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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[gjvvu]

Hi Mitchell,

I was wondering if the ground level timber deck needs to be connected to the house or floating? I'm planning to build the deck on partially concrete pad and partially on the lawn. thanks.

Jacky

[MitchellMc]

Welcome to the Bunnings Workshop community, Jacky (@gjvvu). It's wonderful to have you join us, and many thanks for your question about deck construction.

A deck doesn't need to be connected to a house but is done to increase its stability or out of necessity. You'll commonly see raised decks connected to a house via a ledger board when there is no room for concrete foundations next to the wall.

We look forward to following along with your project. Please let us know if you have more questions.

Mitchell

[jb22]

Hello,

What are the regulations when building a low-level deck? 

Can you use timber substructure/frames, or do they have to be steel if they're on ground level?

We're building an ekodeck flush with our pool coping tile. The pool fence spigots also need to be drilled into the ekodeck/substructure. 

Wondering what the best materials would be and within the building code/regs.

[MitchellMc]

Welcome to the Bunnings Workshop community @jb22. It's brilliant to have you join us, and many thanks for your question about building a low-level deck.

The best place to start for regulations in your area would be with your local council. If your deck requires a permit and approval, then the council will be able to assist with information on what suitable building materials you can use. You'll find the regulations for timber-framed residential construction in the Australian Standard AS 1684.
 

Typically you'll need to use material fit for purpose. This is not only to comply with any regulations but also to ensure the structure's longevity. So, if your deck frame is going on the ground, that would require H4 treated Pine or a suitably treated steel structure. If the deck is above ground, that would be H3 treated Pine. 
 

Check out this helpful series on How to build a deck. Your first step after consulting with the council will be creating a plan. Once you have mapped out your deck's perimeter, decided on its height, and what materials you'll use for the substructure, then we can assist with advice on the layout.

Please let me know if you have any questions.

Mitchell

[peetee]

Hi .
i watched one of the “how to build a deck” videos. It says you should lay every 5th board out. Why is that? Can you not just lay the boards one at a time? Thanks 🙂

[EricL]

Hello @peetee 

Welcome to the Bunnings Workshop community. Thank you for joining the discussion.

Laying the fifth board down is a technique that builders and carpenter's use, they refer to it as grid boards. Grid boards are every fifth decking board that is screwed down first, and then the spaces in between are filled in afterwards. This way the decking panels can be spaced in-between each grid board evenly. 

Since decking panels are not 100 percent straight laying them out in a grid pattern compensates for the imperfections and gives you a more uniform finish. If you were to rely on the panel's straightness, you'll notice that by the time you reach the middle of the deck there will be a noticeable lean to the layout of the decking panels. 

If you are planning to build your own deck, please make sure to post an update. I'm sure our members would be keen to see what your deck looks like.

If you have any other questions we can help with, please let us know.

Eric

[NoviceDecker]

Thanks for your article Adam W. 😀

I'm building a deck at the rear of my unit. Deck size to be 7300mm long and 2850mm across to be built on soil. So i have quite a few questions.

Checking my understanding from your article and my general assumptions (are they correct?):-

• The distance between the ground and bottom of the deck needs to be about 400mm to allow for enough ventilation from damp and moisture.
• I plan to slope the drainage from left to right with 50mm landfall away from the house.
• I intend run ag pipe at the bottom of the fall into my storm-water drain.
• If I use hardwood that i need to use F14 certified materials with corrosion resistance or stainless steel fixings.
• I intend to run a ledger board along the back of the house against the boards that protect from things underneath the unit.
• I imagine its best to concrete the footings into the soil.
• I have already checked at dial before you dig (BYDA) that there are no cables, electricity or gas mains in the area.

Questions:

• What type of footing is best to use on a project like this?
• How deep do footings need to go into the soil?
• What do i need to consider with soil type? (which regular garden soil for 400mm down and then mixture of clay and sandy loom.
• For 7.3 metre joist, i imagine i still fix bearers 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. or do these measurement change or are they fixed?
• Why are noggins so far apart at 800mm?
• What type of paint is good for protection of the lower sub-structure?
• What size do my bearer need to be? I'm assuming 120mm x 70mm may not be big enough to hold the load.
• I'm not sure whether to use ecodeck (which looks too plastic, could be better), merbu or spotted gum or something else? What do you think the best option is?

I intend to watch the video series after this, however i'm just starting out with planning.

Thanks for your help in advance?

ND.

[Akanksha]

A warm welcome to the Bunnings Workshop community, @NoviceDecker. We're delighted to have you join us and trust you'll find our site a handy resource containing inspiring ideas and useful advice for your home improvement and garden projects. 

@Adam_W's popular guide is definitely a great place to start as you begin planning your deck build. I'm sure our resident Bunnings D.I.Y. expert @MitchellMc will be happy to clarify your doubts and assist you with your questions as soon as he can.

While waiting, can I please request you though to provide any photos of your unit's rear area where you are planning on building your deck? This will help Mitch and our other members get a better idea of what you're working with, enabling them to provide more tailored advice suited to your needs. Do let me know if you need a hand with uploading photos. Happy to assist.

In case you haven't seen these already, I also recommend having a look at our Top 10 most popular low-level deck projects. This collection contains some excellent projects which might serve as inspiration. 

Do keep us updated as your deck project progresses. We'd love to be part of your journey and see the final result.

Akanksha

[MitchellMc]

Hi @NoviceDecker,

I'll try to answer those questions for you.

Concrete footers would likely be the most appropriate option if you are constructing an elevated deck with 400mm of height under it.

The size of the footer is directly related to the soil conditions and type in your area. This is a grade determined through testing, not something where the size of the footer can be determined from your description. It's best to check with your council for local building requirements or engage with a professional in your area to assess the soil and make foundation recommendations.

What size timber do you intend to use for the joists, as this will determine how regularly they need to be supported? Typically, our members use 90 x 45mm Pine, and over your 2400mm width, it would need two supports in the middle so it doesn't span more than 1000mm.

Noggins tie the joist structure together. Depending on how you build the structure, they might not be needed. So, a spacing of 800mm would be fine in most situations.

I'd recommend using a bitumen-based paint for the sub-structure, similar to Gripset Betta 1L Waterproofing Membrane Bitumen Rubber.

Your bearer size directly relates to how far apart you place your posts and foundations. 120 x 70mm is certainly large enough if it is supported adequately. It's best to reference the timber supplier span tables for how often the timber needs to be supported.

Perhaps you might like to check out some Ecodek in person at a store. I think you find it doesn't look like plastic and comes with some wonderful benefits over traditional timber. However, the choice of decking material comes down to your preference and desired appearance. Many of our members choose to go with Merbau as it's readily available and reasonably budget-friendly, but if your budget extends to timbers like spotted gum, that's a wonderful choice, too.

Please let me know if you have further questions.

Mitchell