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Hello
I am looking to build a deck at the front of our home which is a converted steel shed on concrete slab. The deck is quite large 12m x 5m overall and it sits under a pergola that has just been completed. (see attached drawings and images.)
I am concerned mainly about the deck's height from the ground which at the lowest point would only be about 160mm and at its highest point 885mm (this is fine; I will do steps at this section).
The lowest point, being 160mm from ground to top of deck leaves only 140mm for bearers and joists, with the bearer sitting on the ground (using a 140mm bearer) and the joists flush to the top of the bearer. I expect that I need the bearer to be a minimum of 75mm from the ground.
Is that correct?
What should I do to make this workable?
Is a 140x45mm H3 timber bearer suitable? or should I go to a steel purlin/box section?
Should I dig it all out to increase the space underneath? how much?
about the number of stumps
You are definitely on the right track in thinking that the height at the low end is the real challenge @Minyarra. With only 160mm from ground to finished deck height, there is simply no practical way to build a compliant and durable deck without excavating. A 140 x 45 H3 bearer cannot sit on the soil and still have the clearance it needs. H3 is only rated for above-ground use, which means it must have separation from soil and good airflow. Even with excellent drainage, you still want a gap under the bearer. If the underside of the bearer is only a few millimetres from damp soil, it will deteriorate quickly.
Realistically, you should plan to dig down at that end. If you can create 50 to 75mm of clearance under the bearer, you are in a workable zone, especially if you seal the underside with bitumen paint for extra protection. That minimum gap only works if the drainage is excellent, so grading the soil and giving water a path to escape is vital. Many builders lay a bed of drainage gravel over compacted soil so that any moisture that gets through the decking can drop straight into free-draining material rather than sitting under the timber.
The more clearance you create, the better the deck will perform. This is also why you should avoid closing the sides with decking boards. It looks neat, but low decks depend heavily on airflow, and boxing them in traps moisture. Leaving the sides open will help the framing last as long as possible.
You may find that the simplest approach is to excavate a strip along the high side, perhaps 200mm or so, install treated sleepers to retain the edge, and then level the soil across the footprint. That gives you a consistent working height for your stumps and bearers and allows you to add drainage gravel across the whole area.
Switching to steel is an option, particularly at the low end. Properly coated steel will tolerate low clearances better than H3 timber, although it can still corrode over time if moisture sits against it. In practical terms, both materials need thoughtful drainage. Steel in the lowest zones and timber where you have more height can be a good compromise.
Excavation really is the key here. Once you give yourself the clearance, everything else falls into place, and you end up with a much longer-lasting deck.
Please let me know if you have any questions.
Mitchell
Hello @Minyarra
To help with the protection of your bearers from heavy moisture, I suggest painting the bottom section of the bearer and a bit of the side with Gripset Betta 1L Waterproofing Membrane Bitumen Rubber. I recommend covering the top of the frame with CAgroup 50mm x 20m Bearer Protector G-Tape. This will protect it from sitting water on the surface. I'm a bit of a conservative when it comes to post spacing, I propose pulling the posts back to 1400 spacing and adding another row on both sides of the decking frame. The 140 x 50 should be strong enough as bearers.
Let me tag our experienced members @Nailbag, @Dave-1 and @AlanM52 for their recommendations.
If you need further assistance, please let us know.
Eric
Hi @Minyarra,
Maybe Concrete Foundation Blocks.
Which should reduce the amount of excavation.
If you bury that @ 75mm maybe you can fit in the 140 x 45 bearer.
Buy a few and have a fiddle with them.
For that space in that area I am a bit concerned you are using post stirrups.
Do you intend to put a roof on that and if that's the case ignore the above.
If not, I recommend set those posts in concrete.
Cheers
Good morning @Minyarra
I would go for excavating out the low end as well, Having a decent airflow that will keep the timbers drier would be my aim 
I also would go with @EricL's suggestion about spacing, I have walked on too many decks that feel mmmm spongy? so something that will be less likely to bounce.
Cant really add to @MitchellMc and @EricL's suggested course of action
I think it will be a beautiful deck when you are finished and would love to see some pics in a project.
Dave
