Is Bunnings 42mm x 42mm Merbau suitable for joists for low level decking?

[Henry1]

Hi all, I'm doing some low level decking and wanting to use the Bunnings 42mm x 42 mm (pencil rounded) Merbau timber for the joists. They will be on a concrete slab but I'm really limited to the height, hence the need for the 42mm x 42mm.

I propose Bunnings mini feet for the bearings. 

I'll be applying Ekodeck boards (180mm width) on top with 6mm gaps. Area is 22m2. 

Height, off the concrete, for the joists will range from 10 mm to 40 mm (Yes, very low level). 

I'll be applying bearer protector (CA group) G-tape onto the tops of the joists.

The concrete has a drain in the middle with slope towards the drain from each side. 

Most of the decking (say 2/3rds) is permanently in the dry, under cover. The exposed area cops the Perth weather. 

The merbau timber is rated to Class 1. Initially, I was looking at a H4 rated softwood (Radiata Pine) for the joists (due to the low height off the concrete) but from what I can see, the span for them is 250mm and that would involve around 200 bearers.

Wondering what the span capability is for the Merbau hardwood and whether people more experienced than I am, would say it's up to the task given the low level. 

Your experience and recommendations are welcome

[Nailbag]

Hi @Henry1 

Runnings straight down to @JacobZ postings, I would suggest exactly the same utilising the H3 treated 75mm x 45mm as your joists and would consider his frame layout. However in dealing with your concern and conceding you have a centrally located drain, could you use a demolition saw to make cuts towards the centre. This combined with a pneumatic hammer to chisel out channels to the drain which toy could if possible remove and lower it to accomodate the channels. These wouldn't need to be very deep and perfectly made being unseen.

Nailbag

[Henry1]

Good thinking. I might give that a go. Thanks!

[Henry1]

Hi .@JacobZ

Water/weather ingress is on two sides, unfortunately at right angles. That's where all the rotted timber (boards and joists) were from the previous deck. 

I was planning to use the window packers on the lower rise joists (and mini feet on the higher central points) as there's a heap that I've retrieved from the previous deck. But I might see if I can chisel out a drainage channel like Nailbag suggested (to allow for water to not pool). That might be workable.   

[Henry1]

Mmmm, the slope on the two outer edges - which cop all the weather - are quite high insofar as the concrete is concerned. It's quite dished, like a ultra small skate park.

I'll be able to get away with 70 x 45mm on 70% of the joists but will need to use 45mm as a maximum elsewhere. The alternative is to have the boards slope upwards slightly, as they go out from the house.  

I think I'm going to have to use the 40 x 50mm hardwood timber I salvaged from the original deck and place it on the two outer edges and run 5 to 10mm window packers under them. I know this isn't ideal and the downside is I won't get anywhere near as long from the subframe as ideal.   

What's the best way to join 70mm H pine board with a 40mm H hardwood board? I'll need to do this on 7 of the joists. 

Also, do I need to bolt the subframe down giving the weight once finished with the decking. And if so, what could be a minimum I could get away with? 

[JacobZ]

Hi @Henry1,

There is no great way to attach your 70x45 to the 40mm hardwood because it's not how decks are typically built. I suppose the best you could do is rip the 70x45 down where needed and then screw it to the hardwood with long batten screws.

Yes, you should bolt the frame down. Regardless of the weight of the deck, if wind gets under it, it can cause uplift. I would be looking at using at least 3 angle brackets per joist.

Let me know if you have any further questions.

Jacob

[Henry1]

Hi Nailbag

I'm still working on this project and have bought 1 x H3 70 x 45mm board to test it in position for each joist. The first position closest to the door worked (just - with about 2mm spare). But wondering was there any reason (s) why you suggested the H3 rather than the H4 board? Notwithstanding the higher cost/ special order, might the H4 might be better for durability for this job given the ultra low setting?

[JacobZ · Accepted solution]

Hi @Henry1,

Allow me to tag @Nailbag so he is notified of your question. 

I'd say he just mentioned H3 as it is what has commonly been used for low-level decks. H4 is a better option in close proximity to the ground, especially considering the drainage issues you have already experienced.

Jacob

[Nailbag]

Hi @Henry1 

Apologies for the typo confusion. Yes to H4 treated over H3 as @JacobZ correctly picked up as well. 

Nailbag

[Henry1]

Hi .@JacobZ

I'm still progressing on this. I've laid out the 70 x 45 joists and most will need shims rather than the mini feet I was hoping to use. Can I nail (stainless steel) or screw the shims to the underside of the joists. I saw someone do it on a youtube but your comment above suggested angle bracket/ bolt to the concrete.

I could still do the bolt to the concrete - say 3 per joist - but was hoping for an easier way for the rest of the shims.

By the way, when I bolt the bracket to the concrete, would ankascrews be ok? After allowing for the shim(s), what length of the screw do you recommend being in the concrete? 

And I assume the shim(s) are placed not under the actual joist but under the bracket like this diagram     

[MitchellMc]

Hi @Henry1,

Yes, you could fix the shims to the bottom of the joists with fixings. Shims typically go under the joists, not the bracket. If there is a need to shim the bracket, it would be best to use larger brackets instead. Ankascrews would be suitable and you'd need at least 40mm of their length into the concrete, so the 50mm you've shown would be suitable dependant on the thickness of the shims, if you're using them under the bracket.

Mitchell