All about decking

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Decks
Decks
are a common addition to Auckland houses and come in a variety of shapes and sizes
and construction methods and materials. They can range from a small entry deck to a most
elaborate feature deck, and with a little bit of thought and artistic flair, can be a great asset to
any house. This section concentrates on timber decks.
DO I NEED A PERMIT FOR MY DECK?
All decks 1 metre or more above ground level require a permit (building consent). A deck
less than 1 metre above ground level does not usually require a building consent (permit)
but may require resourse consent, and a deck extending less than two metres from the
face of a building does not require subfloor bracing.
Although a deck may not require building consent, it must still comply with the building
code. In other words, it must be built properly. If in doubt contact your
DECKING MATERIALS EXPLAINED
The three most common used decking materials are:
KWILA
90x19
Hardwood - Finished size is usually 90mm x 18mm. Kwila is a redish colour, but once
weathered changes to a silvery-grey colour. It is a very dense and durable timber. Kwila
can span 400mm, which means the joists (the timber the Kwila is nailed to) must not be
more than 450crs or 400mm between each joist.
PINE H3
90x35
Usual finished size is 90mmx35mm. This board, because if its thickness, can span up to
550mm. This means the joists (the timber the decking is nailed to) must not be more that
600crs or 550mm between each joist. Pine decking once weathered has a tendancy to
show little cracks or splits along the grain. This is a natural and accepted trait of pine. This
timber usually comes in two grades. Premium and merchant. Premium is clear with only
small tight knots, merchant is a mixture of grades and containes some big knots.
PINE H3
90x18
Same as above but can only span up to 350mm, which means the Joists (the timber the
decking is nailed to) must not have more than 400mm Crs or 350mm space between each
joist.
All of the above decking examples usually come with one face smooth, and the other face
grooved (grip tread). See Picture below. Other less common profiles are obtainable but not
as readily available.
It is personal preference as to which side of the timber faces up, however if you plan to
spend a lot of time lying or walking barefoot on the deck, grip tread may feel
uncomfortable.
DECK STRUCTURE EXPLAINED
The basic deck consists of
•
Posts either embedded in concrete or bolted to appropriate metal post brackets
embedded in concrete.
•
Bearers, which either sit on top of, or are bolted to the side of the posts.
•
Stringer, ledger plate or bearer plate, bolted or fixed to existing house.
•
Joists, fixed to the ledger plate and on top of the bearers.
•
Decking, as described in previous chapter.
•
Boundary joist. Joist that goes around the perimeter of the deck.
POSTS
Usually 100x100 or 125x125 senton piles. Posts embedded into concrete footings must be
H5 treated, where as posts bolted to appropriate metal (shoe) brackets aand off the ground
need to be H4 treated.
The spacing of posts along the bearer line depends on the structure of the deck, but they
can only span a max of 2000mm without requiring engineers calculations.
A standard footing hole is 300x300 square and 450mm deep. There should be 100mm of
concrete between the bottom of the post and the bottom of the footing hole.
BEARERS
Must be H3 treated, unless very close to the ground in which case they will need to be H4.
The size of the bearer depends on the length of the joists and the span of the bearer, but
they cannot span more than 2000mm without requiring engineers calculations.
example:
Joist span
Bearer span
Size of Bearer
1.45m 1200mm 100x75
2.40m 1800mm 2/150x50
3.40m 1800mm 2/200x50
Stringer
Also called ledger. The board bolted to the house upon which the joists are fixed to or on.
The stringer must be H3 treated and fixed to the building using M12 bolts OR
Dynobolts/anchor bolts if against concrete or block wall.
Pack the stringer out from the building wall to stop moisture becoming trapped.
The spacing between the bolts depends on the span of the joists which are fixed to or on
the stringer.
example:
If the joists span 2m, the bolts should be 1.25m apart, where as if the joists span 6m, the
bolts should be 0.5m apart.
JOISTS
Must be H3 treated unless very close to the ground, in which case they should be H4
treated. Joists are fixed to the stringer either by butting up to and fixing with joist hangers,
or by sitting on top of the stringer and nailing through both faces of the joist.
The joists sit on top of the bearer and are fixed by way of nailing through each face of the
joist into the bearer.
The size of the joists depend on the joist span and the joist spacing.
example:
Joist spacing Joist span Size of Joist
450mm 1.40m 100x50
450mm 1.80m 125x50
450mm 2.40m 150x50
450mm 3.30m 200x50
450mm 4.25m 250x50
450mm 5.00m 300x50
BOUNDARY JOIST
Must be H3 treated.
The boundary joist is fixed to the perimeter of the deck to give a neat finish.
In most cases the boundary joist is decorative rather than structural, therefore another
preference might be to replace the boundary joists with decking boards.
HANDRAIL STRUCTURE EXPLAINED
All decks 1 metre or more above ground level must have a handrail.
A quick overview, a decks handrail must be at least 1000mm in height from the deck.
Any vertical slats cannot have a gap exceeding 100mm, and there must not be anything
horizontal forming a ladder effect. However there are many alternatives, such as solid
handrails lined with harditex and plastered over, solid panelling etc.
Trellis panels can be used, but most 'off the shelf' type trellis that most trellis manufacurers
sell, do not meet the requirements. However, most trellis manufacturers are able to make
trellis to ordered requirements. Depending on the size of the trellis panel, it's relevance to
the thickness of the trellis and the size of the gaps in the hellis which are also governed.
Another alternative is glass, although a very expensive alternative. Glass suppliers will
know the type and thickness of glass required for a handrail, but get ready for a shock
when you find out the price.
diagram and detail of a standard handrail
ABOUT DECKS BUILT CLOSE TO THE GROUND
Often a deck will need to be built close to the ground. This is usually the case when
building a deck on to a house which has a concrete floor. It is usually more costly and
labour intense building a low deck than a deck, say, 1 metre off the ground.
Firstly air needs to be able to circulate around bearers and joists to ensure a long life for
the deck.
If the bearers are near or touching the ground, upgrade them to H4, along with the joists.
(In a normal deck construction, joists and bearers are H3 treated).
Ensure there is a gap between the decking.
Sometimes it will be neccessary to excavate the ground. In doing this, taper the excavation
away from the house to let any water under the deck escape. If this is not possible
because of the contour of the ground and a pool effect is created, then the excavation must
be drained. In this case taper the excavation to a low point and run a drain to a lower point
outside the deck.
The drain can be made by digging a trench, putting scoria in the bottom of the trench,
laying drain coil on top of the scoria and then covering the drain coil with more scoria.
The home handyman should easily be able to do this with a small deck and minimal water
discharge, but a larger area may require a professional drainlay as it may require tapping
the possible water discharge into a stormwater drain.
PRICE VARIATIONS BETWEEN DIFFERENT TIMBERS
At the time of writing (October 2001) decking timber prices including GST are
approxamately: Kwila $4.50 per Metre; 100x40 radiata premium $3.90 per metre;
100x40 radiata merchant grade $2.90 per metre; 100x25 radiata premium $2.55 per
metre;
Most standard decking when dressed is 90mm wide. Allow 12 lineal (running) metres of
decking for every sguare metre of deck area.
The selection of decking boards will also have a bearing on the joist costs. For example,
100x40 radiata decking requires that the joists must be spaced at max 600mm, where as
thinner decking such as 100x25 radiata and kwila require the joists to be closer together
therefore needing more joists hence a higher cost.
The cheapest overall deck structure would be using 100x40 radiata merchant grade
decking, but be prepared to accept a lot of knots.
Deck Anatomy
1 stringers
Also called a ledger plate. The stringer is the piece of timber bolted to the house
upon which the joists are fixed on or to. The stringer must be H3 treated and
fixed to the building using M12 bolts OR Dynobolts or similar anchor bolts if the
stringer is to be fixed to concrete or block wall. The stringer needs to be packed
out from the building at every bolt fixing to prevent moisture being trapped
between the stringer and the building. The spacing between the bolts depends
on the span of the joists which are fixed on or to the stringer.
example: If a stringer is 100x50 in size and if the joists fixed to that stringer span
2m, the bolts that attach the stringer to the building should be 1.25m apart,
where as if the joists span 6m, the bolts should be 0.5m apart.
2 footings
A standard footing hole size for 100x100 or 125x125 post is 300mm x 300mm
square and 300mm deep in firm ground. In soft ground the holes will need to be
bigger and deeper. 100mm of concrete is required as a pad beneath the post.
3 posts
POSTS Usually 100x100 or 125x125 senton piles. Posts embedded into
concrete footings must be H5 treated, where as posts bolted to appropriate
metal (shoe) brackets need to be H4 treated. The spacing of posts along the
bearer line depends on the structure of the deck, but they can only span a max
of 2000mm without requiring engineers calculations.
4 bearers
Must be H3 treated, unless very close to the ground in which case will need to
be H4.
The size of the bearer depends on the length of the joists and the span of the
bearer, but they cannot span more than 2000mm without requiring engineers
calculations.
example:
Joist span
Bearer span
Size of Bearer
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