Standard Raised Deck
Plan your deck structure on paper first. Plan where your posts, purlin structure and joists are going to go.
Using string or line to clearly identify the perimeter of the deck, check diagonal measurements to ensure that your deck will be square.
Mark out the positions of your posts they should be approximately 1.5 – 1.8m. You can use marking paint or chalk for this.
Dig holes approximately 400mm deep. Insert your posts, line them up, brace them. Do not worry about how much they extend out of the ground as they will be trimmed later. Ensure they are plumb (level in the vertical plane.) Ensure that your bracing will hold them in place while the concrete sets.
You would normally order enough readymix concrete to fill all the holes to the top normally 25 Kn strength 10mm chip.
Allow 48 hours for the concrete to set, strip the timber bracing off and clean any concrete off the posts.
Work out your heights from the finished deck level and calculate backwards.
The easiest and quickest way to set levels over a large deck is to use a self levelling laser level on a tripod. (If you are hiring a laser level, make sure it has been recalibrated within the past 6 months.)
The purlin structure will run the same direction as the finished deck boards, which is normally parallel to the house / patio doors.
Bolt 6” x 2” on their edge to either side of the posts, stagger your joists so that you don’t end up with two joints on the same post. (Timber locks can be used instead of bolts as they are just as strong and much faster to use. Pre-drill the 6” x 2” timber first, insert at least two bolts or timber locks.
As an extra reinforcement insert a 4” x 2” dwarf leg between the 6” x 2” and the concrete base and attach it to the post with screws or timber locks. This will ensure that your substructure will last 20 years plus. This will transfer the load to the ground.
Set your levels using builders line to create the fall / slope in the deck (to run water off) lower the builders line off the level at one end by the required amount, normally 50mm in 4.8m will create a fall of approximately 1 in 100.
Once the purlins are positioned and bolted / timberlocked and reinforced with dwarf legs the purlin structure is now complete. The slope / fall has been worked into the deck.
This is now straight forward (ensure that the joists 4 or 5 6” x 2” are regularised – this is very important otherwise the deck could be uneven.)
The joists are laid on the purlins at 90° to the purlin substructure. The joists must be spaced at 250mm (10”) apart (bear in mind that this is the construction of a structure designed to last 20 years plus) fix the joists to the purlins below by side stitching from each side at every joist-purlin intersection using 3” galvanised nails or 80mm stainless steel screws.
Bridging pieces 250mm (10”) should be inserted between the joist to ensure a rigid, strong joist structure.