What timber spans for single layer deck frame?

[Jimmy2]

First time I’ve dropped in here after researching a decking project and finding lots of great info from other people building decks. I’ve done 4 or 5 decks over the years but nothing for about 20 years. So I just want to check a few things.

The attached plan will show you the basic idea for the Merbau decks. These aren’t the final dimensions and are just based on the drawing. The block slopes slightly down to the right and we were thinking of a series of smaller decks (platforms really) that step down to the main deck on the right.

We want to have these close to the ground. So I’m thinking to raise them so there’s around 200mm clearance underneath. I’m also thinking about a single layer frame using 140x45mm H3 – exactly like the fame shown on the low-level deck built by jase2, here:

https://www.workshop.bunnings.com.au/t5/Featured-Projects/Low-level-deck-and-pergola/ta-p/167281

The problem is that span tables don’t cover frames that combine the bearers and joists into the one frame. So if the joists are say at 450mm centres and doubled up where necessary (as per jase2’s design), how far apart should the posts be?

Thanks heaps,

Jimmy.

[MitchellMc]

Hi @Jimmy2,

I love the effort you've put into the design. However, there are a few issues here. I think it might just be that you're confusing the ability of a noggin to transfer support to the next joist. They can't.

Let's start with some of your questions.

There's no need for double timbers, unless you are trying to achieve spans outside of the capability of a singular timber. In my design, there is no span over 1300mm for the 140 x 45mm. 

Decking boards don't need to be attached to any joists that run the full length, they simply need to be supported every 450mm. Bear in mind this isn't how I would normally construct a deck, it's just what I thought you might have had in mind, and I actually like it. How most people would build a deck is as you've just designed, with long joists running the length of the deck.

But, your design could only work if it had a load more support. To start, your two outside joists on either side (for your picture frame) are entirely unsupported; they're just hanging in the air there, with the only thing attaching them being the noggins. This doesn't work. You can counterlever portions of the deck like you've done on the top and bottom; that's fine, but you can't do that when there are no continuous joists running out to the side.

Apart from the joists you've supported in the middle, the four between them have no support at all. Noggins don't transfer support to the next joist; they just add rigidity to the structure.

I've gone through the design and added a mark at every point I feel needs support.

Mitchell

[Jimmy2]

Hi Mitchell,

This is why we have experts like yourself to pick up all those stupid mistakes us DIY warriors make. After reading what you wrote I couldn't believe the silly errors I made – particularly the unsupported areas. What was I thinking! Thanks so much for all the feedback, it makes so much sense.

Looking again at your design, it's way simpler, lighter, better supported and no doubt cheaper. So I'm going to go with it, with maybe a tweak or two. I'll go and draw it up in plan view and run in past you one more time.

Regards,

Jimmy.

[Jimmy2]

Hi Mitchell,

So I've drawn up your design in a plan view. There were a couple of changes. The overall size is a little shorter but slightly wider, so I made an allowance for that.

The only other thing I did was add back in the blocked double outer frame to allow the stumps to be set back from the edge. I want to have the deck look like it's floating without great big stumps showing around it. That reminds me, the stumps running down the 4500 sides are offset to enable them to be attached to the more solid part of the frame with a block behind each one.

That's about it. Hopefully that's all good now.

Cheers,

Jimmy.

[EricL]

Hello @Jimmy2 

Your new decking frame plan looks fantastic. To add to the longevity of your frame, I suggest putting  CAgroup 50mm x 20m Bearer Protector G-Tape on the entire surface of your decking frame. I then recommend painting the bottom and sides of your frame with Gripset Betta 1L Waterproofing Membrane Bitumen Rubber. This will give your decking frame maximum protection and prevent it from getting damaged by trapped moisture under your deck.

Please keep us updated with your progress, we look forward to seeing the start of your decking project.

Eric

[Jimmy2]

Hey Eric,

Thanks for your input. I saw some other projects where they'd applied a membrane to the top of their frame and thought it was a great idea. I didn't realise it came in a tape form which makes application dead easy. The Betta membrane I'll also be applying to the posts, but probably not the sides and bottom of the frame as the water will drain off to gravel spread underneath.

As we start to progress the project, I'll post it's progress. However it might be a few weeks before we get going.

Cheers,

Jimmy.