How to build low profile deck over soil?

[Mdsyd]

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?

[MitchellMc]

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]

 

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]

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]

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]

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]

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]

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]

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]

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]

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.