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Please click on the following links to gain information on each of these
Design topics:
Design Standard
Design Tables
Subframe Timber Strength Specification
Specification of
other Timber Characteristics
Subframe Specification
Decking Timber
Decking Timber Specification
Drawing up Your Deck
Termite Protection
Detailing
Design Standard
The decking project shall comply in all respects with the Building Code
of Australia (BCA).
However, there are additional good practices, which are provided on this
website, which can be incorporated into your design, material selection,
subframe construction and decking installation, finishing and
maintenance to deliver a decking project which performs well over a long
life.
If a discrepancy exists between the information contained on this
website or in any of our technical documents and the BCA, the BCA shall
take precedence.
The information on additional good practices applies to domestic
applications, in non-bushfire rated areas. This information has also
been developed for fully weather-exposed decks, which place vastly
greater demands on structural timber, decking and fixings. As a result,
you will find many recommendations for construction techniques and
materials which are of a higher standard than general practice. In
particular, some attention is given to the requirement for suitable
fixings and the protection of structural and decking timber. However,
for best protection from the weather, for a timber deck, comes in the form of a roof with
wide overhangs to shelter the timberwork from sun and rain.
Design tables
The design tables for posts, bearers and joists in AS1684.2 provide the
required section size to span nominated distances and using timber of a
nominated strength (stress grade or "F rating").
The starting point for design is to have an idea of:
- the spacing which you would prefer of posts in both directions
- the strength of the timber you are wanting to use
- the strength of the timber you can readily access in your area
- the lengths and sizes of timber you can readily access in your area.
Post
spacing in one direction will dictate the span and size of your bearers
and in the other direction the span and size of your joists.
Bearers and joists are commonly designed as continuous spans with the
length of bearer and joist twice the post spacing.
You
can start with your preferred post spacing and F rating and determine
the length and size of bearers and joists, however, long and large
sections are becoming increasingly difficult to obtain so we would
suggest you have an idea of what you would like then discuss what F
rating, length and size combinations your local timber merchant can
readily supply and then work backwards to determine the post layout.
Post spacings of 2.4 to 3m are common, providing a balance between the
number of posts required and keeping the timber sections to a reasonable
size for availability and economy..
Subframe
Timber Strength Specification
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,
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.
As softwoods are likely to be
reactive to the presence of moisture (unless treated with an integral water
repellent - which is
rare), softwood structural timber shall be 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.
Specification of other timber characteristics
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.
Subframe Specification
Supply of suitable unseasoned structural hardwood should be ensured by
specifying:
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Stress grade,
Appearance quality (if required)
Durability rating
Maximum tangential shrinkage (for stability)
Treatment level (H3/H4/H5 depending on
application, if section
contains sapwood)
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or |
Stress grade,
Appearance quality (if required)
Timber species (selected for appropriate
durability and stability)
and
Treatment level (H3/H4/H5 depending on
application, if section
contains sapwood) |
In relation to treatment of unseasoned hardwood, check with your timber supplier
whether the mill automatically treats any sections containing sapwood.
If so, you should receive the treatment of the pieces which would suffer
deterioration of the sapwood for no additional charge - in the case of
spotted gum the sapwood is susceptible to deterioration from decay and
also from insect attack (lyctid beetle) and sections containing sapwood
should be treated if they are to be saleable - in Queensland this is a legal requirement.
If you ask
specifically for treatment you may be charged for treatment of all the
pieces, even that part which would have been treated anyway. If
sections containing sapwood are automatically treated, then the only
benefit in asking and paying for treatment of all the pieces would be
colour consistency. More detailed information
on requirements for timber decking is
provided on the "Materials" page
of this site, including suggestions for suitable subframe species.
Supply of suitable structural softwood should be ensured
by specifying: F7,
seasoned softwood and treatment level (H3/H4/H5 depending on
application).
Suitable structural timber can be sourced in high and
lower durability hardwoods 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.
Decking
Timber
Decking timber is available in a variety of species,
including Australian and imported hardwoods and softwoods.
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 shall be
seasoned and combine suitable strength, high durability and
stability ie it will not shrink excessively initially, it will last a long
time and will be relatively stable, performing well over its life.
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 (outside above ground classification) treated to H3
- for in ground or close to ground applications use
durability 1 (in ground classification) treated to H4 or H5, depending
on severity of conditions.
It is assumed here that the decking timber
offered by the timber supplier is of appropriate strength for the joist
spacing. Standard decking offered for domestic applications in
Australia should span the common domestic joist spacing of 450mm.
For greater spans, a thicker commercial decking may be required - refer
to your timber supplier for advice.
Decking Timber Specification
The
supply of suitable decking timber should be ensured by
specifying that the decking be:
seasoned
of a single or range of specific species (selected for appropriate
durability and stability) and
treated (to the appropriate hazard level if the section contains sapwood).
Appearance quality (select or standard grade) may also be specified.
More detailed
information on requirements for timber decking is provided on the "Materials"
page of this site, including suggestions for suitable species.
Drawing up
Your Deck
Floor Plans
Draw several floor plans of the deck to scale (e.g. 1mm
= 20 to 50mm) until you are satisfied that you have drawn the concept to
a size and shape that best suits your overall requirements.
Deck Height
Calculate the relative levels of bearers, joists and decking so that the
finished level of your deck is lower than the internal house floor by
25mm to prevent water from your deck running back into the house see
Figure 5 on the "Construction" page. Take into account that unseasoned subframe timbers
will shrink as they season, with the finished level, when seasoned,
being lower than when first constructed.
Positioning of Posts, Bearers, Joists and Handrails
Choose the type of handrail for your deck. Consider how it will be
attached to the post and whether the post will be a continuation of the
support post or be freestanding and bolted to the bearer (only suitable
for supporting handrail and balustrading - posts supporting roof loads
must be a continuation of the support post - refer BCA for details).
Consider whether the bearer will be attached to the front or back of the
post and whether it will be attached to the face of the post or
checked-in.
Draw an end section of your proposed handrail configuration to confirm
that the post, bearer, joist, decking and handrail configuration can be
constructed as envisaged.
Posts
Mark the posts in at the corners. Then mark in an economical grid of
intermediate posts derived from AS1684.2 and your knowledge of what length and
size combinations are available in your area, as discussed above.
Bearers
Draw in the rows of bearers as broken lines connecting the posts and
write in the size as derived from AS1684.2.
Note that the decking boards will be oriented in the same direction as
the bearers. This may be an important consideration in deciding on the
direction that your bearers, joists and decking run. Unless other design
restrictions dictate the orientation, where possible, design your bearer
and joist orientation so that the decking boards are laid in the same
direction as the longest open face of the deck exposed to the weather
and not as a large number of same length boards across the deck. If a
section of deck becomes weather damaged it will then only be necessary
to replace some runs of boards rather than replace all the boards or cut
and replace the damaged ends of the boards, which is a less than ideal
repair job.
Note that Bearers and joists are not to carry roof loads (i.e. any roof or
pergola supports are to be directly above the deck supports).
Joists
Draw the rows of joists as solid lines at 90 degrees to the bearers and
spaced at 450mm centres for standard domestic decking. Write in the
joist size, derived from AS1684.2 and spacing.
Wider joists (ie 50mm unseasoned hardwood, 42mm seasoned hardwood or
45mm seasoned softwood) shall be used to allow the placing of fixings in
a formal staggered alignment/pattern, thereby reducing the likelihood of
split joists. Wider joists are also better for joining of decking boards
over the joist as the fixings can be placed further from the end of the
board, thereby reducing the likelihood of the fixing splitting the end
of the board. Narrower joists (38/35 mm wide) are available and can be structurally
adequate at an appropriate depth but are not recommended.
Note: Do not position joists above the support posts. Posts normally
only shorten marginally as they season. However unseasoned bearers and
joists will shrink and as a result the whole deck will settle. This is
normally not noticeable or an issue if low shrinkage subframe timbers
have been used, as recommended. If the post has been cut off level with
the bearer initially, and the joist has been positioned above the post,
then as the bearer shrinks, the joist will be lifted off the bearer.
If looking at having timber decking as part of a steel-framed home, it
may be necessary to incorporate timber joists into your steel system as
direct fixing of timber decking to metal joists with self drilling
screws may result in a weakening and eventual failure of the screw in
the long term. Please refer to your steel framing supplier for further
information on how timber decking can be incorporated into the design.
Decking
Calculate the approximate quantity of decking required from the formula:
(Width of deck (metres) x 1000 x Length of deck (metres)) x 1.1
Width of deck board + gap (mm)
Note: The 1.1 multiplier represents 10% added for wastage in cutting
random length decking to fit the set spacing of joists.
Bracing
Bracing must be provided for stability of the deck in accordance with
AS1684.2.
Termite Protection
Termite protection shall be provided in accordance with the requirements
of the Building Code of Australia.
The timbers advised as suitable for structural timberwork (under
Materials) are resistant to termite attack either through durability
of the heartwood or preservative treatment.
However, in addition to the deck potentially being subject to termite
attack, the deck may become a means of access for termites to the
dwelling to which the deck adjoins and termite protection must be
incorporated into your design particularly as untreated softwood is
commonly used as a framing timber in houses. Termite attack of
non-structural timbers such as architraves can be worrying and costly to
repair but termite attack on untreated framing can affect the structural
integrity of your home and be very expensive to repair.
Termite protection may take the form of combinations of the following:
Keeping the under-deck and surrounding area dry;
Keeping gardens and landscaped or mulched areas away from the deck;
Using termite resistant timber species;
Performing perimeter chemical treatment, and/or treatment of the sides
and base of any post hole
Installing termite barriers such as metal ant capping eg. where the
deck joins an adjacent structure;
Performing regular competent physical inspection of the structure and
inspection points;
Designing in gaps between members at critical and easily accessible
locations e.g. setting posts on metal brackets with a 75mm gap between
the post and concrete or designing a separate, free-standing deck with a
75mm gap to any adjacent structure.
Note: Termite barriers and gaps will not necessarily stop access by
termites as physical or chemical barriers can be bridged or breached.
Often, access by termites in susceptible timbers is through the
timber, eating from the inside, rather than along it, so that their
presence is not obvious until the later stages of attack. Termite
barriers and gaps make any access noticeable, by forcing the termites to
build a bridge or tunnel, where it can be seen, over the barrier or gap.
Detailing
Decks shall be detailed, wherever possible, to avoid moisture
traps between timber surfaces. Moisture soaking into timber
surfaces or end grains, which can not then dry out easily, can lead to
premature deterioration. This is particularly important where the
structure is to be painted and moisture cannot dry out easily because of
the paint film on the area surrounding the timber surfaces.
Locations where this is likely to be a problem include bearer to post
connection, housed ends of step treads and checked housing in the step
stringer and balustrading fixed to the face and bottom of a hand rail.
If checked joints are preferred they should be sealed with a suitable
heavy bodied timber sealer or paint before the timber surfaces are
connected. All
cut ends should be sealed as end grain is much more absorbent than face
grain and is prone to cracking over time from the absorption and release
of moisture eg from rainfall.
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