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

[mg2]

Thanks @Dave-1, @Jewelleryrescue and all. I appreciate the vote of confidence!

Here is what they currently look like. Approx 450mm high and 2.8m wide. Turns out that currently the composite is sitting on top of 22mm treated pine. I figure I could replace the tops using 70 x 35mm framing pine secured using countersunk concrete screws and the merbau sitting on top of that, so it won't be any noticeable change in height. (Plus 35 frame + 19mm merbau means I can get 50mm decking screws in)

With 140mm boards, the tops with a picture frame will overhang the concrete, so to support the overhang I'll dynabolt lengths of 70 x 35mm pine to the sides, then fit another run over them as a fascia to conceal the framing.

Sound about right?

Also if I go in with a picture frame, on the corners that will be mitred, I be leaving the same 6-7mm expansion gap between the diagonals, right? Or should I not do the picture frame like that but instead just keep them at 90 degrees with the gap in between?  I looked through the other posts about picture frames but couldn't see the answer to this question - apologies if I have missed it

[Dave-1]

Evening @mg2 

Going through your answer, I am not sure if a picture frame would be easy to do plus a skirt that matches up.

How about a straight skirt either side and a butt end at either end. Once the lot has been oiled it would match 

Or

An overhang by 30mm either side? So you wont need a skirt?

I am thinking of something simple and sturdy more then fancy as schoolkids can be mmmm hard on furniture

The gap between the slats is what I would leave as well.

Dave

[EricL]

Hi @mg2 

Although the picture framing is a nice touch, I worry that the kids will be standing on this short piece of timber, and if it is only lightly supported might bend and pull out. I propose keeping it simple with the boards going straight end to end. In this manner they can stand on any portion of the seat without the panels giving way. 

Eric