How to build a timber deck using Tuff blocks over soil?

[shriya]

I am looking to build a deck with an open pergola in my backyard using timber, tuff blocks and concrete blocks. It will need to be a low lying deck due to natural slope of the backyard towards the house. Currently, the backyard has hedging on all the sides(we plan to remove them from 2 sides), hills hoist in the center (removed), and a concrete path that leads from the door to the hills hoist and goes around it- which has been taken out but we would like to reuse if possible. I've attached the current yard with the elements mentioned and the future yard with the the deck. Please advise what materials and tools are needed, spacing, and how to plan for the drainage.  

Current backyard

Current backyardFuture state with deck 

[JacobZ]

Hi @shriya,

Welcome to the Bunnings Workshop community, it is wonderful to have you with us.

Firstly, is it possible to get some photos of the area you are planning to install the deck? Your plans are brilliant, but oftentimes, regardless of how good they are, they do not convey everything and seeing things can help us notice things that plans can not convey.

The next question I have is, how much height do you have to work with? Oftentimes, decks are constructed so that they sit flush with the bottom of a door sill, or under a windowsill. This height restriction will always have an effect on the design and materials chosen for the deck, so it is an important detail for us to look at.

Low-level decks are definitely possible, but the limited ground clearance that is inherent with them is not ideal, as it limits airflow beneath the deck. This can cause premature timber rot and associated issues with your deck. You'll want to look at ways to maximise the airflow below the deck, which ties in with how much height you have to work with. Ideally, the deck should have as much ground clearance as possible, so things like excavating the ground and using materials that maximise ground clearance are important.

Seeing that the ground naturally slopes towards the deck, we should also look at ways to add drainage around and beneath the deck. Seeing that there is a stormwater drain nearby is good, because it means you can install agi pipe beneath the deck and have a plumber connect it to your stormwater drain. Maximising the ability for water to drain beneath your deck, combined with maximising the clearance from the ground, will be the two most important things we need to look at before selecting materials.

There is no one single way to build a deck, so we need to look at the whole project before selecting materials. H3 Framing is commonly used, and in low-level decks, H4 Framing, which is treated for use in close proximity to the ground, is ideal. Once we have some details about the ground clearance and drainage in that area, we can look at more specific materials.

In terms of tools, you're going to want a mitre saw, a drill with drill bits and driver bits, and a spirit level for constructing the deck. For ground work, at minimum, you'll need a shovel and a tamper.

You might like to have a read through some of the following articles for some further information and guidance -

• How to understand deck components
   
• How to understand deck span tables
   
• How to choose the right timber for the job
   
• How To Build A Deck
   
• How to build a deck.

When you can provide further details, we will be happy to assist with your project further.

Jacob

[shriya]

Thanks for the prompt response @JacobZ 

Regarding the height, the doorstep leading into the backyard is 300 mm high from the soil. Hence the deck will sit below it so that water doesn't go into the house.

Regarding the drainage, the aim is to put a french drain 1 metre from the house walls and a gutter that goes around the deck into the stormwater drainage. 

Current challenge is to cut the bushes that lie close to the house walls as it sits where the french drain should be. Hence the french drain will be placed after that. Furthermore, the stormwater drainage is a concrete column which means to connect the agi pipe, a hole has to be made at 1 feet depth. If not, the french drain/agi pipe will redirect the water away from the stormwater drainage to the other side and into the soil. Atleast the water is redirected away from the house walls. 

I have shared before and after pics. So far I have lifted the concrete path, removed the grassroots, and had an attempt at uprooting the bushes (reciprocating saw failed as it is a bit inaccessible and takes a long time if I do a horizontal cut. Any suggestions appreciated). Next task is levelling and drainage. 

Should I maintain the natural slope, put gravel on top and level it? And then place the tuff blocks? Or should I make the top soil also flat and then put gravel followed by the tuff block?

While the top soil slopes down towards the house , the deck will slope down away from the house? 
And lastly, what materials do we use? How do we plan for the spacing between the tuff blocks?

The images of the yard before(with the concrete pathways and hills hoist) and the current image with the turf layer removed is attached.

[MitchellMc]

Hi @shriya,

Thanks for the update and the detailed information. 

For the bushes near the house walls, if the reciprocating saw isn’t reaching, you might consider using a small pruning saw or even a manual bow saw for tighter spaces. In some cases, carefully cutting the roots in sections and then levering the bush out with a garden fork or crowbar can work better than trying to cut everything horizontally. Patience and small incremental cuts usually beat forcing one big cut in awkward spots.

Regarding levelling and the slope: since your topsoil naturally slopes towards the house, but you want the deck to slope away for drainage, it’s best to roughly flatten the topsoil first, so your base is consistent. Then lay a layer of compacted gravel to create a stable, level platform for the TuffBlocks. You can maintain a very gentle slope in the gravel layer away from the house to encourage drainage. The TuffBlocks sit on this gravel bed, so having a stable, level base is important to prevent movement over time. If the soil is very soft or uneven, don’t rely on just the topsoil, compacting and using gravel will give your deck longevity.

For materials, H3 treated timber is recommended for all joists and bearers, with deck boards suitable for your desired finish. Concrete pavers can go under the TuffBlocks for additional stability if the soil is soft.

For spacing TuffBlocks, it depends on your joist span. Typically, 90 x 45 mm joists can span around 1.5 m between supports. For your deck, place TuffBlocks roughly every 1.5 m along the length of each joist and every 900–1200 mm across for bearers, depending on your timber size. Once you set your blocks on the gravel, use a spirit level to make small height adjustments to keep everything flush and stable. 

Please let me know if you have any questions.

Mitchell