How to build low profile deck over soil?

Hi everyone. I’m hoping someone could help me on this.

I’ve been thinking to deck the whole area of my backyard (see photo). The area is around 25 m2. I have a few questions:

1. Should I still dig the grass or just put a weed mat on top?

2. The clearance from the finish level (tiled area) is around 130-150mm. I am planning to use 90x45 H3 for my joist and bearers and Merbau for my decking. I will also use Builder’s Edge Pedestal and put some pavers and blue metal underneath to play with my level as some areas are not even.

Will this be okay?

3. In terms of connecting the timber to the existing concrete, what is the best way to do it?

4. Would you recommend to connect the deck to the blockwork retaining wall as well?

5. I will space the joist at 450mm and planning to put the Builders’s Edge Pedestal at 1000mm spacing. Is this enough?

6. What size of screw should I use for my timber and when I bolt the timber to the concrete?

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Comments

Jewelleryrescue

Hi @Mdsyd

The whole area is flatish I would seriosly concrete that area as your new deck as concrete is zero maintinance and tile over the top if you wish add timber look alike tiles super sexy these days. No maintenance.

Consider a timber deck so low? constant maintenance and digging it out with complex ground plates and timber structure/ Timber mildew and rot so low as most decks have a minimal breathing space to stay dry.

A concete slab will go between your wall and existin slab just drop the height to allow a nice tile on top.

MitchellMc

Welcome to the Bunnings Workshop community @Mdsyd. It's wonderful to have you join us, and many thanks for your question about constructing a deck.

You might like to start by reading through this guide: How to build a low-level deck.

You're working with some very limited height there. The portion undercover over the tiles will have 20mm beneath your joists once you account for the 19mm decking and 90mm joist height. We've seen quite a few members build decks within such limitations. My concerns lie more so with the height of your joists above the turfed area. Building a deck directly over soil can cause significant moisture issues, as pointed out by @Jewelleryrescue. You'd need to excavate the area, remove at least 100mm of soil, place down weed mat and then add drainage gravel so water can pass through the deck and easily drain away. The bottom of the joist should be coated with bitumen paint to help prevent the early onset of decay. It would be a good idea to leave the sides of the deck open and decking boards should be gapped extra wide at around 6-8mm to help with airflow.

Using pedestal feet over the tiled area and also on pavers over the soil area would be suitable.

Provided you frame in the deck and add noggins between your joists for rigidity, connecting to the tiles, concrete, or blockwork is unnecessary, as a floating deck/platform will suffice.

Your spacing is adequate with joists at 450mm and support for them at 1000mm.

For connecting your frame and noggins to your joists, I recommend Buildex 12g x 100mm 8 TPI Climacoat RapidDrive Treated Pine Screws.

Here's another helpful guide on How to build a deck.

Please let me know if you have any questions.

Mitchell

Mdsyd

https://www.workshop.bunnings.com.au/discussion/comment/37686#Comment_37686

Thank you for your prompt response. On aside note this is now my favourite forum for DIY. People are helpful and nice.

in regards to your comment below. Does that mean that there’s no need for me to bolt the bearers to the existing tiled/concrete undercover area and retaining wall at the back?

“Provided you frame in the deck and add noggins between your joists for rigidity, connecting to the tiles, concrete, or blockwork is unnecessary, as a floating deck/platform will suffice.”

Just to clarify, should I am for 5-10mm finish level below the exisiting concrete/tiled area or I should just flush it and same level?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/81072#Comment_81072

With no attachment points, you'd be creating a floating deck @Mdsyd. With a floating deck, there is no need for attachment points. To see an example, check out this guide on How to build a deck that entirely floats on top of pedestal feet. So, in answer to your question, you would not need to bolt the joists to the existing tiled/concrete undercover area and retaining wall at the back.

Could you please provide a little more information on your last question? I'm not sure if I quite follow. "Just to clarify, should I am for 5-10mm finish level below the exisiting concrete/tiled area or I should just flush it and same level?"

Mitchell

Mdsyd

https://www.workshop.bunnings.com.au/discussion/comment/105206#Comment_105206

Sorry for the confusion.

I want to know if I should flush and make the decking level the same as the current tiled finish level or should the decking level be a bit lower (5-10mm)?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/119738#Comment_119738

Oh, sorry, @Mdsyd. So you are not decking over the tiled area but only over the grass? If so, I'd recommend the decking be the same height as the tile. A slight difference will cause a trip hazard.

Mitchell

Mdsyd

https://www.workshop.bunnings.com.au/discussion/comment/128554#Comment_128554

Yes that is correct. Tiled area will not included.

few clarifications now that is it clear than I will not include the tiled area:

1. Does the method of not bolting the deck to the tiled/concrete area still applies?

2. Do I still need to tie it to the retaining wall?

If i remove the grass the level is around 150mm clearance from the ground. Thinking dig further 30-50mm to allow for roadbase and weed mat.

let me know your thoughts.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/134210#Comment_134210

All still applies @Mdsyd.

I'd encourage you to read through this guide on How to build a deck. Within it I show how to build a floating deck that doesn't need to be attached to anything. You can use the same methods for your project. No need to attach to the tiled/concrete area or retaining wall.

Digging down a further 50mm and adding weedmat and roadbase is essential, and I'd highly recommend you do so, or you'll have a very limited life to your deck due to the close proximity to the ground.

Mitchell

Mdsyd

https://www.workshop.bunnings.com.au/discussion/comment/137785#Comment_137785

Thank you.

Lastly, There is a section on my bearers that will need to be spanned in two sections. What is the best way to join or sister bearers and joists? Would it be the same process for both?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/140218#Comment_140218

Hi @Mdsyd,

If they are hard up against the slab, you might just like to Dynabolt through their ends into it for support. Alternatively, space them away from the slab enough to use a pedestal foot under the join. If you were to do this, it would be best to use some Pryda plates on either side of the joint for support. Spacing the bearers away from the slab might be more involved than bolting them all to the slab. As mentioned before, you don't have to bolt them to the slab, but it might actually be the easier option.

Mitchell

Mdsyd

https://www.workshop.bunnings.com.au/discussion/comment/141964#Comment_141964

Thanks @MitchellMc . So i removed the grass and this is the site now. I’m planning to use Tuff Blocks as support and 90x45 joist and bearers. Current level is around 180mm to finish tiled level.

When it comes to soil prep, I’m thinking to cover this with weed mat and put some drainage gravel and level it from there or should I put road base and compact it? What would you do in this situation?

There is a french drain and connected to the stormwater underneath.

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/143243#Comment_143243

Hi @Mdsyd,

Assuming you use 19mm thick decking boards, I suggest you go slightly deeper before preparing your base.

With the hardware you have specified, the Tuffblocks will raise the 90mm joists and bearers 50mm off the substrate, with a 19mm decking board on top, coming to a total height of 159mm. This only allows 21mm for your compacted base. I would suggest that this base be at least 100mm thick to support the structure's weight and allow for adequate drainage.

I'd suggest using GRUNT Geotextile Membrane over the soil instead of a weed mat, as it allows for better drainage through the material while suppressing weeds and keeping the soil and gravel separated.

Once your geotextile membrane is down, you can use either road base or gravel as a compacted foundation, but there are advantages to drainage gravel that would make me suggest you use it.

Road base is better for compaction, but its small aggregate size means water can erode it more easily than drainage gravel. Drainage gravel will not be eroded as easily but cannot be compacted to the same level as the road base.

As drainage is so important under low-level decks, my suggestion would be to use drainage gravel compacted and levelled with a Trojan 205mm Tamper and spirit level in 50mm layers.

You might like to check out How To Prepare And Lay a Base For Pavers as the process is very similar to what you would do for your deck.

Let me know if you require any clarification or any further assistance.

Jacob

Mdsyd

https://www.workshop.bunnings.com.au/discussion/comment/144184#Comment_144184

I’m thinking to just dig a trench or hole to where the tuff blocks support will go and will add gravel underneath it.

is that possible?

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/144876#Comment_144876

Hi @Mdsyd,

Just to be clear, there are no bearers in a single-level, low-level deck. There is a frame and joists. So, you have 180mm - 19mm decking - 90mm joist, which leaves you with 71mm under your joists. That's slightly better than @JacobZ's concern above. You might wish to remove another 30mm of soil, lay 30mm of drainage gravel and compact it.

Gravel needs to be added over the entire area to help with drainage. Compaction also works better over the entire area instead of just beneath the supports.

Mitchell

Mdsyd

https://www.workshop.bunnings.com.au/discussion/comment/145413#Comment_145413

Thanks. Will take than onboard.

on the other hand, I haven’t added drainage gravel etc and I tried to water this area to make sure the french drain underneath is working. Water is stagnant (heavy clay soil). Although i can hear and see water is dripping from the stormwater connected from the french drain.

i guess what I want to know is if it this is normal since I haven’t put any gravel? When there was a grass on top there were no collection of water it just drains aways quickly.

EricL

https://www.workshop.bunnings.com.au/discussion/comment/145833#Comment_145833

Hello @Mdsyd

If you are getting small puddles on the ground, this is where levelling the surface comes into play. Using a rake, break up the surface to remove low and high spots on the ground. Your goal is to create a flat shallow slope so that the water is directed towards your drain.

Once you have the surface nice and level with the shallow slope, I propose using a tamper to compact the loose soil. Once you've finished compacting the area, there should be no more low spots for water to pool into.

If you need further assistance, please let us know.

Eric

Mdsyd

https://www.workshop.bunnings.com.au/discussion/comment/146154#Comment_146154

Thanks Eric. I’ve sorted it out. french drain is working properly now.

By the way just a thought, if I decided to use 70x45 joist size l, what is the suggested spacing in between joist? I understand 90x45 is more appropriate but I just don’t have the space and means of dumping and removing few mm of soil on top to go dow to the level I need. Going to 70x45 will resolve my issues.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/146426#Comment_146426

It would not be advisable to use 70 x 45mm timber on your deck @Mdsyd, as 90 x 45mm is already quite small in calibre. If you did, they'd still be spaced at 450mm centres, but you'd have to support them more readily than 90 x 45mm, perhaps every 750mm.

If you can't fit 90mm joists above the soil, it would be worth rethinking your project and having an appropriate amount of soil removed from the surface.

Mitchell

Mdsyd

https://www.workshop.bunnings.com.au/discussion/comment/146642#Comment_146642

Hi Mitchell. What is the best way to build around the stormwater drain? Am I allowed to run a joist on top and put a tuffblock? I’m not sure if I can do that. Would you mind marking the joist and support if you were to build it?

Disregard the pedestal sitting on top of it as I just placed there temporarily. The size of the grate is 600mm x 600mm.

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/146821#Comment_146821

Hi @Mdsyd,

It looks like you ended up going with the 90x45 for your framing. Is this correct?

There's no issue with running a joist over the top of the grate, but you shouldn't place a paver and toughblock on it. This would be transferring the weight of the deck onto something not designed for this purpose.

If you could advise the dimensions of this space, I may be able to advise a slight redesign that would allow the deck to be fully supported and fall in line with advisable bearer spans and advisable joist spans.

I've done a very basic mock-up of a potential solution using the 1200mm level as a scale for size. The red line is an additional bearer, and the yellow dots are decking supports.

You would then run the joists perpendicular to the bearers as you would have originally, they would just be split into two separate sections.

This should work based on the size of the level, but the overall dimension would need to be confirmed.

Allow me to tag some of our helpful members to see if they have any other ideas, @Dave-1, @DIYGnome , @Nailbag.

Let me know if you have any further questions.

Jacob

Mdsyd

https://www.workshop.bunnings.com.au/discussion/comment/146969#Comment_146969

Yes 90x45. Size of the corner is 1100 x 1250mm.

Yes, I’m aware not to put something on top of it. I only place the paver and builders edge to help me lift it while I screw the other side.

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/147084#Comment_147084

Evening @Mdsyd

Going through your project I have some concerns over covering that drainage grate. From memory councils are like the waterboard and dont like having their drains covered. I wonder if you could factor in either the deck being able to be opened like a trap door or another grill that can sit in/on your wooden frame to allow inspection and that can be lifted so they can access the drain if needed. And yeah as soon as you cover it is when they will need access lol It happened to me at a sewage inspection point that I was trying to grow grass over as I saw it as an eyesore... They came out in the middle of the night and stripped the grass and cleared a blockage without a concern. Thats why I am suggesting that access can be made to that drain cover.

Dave

Nailbag

Hi @Mdsyd is your intention to deck over the grate as this wouldn't be permitted by any council. At the very least I would advise making that area a non-fixed removable hatch slightly larger than the grate dimensions but still inline with your decking boards.

Regards, Nailbag

Mdsyd

https://www.workshop.bunnings.com.au/discussion/comment/38558#Comment_38558

all done with my backyard reno

EricL

https://www.workshop.bunnings.com.au/discussion/comment/82153#Comment_82153

Hello @Mdsyd

Thank you very much for posting that update on your decking project. It looks very well done and your levelling with your concrete patio is superb.

Many thanks for sharing.

Eric

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