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What timber spans for single layer deck frame?

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.

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Comments

JacobZ

Hi @Jimmy2,

Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.

You'll have to look at two separate tables that work with each other. This Bearer Span Table alongside this Joist Span Table provides all the info I use to design decks.

I think a lot of the confusion can come from knowing what makes a bearer a bearer and a joist a joist.

A bearer is vertically supported by a post, whereas a joist is supported by a bearer. A joist can sit on top of a bearer and be attached with a joist strap or be attached on the same level with a joist hanger.

140x45 H3 Pine used as a bearer can span 1400mm between posts and support joists at 450mm centres. 140x45 H3 Pine used as a joist, can span 2600mm between bearers.

Let me know if you require any further assistance, more than happy to help.

Jacob

Jimmy2

https://www.workshop.bunnings.com.au/discussion/comment/43993#Comment_43993

Hi @JacobZ

Thanks for the quick reply. I think you might have missed what I was talking about. I know exactly what the difference is between a joist and a bearer and their functions. However as I said, I'm doing a combined single layer frame. The joists and bearers are combined into one. Please have a look at the link I provided:
https://www.workshop.bunnings.com.au/t5/Featured-Projects/Low-level-deck-and-pergola/ta-p/167281
You'll see @jase2 has done the same thing with his deck. So I guess the big question is how did he determine his spans.

Thanks

Jimmy.

jase2

https://www.workshop.bunnings.com.au/discussion/comment/84109#Comment_84109

Hey Jimmy

Interesting plan you've got. Here's the span table I used - with the floating deck and hidden posts, I needed to be a bit creative. I basically used 140x45 for all the bearer/joist combos, but with spacings for my spans and cantilevers based off the 120x45 measurements in the below span table, so that everything was a level of strength higher than they needed to be. My basic philosophy was that the span tables were the maximum allowable so if I go one level up then it will be extra strong. Plus I threw in some extra posts based on initial feedback on the webpost you linked to, cause it never hurts to have extra posts.

https://www.demak.com.au/wp-content/uploads/2013/08/tabma_span_tables_v1_0_june_2007.pdf

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/84109#Comment_84109

Hi @Jimmy2,

In @jase2's project, for simplicity's sake, they have five joists that span across the deck and the decking is run perpendicular to those. Between those, they have noggins. Theoretically what you have explained is possible, to have a series of bearers running across the span and have joists between those at the same height, but that would mean, at some points, running decking boards over the top of bearers that are running in the same direction as them. We typically don't do this in deck building as it creates locations where water can be trapped.

Regardless of how you are constructing the deck, the span tables work the same. They dictate the capability of a certain size of timber to span from point A to point B. Whether A and B are between bearers or between the frame doesn't matter. If you can take one of your decks, perhaps the largest one and draw a sketch of how you will place these joists within the bearers, I can look at it for you and tell you if it will work.

Please let me know if you have any questions.

Mitchell

Jimmy2

https://www.workshop.bunnings.com.au/discussion/comment/107054#Comment_107054

Hey jase2

Thanks for the tips and the span table. That'll help me a lot in getting things figured out.
I'm going to draw one up and post it to see what @MitchellMc thinks of my design – which will be based on yours.

Jimmy2

https://www.workshop.bunnings.com.au/discussion/comment/107056#Comment_107056

Hi @MitchellMc

Brilliant thank you. I get your comment about running the boards in the same direction as a few of the bearers. But that was what @jase2 did with his design and surely everyone that goes for a top feature 'picture frame' board around the edge, or a fascia board would run into the same issue.

I've attached a few reference photos that all feature edges where water could be trapped under an edge decking board. If I seal the tops of the offending end bearers with bitumen paint and the underside of those boards, it would minimise water issues.

I'd very much appreciate you checking over my framing plan, that would help me a lot! I'll get onto it ASAP.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/120744#Comment_120744

It sounds like you're on the right track @Jimmy2. There's some terminology that gets' interchanged with a single-level and dual-level deck. You might call the timbers bearers and joists, whereas I might call them joists and noggins. There's also not one way to build a deck. At the end of the day, provided the design adheres to basic building standards and span tables are used to ensure the calibre of timber is suitable, it is just about minimising costs for the project.

Once you've done that sketch, I'll be able to provide feedback on whether it could be done more simply or whether you're spot on.

Looking forward to seeing it.

Mitchell

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/120744#Comment_120744

Hi @Jimmy2,

Was this similar to what you were thinking? If so, I believe it would work. I've used 140 x 45mm supported twice over the 3966mm width as the bearers and 90 x 45mm spaced at 450mm as the joists. Bearers are spaced every 1200mm across the 7100mm length.

Please let me know if you have any questions.

Mitchell

Jimmy2

https://www.workshop.bunnings.com.au/discussion/comment/129217#Comment_129217

Hey Mitchell,

Wow, I wasn’t expecting you to go and design it for me! Thank you.

What you’ve designed is very similar to what I was thinking. But I do have a couple of questions.

First, does your design not feature any double timbers fastened together?

Secondly, because of the single frame design (bearers and joists together), your design means that all of the joists are effectively great big noggins. Therefore for strength and rigidity the decking boards aren’t fastened to any joists running the full length.

So, I’ve been busy drawing up what I have in mind that uses the lateral strength of the long joists to fasten the deck to. I think it might be ‘over spec’.

Continuous spans between the stumps are 1545mm with a 500mm cantilever over each end and a 400mm cantilever over each side.

The joists are at 430mm centres.

Additional double joists run down the centre and are supported by extra stumps. There are also additional doubles down each side. It might be overkill?

Noggins are in four rows that are 1550 apart.

The blocked double joists allow for the corner posts to be concealed and a fascia installed to hide underneath.

So see what you think and please tweak, delete, alter or change anything you think is unnecessary.

Thanks,

Jimmy. Main Deck Framing.png

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/134596#Comment_134596

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

https://www.workshop.bunnings.com.au/discussion/comment/138062#Comment_138062

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

https://www.workshop.bunnings.com.au/discussion/comment/140414#Comment_140414

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. Main Deck Framing 2.png

EricL

https://www.workshop.bunnings.com.au/discussion/comment/142119#Comment_142119

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

https://www.workshop.bunnings.com.au/discussion/comment/143342#Comment_143342

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.

JesseS6

https://www.workshop.bunnings.com.au/discussion/comment/144254#Comment_144254

Hi @Jimmy2 ,

What did you use to produce these drawings? I'm looking for something to do this and not sure which way to go

Jimmy2

https://www.workshop.bunnings.com.au/discussion/comment/144957#Comment_144957

Hi @JesseS6 ,

I just used Adobe InDesign.

I didn't want to use one of the complex design programs that architects and draftsmen use so I went with InDesign for it's ease of drawing simple diagrams. Plus you can export in a range of formats, you can overlay photos and other info and heaps more.

Hope that helps.

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