How high should Merbau decking be for adequate ventilation?

[mg2]

Hi team

I've been roped into volunteering for the local primary school to do something about seating. They currently have concrete blocks with composite decking on top which has seen better days. Decision has been made to replace the composite with merbau.

That's fine, though the merbau should be sitting on top of joists for the usual reasons

But - I can't really have the boards up from the concrete by much, otherwise the bench seating will end up being too high for the little ones.

Will raising them from the top of the concrete with 35mm treated pine be enough? If I do a vertical run of boards around the side to conceal these "bearers",  am I going to be defeating the purpose of raising them that little amount in the first place?

[Jason]

Good on you for volunteering to help out. And good on you for making your first Bunnings Workshop community post @mg2!

Our resident Bunnings D.I.Y. expert is offline at the moment due to Queensland's wild weather, so let me tag a few other helpful members to see if they can kick off the discussion for you: @TedBear, @Jewelleryrescue, @Nailbag and @Dave-1.

The guide How to build a low-level deck should also be helpful.

Jason

[Jewelleryrescue]

Hi @mg2 

Yes 35mm x 75mm treated pine will be plenty to allow the timber to be fixed into the concrete and at the same time have enough thickness to allow the merbau to be fixed to the same timber. (counter sink the cement fixings below the pine timber top level as a tip) Be mindfull of the merbau fixing screw lengths as usually purpose made decking screws, they are 50mm to 65mm long and may be too long and try going right through the merbau and the treated pine into the concrete so just use 40mm treated pine screws.

Adding a vertical merbau timber edge will give a nice touch and say you're the right person for the task at hand and has added attention for detail.

Doing a picture frame merbau on top of the treated pine will also visually add to your project.  Allow the picture frame to also cover the side egde boards to hide the tops of them 

No one will real comment of your extra detail but in their mind's eye it will be noticed as they are used to seeing it in glossy photos and you will know you did the higher quality job which counts the most. 

[Dave-1]

Afternoon @mg2 

Being roped in means someone thinks your skill levels are resonable!

Any chance of a photo of the seating? And maybe some measurements?

Another question is how did they attach the composite decking to the concrete blocks?

I had some low rise gabion walls that I installed a timber top to where I used effectivly merbau slats that I screwed the boards to. Example below

Gabion wall bench seats 

You could do the same type of deal instead of the pine joists. Tho you would have to be cautious about screwing through the merbau slats and then the 22mm thin joists. You are only gaining 13mm tho.... Mmmm If you have access to a little one of average height maybe get them to sit on one and then raise the seat by 35mm and see if the angle of the leg at the knee is uncomfortable? It may not make a lot of difference (the 35mm gain in height)

The 35mm height I dont think will be a problem, but worth checking.

Dave

[Nailbag]

Hi @mg2 

Generally you won’t need a lot of clearance at all. But it does vary depending on application. 

Your best bet is to consult with a local building inspection company for local guidelines. It won’t be anything complicated, just some basic requirements. 

Nailbag