How to build low level deck over concrete and grass?

[rwa1994]

Hi all,

I am planning to build my own deck out the back of my new house. I have some plans and some questions with the plans. 

My questions are:

1. What spacing do I need for my bearers, joists and stumps?

2. I only have 130mm clearance between concrete slab and doorway entrance. I am going to create a single layered structure that I saw on someone else’s post but am I able to use metal stirrups like this one and just dynabolt it to the ground and put the wood straight on the concrete?

3. Am I able to use 90x45 H4 treated pine for my structure? I can't use 140mm as it won't fit clearance.

Thank you!

 

[JacobZ]

Hi @rwa1994,

Thank you for your questions about constructing a low-level deck at your home.

Your joists will need to be spaced at most 450mm apart to sufficiently support the decking boards. You can use 15 joists like you have in your design, but reducing to 14 is possible within this allowable spacing.

Using the brackets to bolt the deck to the concrete is fine, but it's not advisable for timber to be in direct contact with concrete. Concrete is porous and susceptible to the phenomenon of rising damp. If timber is in direct contact with concrete, water will rise through the concrete, into the timber causing rot, mould and greatly reducing its lifespan. You can line the bottom of the joists with CAgroup 90mm x 30m x 500um Plascourse or paint them with Gripset Betta Waterproofing Membrane Bitumen Rubber to reduce this, but ideally the deck would not be in direct contact with the concrete.

I've created a render that would allow the deck to be raised from the concrete to create an air gap while still being in line with the Hyne Joist Span Tables. This would require you to use 4 brackets per joist and a 45 x 90mm Pryda Joist Hanger at each end of the joist. Assuming you use a 16mm decking board and 90x45 timbers, this will allow a 24mm air gap beneath the frame of the deck. While still extremely low, the timber will not be in direct contact with the concrete, so there is some ability for the timber to dry out.

There are no issues using 90x45mm timber as long as it is H3 treated, and everything is adequately supported.

Allow me to tag some of our helpful members to see if they've got any further advice to offer, @Dave-1, @Nailbag, @Jewelleryrescue.

You might also like to check out How to build a low-level deck for some further guidance and 25 inspiring low-level deck builds for some inspiration.

Let me know if you have any further questions or would like further clarification of my above points.

Jacob

[Dave-1]

Morning @rwa1994 

The design looks solid, the only thing I can suggest is to factor in where your water drains and make sure it will work to take away water from under the deck. Bear in mind also where your stormwater pipes will be running, Try not to have your piers over the area-ish  

If you are going to put some deck lighting in it may also be an idea to walk through Bunnings and  check out the styles that are available.

Dave

[rwa1994]

Hi @JacobZ and @Dave-1 ,

thanks so much for your help! This is awesome.

I do have a couple of follow up questions, sorry! 

1. What brackets would you recommend? 

2. The concrete slab is 3120x3720 with a 465x465mm brick pillar in the middle. For the grass portion of the deck, can I just dig the holes and use concrete like in this video? 

Thanks again guys! 

[Dave-1]

Morning @rwa1994 

Your concrete slab looks almost level with your lawn, Id be tempted to remove some soil to ensure it dosnt touch the joists. 

I prefer stirupps into concrete but the posts in the video are fine as well.

Supporting the bearers over the concrete. How much space will you have between your floorboards + the joists and then the concrete. I have seen people use the Right angle bracket to support the joists. Carinya 100 x 100 x 40 x 6mm Galvanised Heavy Duty Angle Bracket There are various sizes and configurations available. You may need to drill a hole in the bracket if the existing holes are too close to the timbers edge (within 20mm of the timber)

Thinking about the concrete patio and maybe not wanting to drill holes in the concrete I am wondering how much height you have and if you can use another method of support.

Pool deck stage 1 

Pool deck stage 2  

These are my decking project just in case it helps. There are a load of very similar projects like yours amongst these pages with the same minimal height type issues and how others have solved them. Definently worth a wander through.

Dave

[JacobZ]

Hi @rwa1994,

Great advice here from @Dave-1.

The brackets Dave has mentioned would be good.

Timber posts in concrete are fine, but make sure you coat them in bitumen paint like they do in the video. You can also use stirrups which would keep the timber up and away from the soil so moisture issues wouldn't occur.  

Let me know if you have any further questions.

Jacob

[rwa1994]

Hey @Dave-1 

Thanks so much!

I have 130mm clearance minus 90mm for wood and minus 19mm for the decking wood will leave me with 21 below the frame for stirrups and airflow. 

I am happy to drill into the concrete. My initial plan was drill into the concrete and then dig holes in the grass to put posts with quick set concrete and make it levels across the surfaces. 

Thansk again!!

[Dave-1]

Afternoon @rwa1994 

A suggestion for anchors for the concrete would be Ramset 10 x 60mm Galvanised AnkaScrew Masonry Anchor They also come in boxes which are a lot cheaper.

Dave

[rwa1994]

Thanks Dave! 

[Nailbag]

Hi @rwa1994 Looks like both @JacobZ and @Dave-1 have provided pretty much everything for your sub-floor. Mu only advice would be to ensure a min of 50mm clearance from the underside of the above lawn area joists. Suggest 450mm deep stump holes using 90mm x 90mm Cyprus primed with bitumen paint.

If you plan on installing any deck lights now is the time to run the fig 8 cable. to each point. Just make a loop of around 300mm with a taped drawstring to pull it through once the deck is down.

Being at ground level I would also recommend using Protectadeck across all joist as an added level of protection for the deck boards.

Nailbag