Need low-level deck advice

[nickb11]

Hi,

I'm planning to build a low-level composite deck in my alfresco area, directly over existing concrete. I've gone through a few of the low-level deck guides shared here, but I was hoping to get some feedback or recommendations on my proposed plan.

I've attached a screenshot that outlines what I intend to do. Any advice or suggestions would be greatly appreciated before I move forward with the project.

Thanks in advance!

[JacobZ]

Hi @nickb11,

A warm welcome to the Bunnings Workshop community, it is fantastic to have you with us.

You've got some solid plans so far, nice work!

What you are planning, with the angle brackets, is usually only something that comes up when you are very limited in height. I am assuming you have limited height for your deck. Is that correct, and if so, how much height have you got?

Looking at the plans, there are three main things I would suggest considering.

The first is to not bolt the timbers to the wall. This is because the concrete slab and the wall will move in different ways. By attaching your deck to both, it will have different forces pulling in different directions, which can cause twisting and buckling of the timber. Realistically, bolting the deck to the wall doesn't really add much value anyway, so I would just leave them separate.

The next thing I will note is the angle brackets. The angle brackets should be more about attaching the joist to the concrete than giving it structural support. This is because the load is being supported by the two coach bolts driven into the timber rather than the angle bracket. A simple solution to add support is to add packers like these 10mm Packers underneath the joists. That way, the load is transferred straight down, as opposed to sideways through the coach bolt, then down through the angle bracket. It is a much better method of supporting the joists.

With extremely low-level decks where there is very limited airflow, I'd usually suggest using 90 x 45mm 4.8m MGP10 H4 Treated Outdoor Structural Radiata Pine as opposed to H3 timber, as it is treated for use in direct contact or very close proximity to the ground.

Outside of these things that you should consider, you have some solid plans that utilise the correct hardware and fit within the span limitations of the timbers.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob

[nickb11]

Hi Jacob,

Thanks for your detailed information and for reviewing my plans — much appreciated.

Yes correct, I have limited height. From the concrete to the top of the brickwork (where the sliding door sits) is 130 mm. The composite decking is 25 mm thick, which leaves me with 105 mm for the joists. That’s why I was considering using 90x45 mm treated pine, with a 10-15 mm gap between the joists and the concrete.

1. Not bolting the timbers to the wall makes perfect sense — I’m not sure why I thought it was necessary in the first place, so thank you for pointing that out. Would you recommend I use some angle brackets on the bearers as well? (the two pieces of timber spanning 4450mm) or just the joists?

2. Just to clarify, are you suggesting I add packers along the entire length of the joists, or only besides the angle brackets (where they’re fixed)?

3. Do you think I could get away with using H3-treated timber and a bitumen membrane for waterproofing, instead of H4? as it is double the cost of H3, so I'd prefer to avoid it if possible.

Thanks again for your advice and recommendations.

[JacobZ]

Hi @nickb11,

No worries, I am happy to help.

To answer your question about the packers, they would need to act as support for your joists like a post would in a standard deck. As your 90x45mm joists, according to the Joist Span Table, are capable of spanning 1400mm between supports, you would need to sit them on packers every 1400mm at most. If you set the joists on packers at the location of every angle bracket you've shown on your plan, you will be fine. You will also need to add packers underneath the two bearers at the top and bottom of the deck. When acting as a bearer, according to the Bearer Span Tables, a 90x45mm timber needs to be supported every 900mm, so they would need a few more packers than the joists.

H3 timber is fine for external use, but it is not good when it is constantly damp and unable to dry out, which is likely with limited airflow under low-level decks. On the other hand, H4 is treated for in-ground use, where it has next to no ability to dry out. 

Ultimately, the choice is yours, and H3 timber with a bitumen membrane will still last for a good amount of time, but I always think it's important to note that there is a better option than H3 for these low-level decks because there is such limited airflow and ability for the timber to dry out. 

Let me know if you have any further questions.

Jacob 

[nickb11]

Thanks Jacob. Appreciate your help!