Why should
fixings be placed in a staggered alignment?
Fixings placed in
a nice straight line certainly look neat, but have a greater tendency to
split hardwood joists - we are not sure if this also applies to
softwood joists.
Fixings act like little wedges, exerting
prising/splitting
stresses on the joist. Where distance along the grain between the
fixings is small, such as where fixings have been placed in a straight line, the
splitting stresses can join up and the joist may develop a split which runs
from fixing to fixing and which can be seen if you look between the deck
boards. (refer image).
Staggering the
alignment of the fixings increases the distance between them, in the
direction of the grain, so that the
splitting stresses are unlikely to join up.
Why are split joists a problem?
Split joists are likely to result in performance issues and dissatisfaction
with asp
ects
of the project:
• in the near term, from
- a reduction or total loss of hold-down
forces from the fixings, allowing movement and distortion in the decking;
- nails “walking-out” of the deck as the
decking moves under foot traffic (refer image) causing possible safety
issues; and
• in the long term, from
- premature deterioration of the joist
through rotting as water enters the split and is not able to dry out
readily (refer image of split joist with 10 years exposure in South east
Queensland ).
The possibility that joists may split
from the stresses caused during fixing of the deck can be significantly
reduced by placing fixings in a formal staggered alignment/pattern, offset
by approximately 10mm (approximately 5mm each side of the centerline of a
wide joist). In addition, joists should be pre-drilled when using large
diameter or deformed shank fixings to reduce splitting stresses or to allow
hand nailing.
Why should I use wider joists?
Wider joists, i.e. 50mm unseasoned hardwood, 42mm seasoned hardwood or 45mm
seasoned softwood, should be used to allow the placing of fixings in a
formal staggered alignment/pattern, to reduce the likelihood of split
joists, while also maintaining the required edge clearance to avoid breaking
out the side of the joist. Wider joists are also better for joining of
decking boards over the joist. On a wider joist, 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.
Why should materials and construction methods be of good quality but also
"suitable" for external conditions?
The external environment has a severe effect on building materials and the
materials used in a fully exposed timber deck must not only be of good
quality but suitable for the application eg only hot dipped
galvanised bolts are both “quality” and “suitable” for external applications
(stainless steel is recommended for extreme conditions) - zinc plated bolts may be a “quality” product but they are not
“suitable” for external use as the zinc coating will corrode from the
surface quickly in contact with treated timber commonly used externally.
The unprotected surface will then rust quickly leading to premature failure.
Similarly, there are many different types of timbers available for decking
with many different preservative treatments.
Decking timber should be manufactured from timber of suitable quality,
durability and stability and treated to the appropriate hazard level for the
application i.e. so that 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, with lower moisture content recommended for drier inland areas
• 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 classification) treated to H3 and for close to or
in ground
applications use durability 1 (in ground classification) treated to H4 or H5
as required for the conditions.
Refer to the "Materials" page for more detailed information regarding
suitable decking for weather exposed conditions.
Other "suitable" materials and practices include:
• Using only hot-dipped galvanised or stainless steel, (depending on
severity of exposure) bolts, brackets and nails (zinc plated bolts and nails
are not suitable for external use and brackets manufactured from lightly
galvanised plate are not suitable for fully weather exposed
conditions. Stainless steel fixings shall be used in corrosive conditions
such as in coastal areas and around pools).
• Using twist shank nails for greater initial and longer term hold-down.
• Sealing any cut ends and housed joints with a timber sealer, consistent
with other finishes used.
Should I lay
reeded decking "reeds up" or "reeds down"?
How to lay reeded decking - with the reeds up or down
- is a question that splits the decking community into 2 views (logically).
One view is to have "reeds up" for slip resistance and the other view is
"reeds down" for improved airflow between the joist and underside of the
decking thereby extending the life of decking and joist.
In determining the right answer, it may be useful to
consider why reeds were originally machined into one face of a decking
board. I don't have any definitive information on when reeded decking
was first produced or why but an explanation put forward by a respected
industry figure is that reeds were first added to imported decking as an
additional manufacturing step to make the decking a more elaborately
manufactured product thereby avoiding trade tariffs, allowing the decking to
be imported and sold at a lower price. The slip resistance and airflow
arguments then become justification for reeding rather than the reason for
doing it in the first place. Over time, reeding became expected by
consumers and offered by manufacturers as part of the general decking
offering.
Don't know if the above is true but what we do know
is:
- In fully weather exposed conditions, reeded decking
will absorb more water when it rains as water sits in the grooves rather
than running off. Hence, it will stay wet longer and will be more
likely to develop mould and deteriorate faster than traditional decking of
the same type. As the top face holds moisture longer, reeded decking
may also be more prone to distortion than traditional decking.
- Dirt and leaf debris (and mould) will tend to be
held in the grooves, making a more slippery surface.
- Parallel to the reeds (along the deck), the deck
will be more slippery than traditional decking (physics - function of
friction and surface area).
- Perpendicular to the reeds (across the deck), the
deck will have some improved slip resistance but in the wet any deck will be
slippery and any improvement will be negligible. Take into
account potential mould or dirt in the grooves and reeded decking will
likely be slipperier.
- Nailing or screwing into the reeds is likely to
cause splinters to flick up which can be a safety hazard. Nailing
reeded decking requires more careful fixing into the groove and in weather
exposed conditions this can lead to accelerated deterioration at the fixing.
It would be important to trial your fixing method on some samples to make
sure you are happy with the result. Bullet head nails would be the
neatest fixing but not suitable for softwoods and may not hold hardwood in
the long term as the timber deteriorates around the nail.
Taking all the above into account we would advise
to lay reeded decking "reeds down".
Reeded decking may be OK laid "reeds up" in dry
conditions (under roof protected from rain), in species not prone to
splintering where foot traffic is perpendicular to the reeds, where
particular care is taken with fixing to avoid splintering at the fixing and
regular maintenance is performed to remove any dirt and mould build-up.
In the real world where most decks are
under-maintained, in weather exposed conditions, decking laid "reeds up" is
likely to give more trouble and deteriorate faster than traditional decking
of the same type and hence should be avoided.
As far as improved airflow is concerned, once the
reeds are forced into the fibres of the joist there is not likely to be any
great airflow improvement - manufacturers may disagree and be able to
provide more information. Sealing the top of the joist and underside
of the decking is recommended for improved life - for details of recommended
methods click on the following link to transfer to the "Construction"
page and follow the links at the index to "Pre-treatment of Decking" and
"Installation of Decking".
What are the long term performance issues you can experience from poor
fixing techniques?
Click on the following link (Fixing
Techniques & Performance) to view our technical note on performance
issues associated with poor fixing techniques.
How to
achieve good decking performance around pools?
Timber decking can provide an attractive surround to
your pool. To achieve good performance from your pool deck it is important to give careful
attention to all the elements of the decking system, particularly:
• Ensure the ground under the deck is sloped or
has a suitable drainage system installed, so that water does not pond under
the deck. Retained moisture in the underdeck area can contribute to
distortion of the decking, premature decay of the decking and subframe and
potentially termite activity.
• Use high durability, stable, high density and
low shrinkage subframe timber
• Use high durability, stable, high density and
seasoned decking timber
There is no indication that quality decking timber,
installed and maintained properly, will not provide good performance over a
long life
Suitable, quality subframe and decking timbers are
listed on the "Materials" page
Softer, more absorbent and less stable timbers are
likely to suffer from checking and distortion as a result of constant
wetting and drying
If you are looking to match the level for your decking
with another surface you may want to use seasoned joists or make an
allowance for the shrinkage which will occur (check with your timber
merchant for advice). 50mm joists will take 18 months to 2 years to
shrink to final depth.
• Use only stainless steel for all brackets, nails,
bolts, concrete anchors and screws
• As an alternative to stainless steel post brackets
you could use H5 treated pine posts or durability 1 hardwood posts (treated
to H5 if section contains sapwood) as in-ground posts. Note that pine
posts will likely have to be especially ordered as standard landscaping pine
posts are only H4 treated. Similarly the hardwood posts may have to be
ordered as hardwood posts are likely to be only treated to H3 as standard.
Ensure that the foundation of the post is appropriate so that there is no
settlement of the post into the ground under the foundation.
• Only bolt an edge support to the side of the pool if
this is approved by the pool manufacturer. If approved, stainless steel
"chemset" anchors should be used in preference to expansion anchors as
chemset anchors will not put pressure on the sides of the pool wall and will
tend to seal the drilled hole.
• Cut the decking ends with a slight backward
taper to reduce the amount of water held between the ends by capillary
action. The slight gap will also aid in air circulation at the ends
• Apply a sealer to the decking ends,
before fixing,
to reduce moisture uptake into the absorbent end grain. A suitable
long term sealer for the ends (note only the ends) would be an exterior
polyurethane applied carefully to avoid coating the sides of the decking
• Apply a sealer all round the decking prior to
laying. The sealer must be compatible with the intended decking sealer
• Use wide joists (50mm unseasoned, 45mm
seasoned) to allow a staggering of the nails (to avoid splitting the joist
• Apply a water shedding membrane to the top of
the joist to reduce the amount of water absorbed into the top of the joist
which is a cause of deterioration of the joist and loosening of the fixings
• Use appropriate size, length and type nails or
screws (this may seem obvious but there are a lot of different type of nails
and screws in the market - many of which are not suitable). Remember
to only use stainless steel fixings
• Stagger the fixings (nails or screws) to avoid
splitting the joist. Position in a formal pattern to maintain an attractive
appearance - don't just install at random
• Only drive nails flush with the top of the
decking - don't punch below the surface (to avoid water ponding in any
depression and to avoid timber damage if using dome head nails). If
the deck structure shrinks a little resulting in the nails just showing
above the decking surface, go around later and drive the fixings flush with
the surface again
• Apply additional sealing coats, in accordance
with manufacturers recommendations, after installing the decking
• Undertake regular maintenance, in accordance
with manufacturers recommendations, to maintain the water repellency of the
decking. The regular wetting from pool water may result in maintenance
having to be undertaken at shorter intervals than for normal exposed
decking.
Images showing pool decking problems -
Left - Decking timber and nails not as recommended -
decking is imported hardwood but not one of the recommended species and
nails are straight shank and nailed in line - decking is distorting and
nails cannot provide sufficient hold down.
Centre - Nails in-line have split the joist.
Nails will not provide suitable hold down and split in joist will hold
moisture accelerating decay of the joist.
Right - Posts are rusting from corrosive pool water
sitting around base of the post. The worst position for metal is at
the edges of the pool where pool splash is greatest.
