How to build a low-level deck

[Adam_W]

A deck is one of the most cost-effective ways to expand your outdoor living space.

A deck can be a real problem-solver too. It can bring sloping areas to level, lift your levels up above damp or difficult spots, or hide old structures such as concrete slabs.

A decking project is one of the first major projects many home-owners undertake. But while decking is usually “big” in scale, it’s not necessarily difficult once you take the time to understand the basics of how they are put together.

A normal deck structureThe issues with an elevated deck tend to be obvious, such as safe fencing and handrails, as well as cross-bracing of both bearers and posts. However low-level decks can pose a range of potential design and installation problems.

Let’s take a look at some of the issues with low-level decks and how you may be able to work around them to create that space you’ve been dreaming of.

What is a low-level deck?

The actual definitions will vary based on material and design, but any deck that has the lowest point (the bottom of a bearer) less than 400mm from the ground is considered a low-level deck.

Why is being close to the ground a problem?

There are three main concerns with low level decks that might lead to your deck having a reduced lifespan.

1. Ventilation – Good air movement is required beneath a deck to prevent moisture from building up and damaging the deck sub-structure and fixtures.

2. Drainage – Surface water needs to flow naturally away to prevent rot and timber movement from swelling. If bearers are too low they may restrict water movement and over time soil will build-up, further hindering water movement. If there is too much moisture and inadequate ventilation decking boards can cup and bow, too.

3. Termites and rotting – If materials are too close to the ground or you cannot easily access them, then the likelihood of attack from termites or the development of wood-rotting fungus is more likely.

When are low-level decks typically installed?

The most common scenario is where a deck is wanted straight outside an entry door of a ground-level slab home. This will generally only give you a depth of around 200mm to work with. Another situation may be one where only a part of the deck is low-level, say, for example, where a home has been built on a cut-and-fill mound and the ground level falls away.

Achieving clearance

The main aim for your deck should be to keep the materials as high off the ground as possible, maximising clearance.

Substructure – Normally a deck is a layered structure. Larger bearers are supported on posts or stirrups – the footings. On top of these bearers, running the opposite direction, are joists. Decking is then laid across these joists. This generally gives you an overall height of around 400mm above the head of the footings.

A single layered structureThis can be avoided by using a single layered structure where an external frame of larger bearers is used to support internal joists. The internal joists are supported by nail-in joist hangers fixed to the larger bearer frame. To prevent the joists twisting, noggins will be required between each joist with one run of noggins for each span section.

As an example: a 2.4m joist would be fixed to a bearer at 0mm, rest on a footing at 1200mm and be fixed to the far bearer at 2400mm. You would run noggins between the joists at around 800mm and 1600mm.

If you had a sloping site, then you can use this structure in the areas with limited clearance then transition to a regular structure as height allows.

Footings or support – The options you select in this part of the structure can make a significant difference in overall height. For example, galvanised steel stirrups can be set in concrete footings very close to the ground and the stirrups themselves only contribute around 5mm of height.

Options include:

• Stirrups bolted down or set in concrete.

• Specialised support blocks such as TuffBlock instant foundations. These are ideal if adding your deck over an existing concrete slab.

• H5 100 x 75mm (minimum) treated pine strip-bearers laid on a compacted sand or gravel bed (see below).

A footing-free structureFooting-free option - An option that does not require conventional footings or supports is to use a floating or strip-bearer to carry the deck frame. Here’s how:

• Profile the soil to allow for drainage away from structures (see drainage below).

• Lay a heavy-duty builders membrane (the sort used under concrete) over the area lapping it up the slab of the structure but stopping short of the actual house or building proper.

• Lay coarse drainage sand or road base and compact to a firm surface. This should be level when finished.

• Lay H5 treated pine strip bearers the required direction. These must be a minimum of 100 x 75mm. Fix joists to these.
  
  

Material considerations

Product selection is also crucial to ensure your low-level deck has the longest-possible lifespan.

Dimensions of materials - Always seek professional advice on the correct size of substructure materials to use. It is critical that all bearers and joists are the right size for the span otherwise you may get some bowing or the deck failing when a beam breaks.

Spans – The larger the timber bearers or joists, the longer the span allowable between footings. As an example: H3, F7 treated pine 90 x 70mm can span 1200mm centre-to-centre on footings as a continuous joist. Increase that timber size to 120 x 70mm and the span increases to 1800mm.

It’s worth noting too that hardwood can take higher loads and provide longer spans at smaller dimensions.  A 1200mm span can be used with F7 hardwood that is 75mm x 75mm.

For a low-level deck you need the smallest materials possible to reduce height so the sacrifice for this is that you will require more footings for support.

Timber – Any timber used must be H3 as a minimum. This means it’s treated against moisture, rot and insect damage. If an area is very damp or the timber is in contact with the ground, then you’ll need H4 or even H5. These are somewhat specialised timbers and will likely require ordering.

Any timber used for bearers or joists must be intended for that load-bearing purpose. For treated pine this means it needs to be a minimum of F7 and for hardwood F14 is the norm.

A steel frame for a new deckSteel – Excellent spans can be achieved with steel frames and it obviously does not have issues with rot or termites, however it may rust if very damp. You can order easy-to-assemble prefabricated steel deck frames made-to-measure.

Additional protection – To extend the life of timber or steel you can use paints to provide an extra barrier of protection. This is definitely worth considering if a low-level deck is beside a pool.

Drainage

Excess water and the resultant humidity is the major risk with any low-level deck. Doing the right groundwork can be a labour-intensive but it will be worthwhile in the long-run.

Installing over a slab - Any existing slab should be falling slightly away from any adjoining structures. Use this fall to your advantage. Along the outside edge of the slab dig a trench at least 200mm wide and deep, line it with filter or drainage fabric, add a thin layer of drainage gravel then roll out a slotted drainage pipe ensuring it has a slight fall to an area convenient for water runoff before covering this pipe with gravel.

A drainage panOn soil - Regardless of the type of footings you use fall must always be away from any structures so you may need to profile the soil accordingly. To ensure the best possible drainage remove at least 50mm of soil ensuring fall is maintained. At the edge of this dig a drainage trench as described above. Over the excavated area lay filter or drainage fabric and then cover this with drainage gravel before installing the edge drain as described above. If you cannot achieve fall to the outside then excavate to create a V-shape with fall from either side towards the centre of the area and install a drain channel through there. When using either of these options, think first about where your footings will be so you don’t interfere with their layout.

Fittings

Dues to the moisture and humidity problems with low-level decks it is absolutely essential that all screws, bolts and fittings (like stirrups and joist hangers) be as a minimum galvanised or treated with other forms of heavy corrosion resistance.
You may want to consider stainless steel fittings and they are essential for any section of deck that is within 2m of a swimming pool.

Final tips

With any decking or drainage project, research and planning will save you a lot of time and stress when it comes time to pick up the tools.

Always seek professional advice to determine the right specifications for timber, paints and fixtures.

If in doubt with drainage issues, call in the experts and have them either provide you with recommendations or have them conduct that aspect of the project.

Feel free to post your low-level questions and projects here on Workshop. We’re here to help.

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