multi-level timber decking

construction note no.6011

As we have seen in the design note 3074: terracing with decks, timber decks are an ideal way of creating a flat surface on a slope without have to do major cut and fill terracing. This Note deals with building raised or split-level decks, Note no. 6010 deals with simple low-level decks, which require less structural support.

The essential difference is that here we are dealing with decks that may be raised partly or entirely off the ground, potentially to some height and often on different levels.

construction method for a simple deck:

1. All timbers should be cut and pre-stained or painted prior to construction (a separate note will be written on this subject) - especially joints.
2. Frame spacing should be between 450 - 500mm; any wider will cause too much bounce in the planking, especially the thinner (25mm) ones. Keep all joists an even width apart across the whole deck.
3. The top joists run at right angles to the direction of the planking! Think about this when you set out your design.
4. Use of sufficient support posts throughout the framework is vital. Whilst with timber it is quite possible to span large areas without interim supports, this will lead to bounce and flex in the structure.
5. Wherever possible on raised decks, use a double depth framework. This is stronger, and actually quicker as it entails far less cutting and fitting. Place posts to corners and at 1.2 - 1.8 m intervals for support.
6. Posts to external edges will often extend upwards to form handrail supports, or even pergola structures. With these lengths, the type of timber chosen is crucial and a laminated timber post may have to be considered in some instances.
7. Posts can be set into the ground but on a highly structured deck, where longevity needs to be maximised, I suggest inserting posts into a metal shoe which is secured to a concrete pad. This makes for secure and accurate positioning. Spiked fence-post shoes which are knocked into the ground are not accurate or stable enough!
8. Build the external frame onto these posts, securing with coachbolts, or preferably stainless steel bolts - two per post.
9. Place internal joists, but do NOT rely on galvanised joist hangers to to give structural strength when securing the ends to the frame. A double frame system as described largely eliminates the need for such things.
10. As construction proceeds, always check with a long spirit-level that the tops of joists are all of an even height - otherwise, plank-laying will be impaired.
11. Hatches for access to services or drains etc. are less likely to be required on a raised deck if there is access from the sides. If they are required, they can be added after the manner described in 6010: simple decks.
12. It is always a good idea to give the entire frame another coat of stain or paint before placing the planking. Note that tops of joists especially, may be visible.
13. If the underside of the deck is visible form the lower areas, you might want to consider cladding the sides with horizontal or vertical planking. Leave air gaps of 30 - 50mm between cladding planks as good circulation or air will be essential for keeping the timbers dry.
14. Handrails and steps will be the subject of another design note.
15. Place a geomembrane under the deck to discourage weeds. Pin this with wire staples and spread 10mm gravel over to a depth of 30 - 50mm.
16. Run any services required, such as water or electricity under the deck. Don't forget to lag water pipes if likely to freeze. This under-deck area makes a good storage space!
17. Stain all deck planks prior to fixing.
18. Start planking from the most visually important end. The full width might require one plank cut to a narrower width, which should be placed near (but not right on) the least visually obvious end.
19. keep an even gap between planks, of about 5mm. 150mm nails make good spacers. Bear in mind that the planks will shrink slightly as they dry out, unless they are from a recycled source.
20. To the cut ends, leave a small overhang of about 20mm over the frame. Sand and chamfer slightly.
21. Secure with bright zinc or preferably stainless steel screws, or a proprietary decking screw. Do not use nails, which are hard to remove later and may split the timbers
22. A final coat of stain or paint should be applied.