How to build low clearance deck?

[L3mon1980]

Building a low clearance deck on a concrete slab 4.6mx7.1m. At the highest point at the backdoor is 80mm. So can I use 70x45 h3 for the joists using packers and concrete screws and if so what would be my spanning capacity for the concrete screws.

[MitchellMc]

Welcome to the Bunnings Workshop community @L3mon1980. It's great to have you join us, and many thanks for your question about constructing a deck.

Using 70 x 45mm H3 timber as joists on their face (45mm high) within your 80mm clearance is feasible, but keep in mind that timber loses most of its spanning strength when laid this way. The 45mm joist and 19mm decking will give you a structure thickness of 64mm.

To ensure the deck remains stable and doesn’t become bouncy, you’d need to support the joists at least every 500mm with packers and concrete screws. The packers can help level the joists on the concrete slab, while the screws secure them firmly in place.

You might like to check out this guide: How to build a low-level deck.

Please let me know if you have any questions.

Mitchell
 

[L3mon1980]

Hi my plan was to use them on the 70mm side so the joists line up with the brickwork and place decking on that so the decking lines up with the sliding door height. And if i could ask what size concrete screws to use?

Thankyou

[MitchellMc]

Hi @L3mon1980,

That would give you a height of 89mm so slightly higher than your highest point. Also, bear in mind that it places your joist directly on the surface, which can lead to early decay as H3-treated Pine is designed for above-ground use, not on-ground use. Having no airflow and water drainage under the joists will lead to issues.

You'd need to use angle brackets and around an 8 x 65mm Dynabolt to attach the joists to the concrete beneath.

Mitchell 

[L3mon1980]

Yeah I was aware of that but was hoping to use bitumen paint on the joists as well as vapour seal would that help in that sense

[MitchellMc]

That would certainly help @L3mon1980. However, it probably won't assist with the water draining from beneath the deck, creating quite a moist and humid zone. 

Mitchell

[L3mon1980]

 this is what I'm building the red rectangle is the existing covered alfresco and the area around it is lower concrete around the house and was going to use the adjustable joists as the joist goes onto the other concrete.

 

So would you advise to use the face of the 70x45 instead? 

[MitchellMc]

Hi @L3mon1980,

Unfortunately, we could never recommend using timber on its face as that's not what structural timber is designed for. You could choose to go that route if you wish. It really comes down to the structural integrity of the deck versus the diminished life on it due to being extremely low-level. With a height of 80mm, consideration should be given to whether creating a deck in this space is appropriate. If you are happy to deal with a potentially diminished lifespan, then you should have the joists on the 45mm edge.

Mitchell

[L3mon1980]

How about if I were to plane off 15-20mm off the 70mm, bitumen paint tape it and vapour seal on top with packers and concrete screws i assume would help the issue of ventilation and drainage?

[EricL]

Hello @L3mon1980 

I understand your desire to build your low-level deck to the proper height. Planing off 20mm will not help as it diminishes the structural strength of the timber. The application of bitumen paint and vapour seal on the surface will help extend the life of the timber, but it will not address the issue of ventilation. Eventually these seals and coatings will break down if constantly soaked in water and heavy moisture.

One other option is to use large 6mm spacings between your decking boards. This will help a little bit with airflow, but without a gap underneath your decking frame, moisture will get trapped underneath.       

If you need further assistance, please let us know.

Eric