Prior to Construction
Ensure suitable
conditions under the deck
The area which will be under a completed deck shall be adequately
graded, drained and ventilated so that the area is dry in normal
conditions and dries quickly after wet weather without water pooling.
Where the ground is excessively damp due to poor drainage or seepage,
sub-surface drains should be installed to allow the moisture to drain
away.
Retained humidity in the sub-floor area resulting from pooled water,
damp soil conditions and/or a lack of sufficient ventilation can result
in swelling, cupping and distortion of the decking, possible decay of
the decking and/or structural framing members, and increased risk of
termite attack.
On-site storage
The decking shall be protected from damage from both moisture and
excessive heat, and packed up off dry ground to allow a free airflow,
while stored on site prior to installation.
Any storage shed or protective cover must be waterproof, but should also
provide for a free-flow of air so that the timber is not affected by
excessive heat build-up or condensation.
Protective coverings must not trap condensation, and outdoor storage
areas should be chosen so that there is no possibility of water pooling
under the covers, which could result in moisture uptake into the boards.
Inadequate covering can result in:
• Water marking, which may remain visible under clear finishes;
• Moisture absorption, which can result in moisture induced movement;
• Excessive heat build-up, which can cause distortion of the timber.
Because of the potential for excessive heat build up, black plastic is
not considered a suitable long-term covering.
Sealing cut-ends and joints
Seal all cut ends, checked joints and timber-to-timber interfaces in
structural timber or detailed timberwork, during construction, to reduce the absorption of
trapped moisture, which may lead to accelerated deterioration of the
timber.
In the case of a structure to be painted, the sealing of
timber-to-timber interfaces during construction is particularly
important, as trapped moisture which is absorbed into end or face grain
cannot dry out through the paint film. Timber-to-timber interfaces
include:
• A bearer or brace bolted to the face of a post;
• A joist end sitting on or butted to a bearer;
• A handrail butting onto a post;
• The face and ends of balustrading fastened to a handrail.
For pressure treated structural timber which is not to be later painted,
a heavy bodied timber preservative such as CN Emulsion is
an appropriate sealer, absorbing into and sealing the end or face grain.
However, CN Emulsion will be noticeable as a darker coloured oily area
around the joint. This darker coloured area may not be suitable for
appearance and will stop paint adhering.
Alternatively, you could use a liberal application of an oil-based
primer/undercoat as a sealer, coloured if required for appearance. An
oil-based primer/undercoat is also suitable for sealing structural
timber which is to be painted.
For detailed timberwork, such as handrail timbers or balustrading, any
cut-ends, and the abutting surfaces should be sealed during
construction. It is also good practice to leave a gap between the end of
the balustrading and the bottom rail so that moisture which pools there
can dry out.
The sealing of handrail components before installation is often
neglected, leading to rotting of handrail and balustrade ends and the
timber around fixings which can be unsightly and in the worst case can
lead to loss of structural and fixing strength, increasing the risk of
failure.
Refer to “Decking” for information on sealing decking.
Construction
The following outlines the "joist-on-bearer" method of construction
where the joists sit on top of the bearers. The "joist in line with
bearer" method, using metal joist hangers to support the joist so that
the tops of the bearers and joists are at the same level, may also
provide suitable long-term performance in weather exposed conditions
where the deck is completely protected from the weather and not in a
corrosive or coastal environment or where hot-dipped galvanized or
stainless steel stirrups and fixings are used, in association with all
other appropriate timber protection and construction techniques.
Please note: The following instruction guidelines are generic
and may not be followed in detail by your local Deck Master, as he
or she will have developed a technique trough formal training and
experience which has been found to be effective and efficient.
If you wish your Deck Master to use specific construction techniques
please discuss this with your Deck Master and have these practices
incorporated into the price which your Deck Master gives you for
construction of your project.
Setting Out
Using a tape measure, string line and level, locate the corners of the
deck, setting up your forms and string lines to accurately mark the center-line of your posts as shown in
Figure I.
Square the corners using the 3-4-5 ratio* or equal diagonals method and
level the string lines. Measurements along these lines will give the
positions of intermediate posts and vertical measurements at these
points can be used to determine the height of the bearers above the
ground. Be sure to confirm that the drawings, profiles and siteworks are
all referring to the same reference points. Also, be sure that your post
positioning takes into account whether you will or will not be checking
your bearers into the post.
*A right-angled triangle is formed if one side of the triangle is 3
units long (e.g. 1.5m), another is 4 units (e.g. 2m) and the hypotenuse
is 5 units (e.g. 2.5m).
Footings
Timber posts should ideally be installed on commercial quality
hot-dipped galvanised brackets cast into engineered concrete footings
or, if necessary, extended into the footings.
Even hot-dipped galvanised brackets can corrode in contact with
concrete, although such corrosion can take a long time to affect the
strength of the bracket. It is advisable to apply an additional
protective coating such as an epoxy paint eg Ameron Amercoat 385, to the
galvanising which will be in contact with the concrete.
Position the bracket vertically so that the timber post will stand clear
of the horizontal bar on the stirrup by 10mm, clear of the top of the
concrete by 75mm and 100mm above ground level (bar clear of concrete by
65mm). See Figure 2.
Concrete around the post brackets so that the top of the concrete is
above ground level sufficiently that dirt will not collect on top of the
concrete (indicatively 25mm).
Trowel the top of the concrete neatly, graded with a “fall” outwards to
allow water to run-off.
Check, re-position and plumb the post brackets before the concrete sets.
See Figure 2.
After the post brackets have been correctly positioned and cast-in,
reposition and level the string lines so that they indicate the outside
edge of the posts and are out of the way of construction.
Timber posts extending into the footings shall be sealed below ground
with Koppers Arch CN Emulsion and set into "no-fines" concrete with a
layer of "no-fines" concrete at the base, to allow water to drain, in
accordance with the engineering design but a minimum of 100mm. Standard
concrete can hold moisture around the post and cause accelerated
deterioration. The top of the no-fines concrete shall be sealed with
mortar and sloped away from the post to shed water. See Figure 2.
Allow sufficient time for curing before placing posts. The top of the
no-fines concrete should be sealed with mortar and sloped away from the
post to shed water. Termite treatment to embedded posts is to be in
accordance with the Building Code of Australia and may include steel
mesh socks &/or chemical treatment to the surrounding ground.
Posts and Ledgers/Pole Plates
After the concrete is 4 days old, bolt the posts to the brackets using
2/12 mm hot-dip galvanised hex-headed bolts, allowing a clearance of
10mm from the horizontal bar in the stirrup to the bottom of the post to
allow any moisture which collects there to dry out. Seal the ends of the
posts before installing them onto the brackets.
Where the house or adjoining structure is structurally adequate to
support one side of the deck, a 100 x 50 mm ledger or pole plate can be
used to support the joists fixed to the house framing with 12mm
hot-dipped galvanised bolts or coach screws at maximum 600 mm centres.
See Figure 3. Note that single skin brickwork, including brick-veneer
may not be structurally adequate to support a deck. Check the
structural aspects with your designer and incorporate additional piers
or freestanding posts into the design if required.
Bearers
Use a level (automatic or water level) to locate the position of the top
of the bearer on the posts. Confirm the positioning of your bearers,
that they are level and in their correct alignment, as this is the last
chance you will have to easily adjust their position and that of the
structure above.
Posts which are not continued as handrail or roof supports (as in figure
5) shall be cut off above bearer level to ensure minimum bolt end
clearances. See Figure 4.
Where the bearer is to be bolted onto the face of the post - mark the
position of the bottom of the bearer and securely clamp a timber block
with a heavy securing nail onto each post at that point to support the
bearer. Ensure that the block and securing nail will support the bearer
before proceeding.
Where the bearer is to be checked into the post - mark the position of
the bottom of the bearer and check out the post the required amount, but
no greater than 1/2 of the depth of the post and leaving a minimum of
35mm of post remaining. Clamp the bearer securely before proceeding.
Apply a timber sealer to the bearer and post where they will be in
contact, as moisture trapped at the interface may lead to accelerated
deterioration. Sealing checked joints is particularly important, as
moisture will penetrate into end grain to a greater degree than face
grain, increasing the risk of rotting.
Place the bearer onto the supporting block or into the checked joint,
hold safely and clamp the bearer to the post. Mark the position of the
bolt-holes and drill through the bearer and continue through the post.
A splay joint may be used where bearers are joined at a post. See Figure
4.
Alternative methods for creating joints in bearers over posts are
described and illustrated in Australian Standard 1684.2 - 4.2.
Bolt the bearer at each post using 2/12mm hot-dipped galvanised cup-head
bolts and washers.
Trim off the bearer ends, flush with the end support posts or cantilever
the bearer up to the maximum allowed in Table 2.
Seal the top edge of the bearer, before placing the joists, to reduce
water penetration and accelerated deterioration. A heavy coat of CN
Emulsion, or one coat of oil based primer plus one coat of paint finish,
applied to the top of the bearer will act as a suitable sealer.
Joists
Seal the underside of the joist, extending out 50mm, where it will be in
contact with the bearer.
Mark the position of the joists on the bearers using a string line and
tape measure. Set out the joists over these marks making sure you place
any curved timber so the bow is up and the ends point down.
Check that the finished deck is level and will be lower than the
internal house floors once the decking has been installed. See Figure 5.
The deck joists may naturally be suitably smaller than the house joists
if the deck span is smaller than the house span. Alternatively, if the
deck and house joists are the same size, the deck joists can be reduced
in depth (notched or gauged down) by a maximum of 1/4 of the depth of
the joist (so that the remaining section is a minimum of 3/4 of its
original depth). See Figure 5.
Fix the joists with 2/75 mm galvanised skew-nails or proprietary
hot-dipped galvanised or stainless steel or epoxy-coated framing
anchors. Trim off the joist ends, flush with the bearer and cover with a
fascia or cantilever the joists up to the maximum allowed in AS1684.2.
See Figure 6.
Bracing
Decks on columns shall be braced to prevent lateral movement in
accordance with AS 1684.2 - 1999.
Decks attached to the house, shall have double diagonal bracing, either:
• Fixed to the underside of the joists with each brace fixed to each
joist with 75 mm No. 14 Type 17 galvanised batten screws, or
• For decks greater than 1800 mm high, fixed between a pair of posts,
parallel to the wall, with the ends of braces bolted to posts with 12mm
diameter galvanised bolts.
Freestanding decks, greater than 1800mm high, shall be braced to prevent
movement in both directions with double diagonal braces between at least
two pairs of posts at right angles (timber and bolts as previously).
Freestanding decks, less than 1800mm high, shall have posts embedded in
the ground to provide lateral bracing.
Bracing is most often provided with either timber or threaded rod
bracing. Timber braces provide a more rigid bracing effect as they
brace in both tension and compression and will not "sing" in windy
conditions.
Decking
Use only seasoned, highly
durable and stable timber species for your timber decking. Decking containing
treatable sapwood shall
be preservative treated to the appropriate hazard level.
Note that not all sapwood is treatable. Timber species with untreatable sapwood, but which
satisfy the requirements for durability and stability shall be supplied
sapwood free.
Decking containing untreated sapwood shall not be used as sapwood is assumed to
be of lowest durability
and will suffer from premature decay. Untreated sapwood may
also be highly susceptible to insect attack.
Refer to "Decking Timber" on the
Materials page for more detailed information.
Pre-treatment of Decking
Pre-treatment of decking before laying may assist in achieving extended
life and performance from your decking. Pre-treatment will reduce
moisture uptake into the bottom of the board, in-service, assisting in
maintaining a stable shape.
Please note that the following advice on application of decking
finishes is for general advice only as each
product type and potentially brand will have its own detailed
surface preparation and application instructions developed by the
manufacturer specifically for the characteristics of the product, so our
suggestion is to
- First decide which finish you wish to use based on the appearance and
longevity of finish and the amount of time you are prepared to put into
initial preparation of the surface and application of the finish and
then maintenance of the finish over time.
- Then download recommendations or call the manufacturer's help
line or visit a retailer for recommendations on pre-treatment surface
preparation and application including sealing of the end grain of
boards.
The following general advice will be useful when read in conjunction
with the manufacturers recommendations. If the general advice
following is inconsistent with the advice provided by the manufacturer,
the manufacturers advice takes precedence.
If intending to use a penetrating oil or stain as a decking finish,
apply an initial coat of the finish to all surfaces , including cut-ends,
before laying. If using a stain finish, be careful not to overload the surface,
and to brush out any excess stain. Refer to “Finishing of Decking
for more detail.
Note: Apply the sealer liberally to the absorbent end grain. An exterior
polyurethane will provide a more effective and longer lasting end grain
seal - applied carefully so as not to coat the faces of the board.
If intending to use a film building decking finish, an initial period of
weathering may be required. Contact your supplier or finish
manufacturer to find out if any pre-treatment is recommended and to
source a suitable end grain sealer as the sealing of cut ends of decking
is always recommended.
Our preference is to use a penetrating or specialist film building
decking finish, rather than a paint finish,
for sealing decking. If a paint finish is required, refer to your paint
supplier for recommendations on sealing and finishing decking.
Whatever sealing and finishing system is to be used, read carefully and
adhere to, the manufacturer’s recommendations for surface preparation
and application of
preparatory and finish coats to achieve the best result.
Installation of Decking
Before fixing the decking, seal the top edge of each joist with either
CN emulsion, a primer plus finish coat or, as we strongly recommend, a
waterproof membrane (such as "Malthoid", a bituminous dampcourse or flexible proprietary joist
sealing strips), for a more effective seal which will also reduce
water entry around the fixing and shed water from the joist. See Figure
7.
Note that CN Emulsion, applied to the top of joist, may be absorbed
upwards, causing an oily mark on the top surface around fixings and at
butted joins in the decking. Regular application of a decking oil or
stain will make any oily mark less apparent, however if this is likely
to be an issue in terms of appearance, use an alternative sealing
method.
As advised in “Pre-treatment of
Decking” (above), apply a suitable sealing coat to all surfaces of the decking,
including cut-ends, prior to laying.
Consider your required edge detail eg covering a fascia capping
board and/or with a small nosing and determine the position of your
outer decking board, but do not fix it initially. Run a string line one
board-width in from the outer edge of the joists. Drive a positioning
nail into each joist along the string line and place the first board
hard against these nails before fixing. After removing the positioning
nails, touch up the holes created in the joist with the chosen sealing
system.
Lay the remaining boards, using spacer blocks to create a 3 to 5 mm gap
between adjacent boards to allow for swelling and shrinking in response
to changes in atmospheric moisture content, water runoff and cleaning.
Note: Do not use nails as spacers as the nail holes can be a source of
moisture entry and degrade.
Small differences between the moisture content of the decking as
supplied and the local EMC can be allowed for by applying a gap at the
lower end of the range if the decking is likely to shrink, or at the
upper end of the range if your deck is likely to swell.
In determining whether the moisture differential will cause movement in
the decking, confirm the moisture content by accurate measurement of the
decking boards supplied.
If significant shrinkage or swelling is expected, decking with
appropriate moisture content shall be sourced or standard moisture
content decking acclimatised on site prior to laying. Boards shall not be
butted together in anticipation of significant shrinkage, or laid with a
large gap in anticipation of significant swelling.
Butt joints shall be staggered so that they do not occur on adjacent
boards.
Butt ends should be cut with a slight backward undercut to assist in
achieving a tight fit (see
Figure 7). The slight gap created by the undercut will also
reduce the potential for premature deterioration of the board ends as
less moisture will be trapped and absorbed into the end grain, and
assist in ventilation of the board ends, allowing any moisture which has
been absorbed to dry out more readily.
Regularly check that the boards are running true by measuring to the
opposite or inner edge of the deck and by running a string line. For a
deck with square ends, one end treatment is to lay boards over-length,
mark with a string line and trim them all off in one neat cut. Another
method is to fix a trimming board at right angles to the direction of
the run of boards and cut each end board to length to suit.
Fixing of decking
Selection of the correct fixing and fixing method is critical to the
satisfactory long-term performance and appearance of timber decking.
In weather exposed domestic applications subject to light foot traffic,
decking shall be hand nailed or screwed with two fixings per board at
each joist.
Recommended fixing method is to hand nail ,or screw, decking to joists.
Nails shall be domed head (DH), twist shank (TS) type in stainless
steel (SS) (preferred) or hot-dipped galvanised (HDG) (depending on
severity of exposure conditions).
For hardwood joists, use 50x2.8mm DHTS nails (as above)
For softwood joists, use 65x3.15mm DHTS nails (as above)
Fixing with gun-nails may also provide satisfactory results - refer
notes below.
Screws shall be stainless steel and of suitable head type, size and
style to ensure secure fixing but with limited shearing failures while
driving eg
50 x 10G SS Decking Screws (refer Product and Services Directory for
details)
Notes on fixing decking
• Twist shank nails provide a stronger hold-down than
traditional straight shank nails.
• Stainless Steel nails provide a longer lasting fixing than galvanised bullet head nails
in fully weather exposed or corrosive conditions.
• Hand nailing with the nail type and size listed above provides strong
initial hold-down and has sufficient history of use to be confident of
long term performance. 50x2.5 stainless steel domed head gun nails are
also readily
available and commonly used for fixing decking however each brand will
have a different configuration to the twisted shank which will affect
performance. Your materials supplier or machinery hire company may
be able to provide additional information on using gun nails for decking
generally and a
recommendation as to performance and suitability for your application.
• T-Nails (eg 50x2.2 Finishing Nails or 50x2.5 Flooring Nails) shall not
be used to fix decking as the zinc plating may deteriorate rapidly and
the shank diameter is insufficient to provide suitable hold-down,
resulting in boards moving or ‘rocking’ and nails working up.
• Plain steel or zinc plated nails shall not be used to fix decking or
in any external application, as they are likely to rust, causing
staining and gradual deterioration of the nail and the timber around it.
• Decking should be drilled with a small pilot hole to avoid splitting.
• If a pilot hole is required in the joists, the hole should be of
suitable size and depth, to provide maximum hold-down force while
allowing either, the nail to be driven without bending and without damaging the
nail head, or the screws to be driven with limited shearing failures.
The need for, and size of, a pilot hole will vary between timbers,
depending on hardness and may not need to be drilled full depth.
Determine the need for, and appropriate size and depth of, a pilot hole
for your timbers by testing with off-cuts of decking and joists.
• To reduce the possibility of splitting the joist, the nail or screw
fixings shall be placed in a formal and controlled staggered alignment
(10 to 12mm stagger, with a minimum of 12mm edge clearance).
Decking fixings placed in a straight line are likely to split the joist.
Split joists are likely to result in:
• a reduction or total loss of hold-down forces, allowing movement and
distortion in the decking;
• nails “walking-out” of the deck as the decking moves under foot
traffic;
• accelerated deterioration of the joist as water enters the split and
is not able to dry out readily.
• Where decking boards terminate or are joined at a joist, domed head
nails should be 12mm from the end of each board, with the board
predrilled to 80% of the domed head nail diameter to avoid splitting
(test on off cuts first and adjust as necessary) . Nails may have to be
skewed slightly inwards to achieve the necessary clearance to the end of
the decking board and edge of the joist.
See Figure 7.
• 50 x 2.8 Galvanised Bullet Head Nails may be suitable for fixing
hardwood decking to hardwood joists in weather protected domestic
applications, where decking is not subject to the stresses associated
with exposure to sun and rain and high volumes of foot traffic and where
the lesser hold-down forces from a straight shank and bullet head nail
may be acceptable for long term performance - refer to your supplier or
nail manufacturer for advice as to suitability.
If galvanised bullet head nails are to be used for fixing hardwood decking to
hardwood joists, as above, the nails shall be in a formal
staggered alignment and driven inward at an angle towards the centre of
the joist (ie as skew nails). See Figure 8.
• Nails, whether domed or bullet head, should be driven only so that the
head pulls the board down tight onto the joist. Nails shall not be
punched into or below the decking surface.
Punching” of bullet head nails will leave a depression in which water
can pool and soak into the timber, accelerating deterioration. As the
exposed surface in the depression is largely end grain, water will soak
into it rapidly but will not dry out readily which will lead to rotting
of the timber around the entry hole and nail shank, and reduction of
hold down force.
“Punching” of domed head nails will also severely damage the timber
around the head, in addition to the potential problems mentioned above.
If the nail head becomes slightly “proud” of the surface, due to the
board or joist shrinking, the nail can be lightly driven, after, say,
6-9 months (under normal conditions),
so that it is again flush with the surface.
For commercial applications use
commercial standard decking and fixings.
