How to build a low-level deck?

[PscottDIY]

Hi Team,

planning on building a deck under our new patio approx. dimensions;

~8200L

~4700W ( can be flexible on this to suit a standard board length to avoid cuts ) 

~150/200H 

ideally wide Boards running out to the grass. 

Most of it can be supported by an existing concrete patio, with I assume some footings on the outer side closest to the lawn.  

Can you please help me with how so set out the frame / bearers / joists  footings etc ? what sizes to use etc ?

Cheers 

[EricL]

Hello @PscottDIY 

Welcome to the Bunnings Workshop community. It's fabulous to have you join us, and thank you for sharing your question about building a low-level deck.

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. These guides will give you a general idea of how to plan your frame and what to take into consideration. But before we go any further, would it be possible for you to provide the measurement from your concrete patio floor to the bottom of the aluminium screen door.

This measurement will be the basis for the size of timber framing you'll be using, the amount of space that will be left under your deck and the type of footings you can use. I suggest having a look at these low-level deck discussions to give you an idea of how to plan your decking frame.

Low-level deck discussions:

• Building a low-level deck over concrete by @DIYImpressions
   
• Low-level deck building advice by @rookabook
   
• Low-level decks over concrete and around tree by @SLFDesigns

I don't wish to overwhelm you with information, so we'll take it step-by-step once you provide the measurements we need.

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

Eric

[PscottDIY]

Cheers Eric , 

that measurement is approx 360mm , ideally want the deck to be about 150/200mm raised from the existing concrete , so a step down from the door ( this aluminum door will be replaced by a large sliding door ) 

[EricL]

Hello @PscottDIY 

Thank you for providing those measurements. Let's do some quick math, if for example you were to use 140x45 along with composite decking with a thickness of 23mm. This will give you a total thickness of 163mm, if we subtract this from your usable space of 200mm you will have a gap of 37mm. Ideally the bigger the gap the better air circulation you have under your deck. Better air circulation means less trapped moisture under your deck and thereby less damage.

However, if you were to use a smaller piece of timber such as 90x45 along with the composite decking you would have a thickness of 113mm. You would be left with a gap of 87mm which is much better for air circulation. I suggest having a look at this discussion - Low-level composite deck by @oninpena. This is a very good example of a single layer decking frame supported by galvanized brackets and Builders Edge 25-40mm Minifoot Pedestal Feet. 

An important thing to consider is in which direction are the decking panels going to be laid. Once you've decided on this it will then dictate the direction of the decking frame. Generally, a gap of 450mm is commonly used between joists, but this is not a fixed rule, and the gap can be adjusted depending on the timber that you use and the space available. 

I suggest using pen and paper first to draw a preliminary sketch of your decking frame. You can see a lot of examples from the links I've provided. I then encourage you to share that sketch with our members and ask questions about its layout. 

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

Eric

[PscottDIY]

Gday Eric,

Back to this one after some other jobs, from the back wall i have used a piece of 90mm and looks like at the highest point of the concrete below i will have approx. min 130mm of clearance for airflow if i used a 90mm H frame, can the bearers and joists all be 90mm high at the same level ? i assumes laminate two pieces of the 90x45 to make a 90x90 bearer ?

[EricL]

Hello @PscottDIY 

If you are planning on doing a single level deck frame, I suggest having a quick look at this discussion - Low-level composite deck over concrete and soil by @oninpena. This is an excellent example of a single level frame with a slopping concrete base. You'll notice that by the time the frame reaches the outer edge, adjustable posts are being used.

If you decide to use two 90x45's and join them together, I recommend using Galvanized Nut and Bolt assembly to secure them together. At this point in your preparations, I recommend drawing up a plan. Once you have a plan, it will help you to estimate your materials list and give you a very good idea on size and shape.

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

Eric

[PscottDIY]

Cheers Eric, 

Do you think 130 mm of airflow at a minimum is enough ? can my job be achieved with 90 x 90 bearers or would 140×45 be more solid ?

Also can you recommend any tools or resources for planning / drawing out the deck ?

Thanks

[EricL]

Hello @PscottDIY 

In an ideal building situation, you would want more space. But I believe 130mm is enough to facilitate airflow under your low-level deck. The technique to using 90x45 framing timber is to use an adequate number of footing supports. This prevents the timber from wobbling or bouncing when weight is applied on it. 

Another measure you can apply to the decking frame is noggins between the joists. This will further stiffen the assembly and give you a nice solid frame. Using larger timber such as 140x45 is possible, but you then have less space for airflow to pass through. You'll also need to use stronger footing for such heavy timber.

Using 90x90 as bearers is possible, but I suggest drawing up a plan first so that you can visually see how you'll be putting the frame together. Especially if you are planning on just building it as a single level frame. 

I use Sketchup for all my renders and my friend uses Tinkercad. These have basic tutorials that come with them so that you'll have an understanding on how to use them. I'm sorry, but I'm not familiar with other rendering software.      

If you need further assistance, please let me know.

Eric

[Jason]

Hi @PscottDIY,

There's some more recommendations on design software from our members in this Best Advice article - What software do you use for drawing plans?

Hope it is helpful. 

Jason

[PscottDIY]

Hi Guys, 

looking to get moving on this as the weather improves. planning at this stage , my layout looks like this;

- 140mm x 19mm wide merbau boards

- 90x45 joists sitting on 8 x 140x45 bearers ( supported with shims and angle brackets on existing concrete patio, and then concrete footing / stirrups at the end where i can dig out the soil ) 

- shims https://www.bunnings.com.au/macsim-150mm-x-100mm-x-15mm-heavy-duty-shim-grey_p1044801

- angle brackets https://www.bunnings.com.au/dunnings-40-x-80-x-40-x-5mm-m12-galvanised-angle-bracket_p1070088 

- i also plan to waterproof the bottom of the bearers with something like this to protect

https://www.bunnings.com.au/crommelin-4l-blackseal-heavy-duty-bitumen-waterproofing-membrane_p035689... 

( see pic of the clearance which will be under the bearer as the concrete slopes away ) 

would be great to get a sense check on this  

Thanks