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Need framing suggestion for low level deck

Hello experts,

I am looking to do decking project (3.8m *5.7m ) to transform my back yard. The deck runs across concrete and the soil and the major portion lies on concrete. Attached is the plan of it.

Height difference between the concrete slab and the bricks under exist door is 60mm. How to achieve framing (driinage , strength etc ) in this low clearance situation ?

back entry.jpg

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Comments

EricL

Hello @nvelaga

Thank you for sharing your questions about your deck installation project. I suggest having a quick look at these guides - How to build a low-level deck by @Adam_W and How to build a deck by @MitchellMc. I'm afraid 60mm is not enough space to build a low-level deck, because your deck is touching the concrete base directly moisture will be trapped underneath and will cause damage to the timber framing and shorten the life of your deck.

I suggest removing the concrete slab so that you'll have enough space for a proper low-level deck. The ground can then be levelled and sloped to provide a fallaway for the water. Weedmat and gravel can be placed underneath to facilitate drainage under your deck.

I also suggest looking at the Bunnings deck building guide:

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

Eric

nvelaga

https://www.workshop.bunnings.com.au/discussion/comment/48086#Comment_48086

Thanks @EricL.

Don't we have any other way without removing concrete ?

If we use H4 treated pine with betta paint , will suffice ?

How much min gap is needed between concrete and joist ?

MitchellMc

Hi @nvelaga,

Joist-size timbers such as 90 x 45mm are only available in H3-treatment for above and not on-ground use. Theoretically, you could use 90 x 45mm joists placed on their face and then add your 19mm thick decking on top. This will give you a height of 64mm. However, what you are creating is an exceptionally low-level deck with no clearance, which results in a high-humidity environment and poor drainage and leads to the early onset of decay. If you are prepared to deal with the consequences, which could be a deck that lasts less than 10 years, then you could choose to implement this design.

Typically, our members would leave at least 10mm clearance under their joists so water has the ability to drain away.

Consideration should be given as to whether creating a deck over the concrete with your limited working height is a viable project. Perhaps you could tile the concrete area and then have those surrounded by the deck.

Please let me know if you have any questions.

Mitchell

nvelaga

https://www.workshop.bunnings.com.au/discussion/comment/48097#Comment_48097

Thanks @MitchellMc , I have remeasured the surface, Can go another 5mm extra so that door wont be blocked.

how can we achieve 5mm clearnace ?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/86096#Comment_86096

Titan 5 x 72 x 100mm 4 Piece Yellow Packing Shims can be used to establish that 5mm gap @nvelaga. The joists will then be placed on top of them and Dynabolts through the face of the joists used to secure them to the concrete.

Mitchell

nvelaga

https://www.workshop.bunnings.com.au/discussion/comment/108363#Comment_108363

Thanks @MitchellMc, thats good idea and great suggestion. It adds atleast a bit of drinage as well.

Could you please help with below

1. How many joists are needed for 5.7 * 3.8 Meter area ?

2. The area includes combination of concrete and regular soil. on concrete as you have suggested I can lay the face of the joist. How to run joists on soil ? Additionally how to attach those joists for that length without compromising on strength ?

3.Which one recommend among these two for soil

a. https://www.bunnings.com.au/tuffblock-300-x-300-x-90mm-instant-foundation-system-deck-support_p2410345

b.https://www.bunnings.com.au/deko-250-x-150-x-150mm-instant-concrete-foundation-block_p1080391

EricL

https://www.workshop.bunnings.com.au/discussion/comment/121527#Comment_121527

Hello @nvelaga

If the joist were facing the 5.7m length for example and you placed the joists at a distance 450mm from each other, you would need approximately 13 lengths. Running the joist over the soil will require an adjustable foot pedestal, but it all depends on how much space you have between the joist and the soil. The support for the joist will need to be very close to each other, this is to prevent the joist from sagging. Neither of those two products you mentioned will work as the joist is facing the towards the flat side.

Eric

nvelaga

https://www.workshop.bunnings.com.au/discussion/comment/129732#Comment_129732

Hey @EricL ,

Thanks for your suggestion. Is there any recommendation on footing to use ?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/134910#Comment_134910

Hi @nvelaga,

You can use adjustable pedestal feet as they will accept the flat side of the joist.

Mitchell

nvelaga

https://www.workshop.bunnings.com.au/discussion/comment/138318#Comment_138318

Thanks @MitchellMc ,

1. How many of such pedestals needed for 1m of timber ?

2. I am thinking to use https://www.bunnings.com.au/70-x-45mm-outdoor-framing-h3-treated-pine-5-4m_p8032136 for joists and bearers ? Does this sufficient or do we need to go for h2 level considering the clearance is less ?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/140583#Comment_140583

Since you're laying the joists on their face @nvelaga, which already is not something that is recommended, you would have to at least go for the 90 x 45mm and not the 70x45mm. H3 is the minimum grade acceptable for exterior use. H2 is for internal use.

For a 90 x 45mm joist on their edge, you'd support them every metre. However, since you are laying them on their face, which significantly reduces their span capabilities, I'd recommend having support at least every 800mm at a minimum.

Mitchell

nvelaga

https://www.workshop.bunnings.com.au/discussion/comment/142237#Comment_142237

Understood , thanks @MitchellMc. Just to confirm bearers in all directions will also need to lay on their face right ?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/143421#Comment_143421

If you're working within the height you've specified, them laying on their face would be the only option @nvelaga. If your working height increases sufficiently once you're off the concrete, you could consider switching their orientation.

Mitchell

nvelaga

https://www.workshop.bunnings.com.au/discussion/comment/144321#Comment_144321

Thanks @MitchellMc for the suggestions, I will reach out to you if i am stuck anywhere.

nvelaga

https://www.workshop.bunnings.com.au/discussion/comment/144321#Comment_144321

Hey @MitchellMc ,

As suggested I will be using 90*45 H3 Outdoor framing treated pine ,

Big foot pedestal for support on soil , these pedestals will be placed on concrete paving slab

for stability.

I have put together a plan on how these materials will be placed on the surface. Do you think this would work interms of strength ?

Deck Plan.jpg

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/144974#Comment_144974

Hi @nvelaga,

That looks well supported.

You might like to add some additional framing where the joists meet the concrete. You should also add noggins between the joists to stabilise them.

Mitchell

nvelaga

https://www.workshop.bunnings.com.au/discussion/comment/145488#Comment_145488

Thanks @MitchellMc for the guidance.

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