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. Decking joists are the basic and the right spacing is important
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 that it is 'square'.
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 between each decking joist. Much will depend upon the support the frame is to get in the construction.
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 decking joist bearers should normally be fixed two per post, and be of dimensions 8x2 (200mm x 50mm).
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.
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.
There are many images showing the various deck frame structures here.
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 threaded end.
Images of various sub decks and decking frames.