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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.
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
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.
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
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
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.
