species with untreatable sapwood but which satisfy the requirements for durability, stability and weathering characteristics shall be supplied sapwood free.
• Because hardwoods only have a small sapwood band the durability (and hence longevity) of treated hardwood is as per the durability of the species from which the timber section is produced.
• Because softwoods have a large sapwood band, the durability (and hence longevity) of the treated softwood is as per the durability of the treated sapwood, provided there is minimal heartwood present.

Shrinkage

If using unseasoned hardwood structural timber, use only low shrinkage timbers ie less than 8% tangential shrinkage. Ideally, a single species, or alternatively mixed species but with similar characteristics, shall be used to ensure that similar shrinkage occurs in all the structural members.

Unseasoned hardwood structural timber from high shrinkage species (tangential shrinkage greater than 10%) is sometimes supplied for the decking subframe. High shrinkage species are subject to collapse, distortion and excessive reduction in section size during on-site seasoning. High shrinkage and widely varying differential shrinkage between species, over time can result in unevenness in the decking surface, unsatisfactory appearance and even structural problems.

If using unseasoned hardwood, confirm with your timber supplier that low shrinkage species will be supplied.

“Surface checking” will occur in unseasoned hardwood as the outside of the section dries out quickly and shrinks while the inside of the section dries more slowly. Surface checks will close up in the long term as the whole of the section dries out. In low shrinkage timbers the surface checking is normally not severe and we would advise against using filler to fill surface checks as the filler will be squeezed out as the checks close over time.

If using softwood structural timber, use only seasoned timber for reduced distortion of the structural members and greater stability and uniformity of the overall subframe.

Stability/Weathering Characteristics

Weathering is the gradual deterioration of the timber surface from the combined effect of exposure to ultra-violet (UV) radiation in sunlight and water from rainfall. The combined effect is for colour change, a loosening of the timber fibres and a gradual wearing away of the surface.

The swelling and shrinking associated with wetting and drying can cause surface and deeper level checking, and exposure to the heat from direct sunlight can cause distortion of decking. The effect of hot, direct sunlight on saturated timber after rain can be particularly severe. The natural properties of some timbers, in resisting excessive swelling and shrinking, surface checking and distortion, make them more suited to weather-exposed conditions than other timbers. Similarly, some timbers have natural properties which are not suited to weather exposed conditions and their use should be avoided.

Maintaining an effective surface coating will improve the performance of the timber work, protecting it from weathering damage.

Suitable Structural Timbers for the Decking Subframe

Structural timbers for the subframe of weather exposed decks should be:

·  highly durable

·  low shrinkage (if unseasoned) or seasoned

·  stable and

·  treated to the appropriate hazard level if section contains sapwood.

Note on durability

- if clear of the ground (>400mm) use durability class 1 hardwoods (outside above ground classification to AS 5604 – 2003 or F7 seasoned structural softwood treated to H3

- if in-ground or close to ground (<400mm) use durability class 1 hardwoods (in ground contact classification to AS 5604 – 2003) or F7 seasoned structural softwood treated to H4/H5 – softwood posts in ground should be treated round posts.

- if subject to regular rainfall and high humidity, such as in tropical areas, use durability class 1 hardwoods (in ground contact classification to AS 5604 – 2003)

Note on shrinkage for unseasoned hardwood – to be less than 8% tangential shrinkage

Note on treatment - H3 for above ground applications or H4/H5 for in and close to ground applications

Suitable hardwoods for structural timberwork include (please refer to the essential notes below):

- For above ground, (in average conditions) (1): Spotted Gum and Black Butt (2)

- For above & in ground (3): Iron Bark, Forest Red Gum (4), Grey Gum (5), Grey Box (Gum-topped Box), White Mahogany, Tallowwood and Gympie Messmate.

Note that the above are “standard trade names" (Technical Pamphlet 1 - Building Timbers - properties and recommendation for their use in Queensland - published by Queensland Forest Service). Ensure that species with similar or local names but with inferior properties are not substituted for the species listed above. Any regional high shrinkage examples of these hardwoods are to be avoided – discuss with your timber supplier.

Structural timber in softwood, shall be seasoned, minimum F7 stress grade, preservative treated to H3 for above ground use or H4/H5 for close to or in-ground contact. 

·The list of suitable timbers in hardwood, above, is not exhaustive. Contact your local timber supplier for information on suitable structural timbers which are available in your area.

·High shrinkage unseasoned timbers and/or low durability timbers and/or timbers with untreated or untreatable sapwood are generally not suitable for structural timbers in weather exposed conditions.

·In some areas, hardwood structural timber may only be readily available in lower stress grades and lower durability. Structural timber in F11 stress grade may be suitable for use in above ground conditions which are not extreme, provided that it is: minimum durability class 2 (outside above ground classification); of low shrinkage or seasoned; well ventilated; not used in conditions of regular rainfall or high humidity and used in combination with other good building practices, particularly the sealing of cut ends and sealing of the tops of joists with a waterproof membrane to shed water away from the joist. For structural timber of durability class 2, these requirements would be necessary to satisfy the performance requirements of the BCA.

·Softwood structural timber also should be well ventilated; and used in combination with other good building practices, particularly the sealing of cut ends and sealing of the tops of joists with a waterproof membrane to shed water away from the joist. Check with your supplier whether any limitations on use, or particular construction techniques, are appropriate for conditions of regular high rainfall and humidity.
 

(1)  These species are classified in AS 5604 - 2003 as durability 1 outside above ground and durability 2 in ground contact. 

(2)  Blackbutt (Eucalyptus Pilularis) is not to be confused with, and should be used in preference to, New England Blackbutt (Eucalyptus Andrewsii) which has a lower durability and higher shrinkage than Blackbutt. Blackbutt is best used under cover. 

(3)  The species are classified in AS 5604 - 2003 as durability 1 outside above ground and durability 1 in ground contact.

(4)  Forest Red Gum (Eucalyptus tereticornis) – sometimes referred to in Queensland as “Blue Gum” - is a high durability & low shrinkage timber suitable for external use & is not to be confused with “Sydney Blue Gum” (Eucalyptus salilgna) which is a low durability & high shrinkage timber unsuitable for weather exposed use.

(5)  Grey Gum (Eucalyptus Propinqua) is a high durability & low shrinkage timber suitable for external use & is not to be confused with “Mountain Grey Gum” (Eucalyptus Cypellocarpa) which is a low durability & high shrinkage timber unsuitable for external use. 

Please click on the following link for information on suitable structural timber for the desking subframe:

Timber for the decking

The decking timber should be manufactured from timber of suitable quality, durability and stability and treated to the appropriate hazard level for the application.

As a minimum, decking timber should be:
• seasoned (less than 18% moisture content to avoid excessive shrinking on the job)
• stable (not swelling and shrinking excessively in response to moisture changes or distorting from the effects of heat or moisture changes) and
• of appropriate durability - for above ground applications use minimum durability 1 (above ground conditions) treated to H3 or for close to ground applications -durability 1 (in ground) treated to H5).

Some timber species are:
• more stable than others when exposed to hot conditions and or moisture, maintaining their shape and resisting distortion and
• more durable than others lasting longer in exposed conditions.

Good decking timber for exposed conditions combines both high durability and stability ie it will last a long time and will be relatively stable, performing well over its life. Examples of species which would be suitable for above ground applications and in all weather conditions include:
• Spotted Gum,
• Ironbark (red and grey),
• Grey Gum (not Mountain Grey Gum)
• Gympie Messmate
• White Mahogany
• Tallowwood and
• Kwila (Merbau).

Ironbark, Gympie Messmate and Tallowwood are durability 1 (in ground contact classification) species and would be suitable for close to ground applications.

Other decking timbers available in your area may be suitable, (seasoned decking with appropriate durability, stability and treatment) – refer to your local timber supplier for recommendations to suit your application ensuring you advise whether the deck will be exposed to or protected from the weather.

If using decking of standard moisture content, in areas of low or high humidity, make suitable allowance for the corresponding shrinkage or expansion which will occur.

If decking is to be installed in areas of consistent extreme low humidity, eg in drier far western areas or in areas of consistent extreme high humidity, eg in rainforest areas, then decking with an appropriate moisture content shall be ordered, or standard decking acclimatised to the local EMC (equilibrium moisture content) by strip-stacking before installation. The equilibrium moisture content (EMC) is the moisture content at which the timber no longer loses moisture to, or gains moisture from, the atmosphere. The EMC varies with seasonal changes, so it is normal to quote a range for a location. Minimal shrinkage or expansion will occur if timber is used with a moisture content within the EMC range for a location.