How to Build a decking frame -
design and construction.
The success of your deck project - whether flat or raised
levels - will depend almost entirely on the way you design and
build your deck sub frame or decking base. If you rush this part
- or are perhaps not up to the job, then your resultant deck
will suffer in the short and long term and at worse, render your
deck as unsafe for use.
The basic construction of the deck base, should include
accurate measurements and a level or fall to suit the finished
deck that you have in mind. However sturdy your decking timbers
are, they will be wasted if laid on a poorly constructed sub
decking base frame.
get the design and building of the deck base right, and the
rest of the deck will be much simpler to complete. A poorly
built sub deck frame will make the rest of the work either more
difficult, or maybe impossible! That is how important the task
of designing and building your deck frame base is.
If you have followed the deck
planning procedure set out on previous pages, then you will be ready
to start building your decking base. Building a good decking frame
should not be beyond the capabilities of the average DIY person. The
main thing is, not to proceed to the next stage, or permanently fix
anything until your are absolutely sure of the dimensions of your frame
and also its squarness.
to Use for Decking Base Sub-deck.
150mm x 50mm (6x2in) pressure treated softwood joists are the
standard timber used by professionals for a good sturdy decking sub
base. The timber joists are normally pre-treated with Tanalith (Tanalised)
at source, and there is nothing to gain - and much to lose - by thinking
that you can simply brush a standard preservative onto the joists. It
will not penetrate, and will cause premature decay of the sub base and
therefore render your deck useless. This standard of timber joist is
suitable for the deck frame of most raised decks - but with additional
support from bearers.
Tanalised timbers normally have a lifespan of in excess of fifteen
years - maybe more than twenty - depending upon situation
All cut ends of joists should be brush treated with a proper 'end
seal'. This is different to ordinary preservative. This will ensure that
the deck joists do not pre-age at the cut ends.
Thinner joist can be used if absolutely necessary for a ground level
deck - especially if there is not enough depth near to the house to use
the standard 6x2in timbers. The smaller joists will need supporting at
shorter intervals than the 6x2 joists.
The spacing between standard joists will depend upon the thickness of
the actual deck boards. 28mm thick deckboards can be laid on
joists with 600mm spacing - though for higher level decks, this should
be reduced to 500mm centres. Composite deck boards will require the
joist spacings to be closer than this. Smaller joists - say 4x2 will
need to be positioned at 450mm or 500mm. Much will depend upon the
support the frame is to get in the construction.
Raised Deck Frame - sub-deck - specifications.
The frame for a raised deck is more or less the same as a ground
level deck - but with added support by way of bearers, and also cross
bracing of the frame and maybe fixing to a house wall. The most
important aspect of a frame or sub-deck for a raised deck, is its
rigidity and ground support system. All fixings for a raised deck must
be of 'professional' standards, and this will mean 12mm bolts and
fixings as norm. Forget about using 10mm - or smaller - bolt fixings.
There are specialist fixings available, and these should be obtained
from a proper decking materials supplier. Do not compromise the safety
of your deck and the users who are going to enjoy it for many years.
Depending upon the height and size, the deck frame for a raised deck
will be supported at intervals with joist bearers fixed to sturdy
support posts that are concreted into the ground. The joist bearers
should normally be fixed two per post, and be of dimensions 8x2 (200mm x
When fixing a decking sub frame to a house wall, the base plate
should be affixed to the wall at 600mm centres (between joists) using a
proprietary wall fixing - sleeve anchor fixings or rawlbolts having
minimum dimensions of 12mm thickness and sunk into the wall by no less
than 100mm (4in). the decking base plate - wall plate - should be
affixed using 50mm galvanised rectangular steel plate washers. If
bridging a damp proof course, then ensure that minimum space between
wall plate and wall of no less than 10mm. Use spacer washers - 4no plate
washers will normally suffice.
Building the frame.
If the frame is being fixed to a wall, then firstly fix galvanised
joist bearers to the base plate at the given centres, fix the prepared
wall plate to the wall, and then it is an easy matter to sling your
joists out from the wall. This method is only suitable where the decking
is to run parallel with the house. The joists can be supported on a
temporary basis, by a joist bearer clamped to posts. The end plate can
be fixed to the joists by means of 2 no 100mm galvanised nails or by the
use of joist hangers if a raided deck.
Whatever the ultimate shape of your deck surface, it will be best to
construct a rectangular base - oversized if necessary, then cut away the
portions not required. Plan this
out on paper to ensure that it will all work out!
In most cases, it is possible - even desirable - to construct the sub
base, then fix it to temporary supports, before concreting proper
support posts into the ground. This will ensure that you have your deck
base square, level and with no bumps or hollows in the joists. This can
be checked by use of a taut string line. If using a bearer or two under
the deck frame, then this should ensure a perfectly level sub frame upon
which you can lay your decking boards.
Normally, the joists on a slightly raised deck can be supported at no
greater than 1.8m between bearers. this is applicable to standard 6x2in
(150x50mm) joists. Smaller joists - 4in x 2in - will need intermediate
support beams - bearers- at 1m ideally or 1.5m maximum. decks which are
raise well off the ground - and not really for this article - should be
supported at 1.5m ideally.
Support posts will be a maximum of 1.8 meters apart in any given
direction, and better at 1.5 meters.
Support posts for most decks should be minimum of 100 x 100mm (4x4in)
and using tanalised or other suitably treated posts. Do not use normal
fencing posts unless you can be assured of a life expectancy of minimum
of fifteen years. The frame should be bolted to the posts using 12mm
galvanised coach bolts, with a 50mm galvanised plate washer on the