Thread: need advice - deck 'subfloor' layout/design

Firstly, a hello to all, I've lurked for a long time and joined today in the hope of some advice regarding a deck I'm planning to build.

SWMBO and I are currently about to start building a house - they should poor the slab in the next week or so, and the intention is to build a deck after handover.

The plan is for the deck to stretch from the back of the house, to the back fence of the block (approx 4.6m) and from one side of the block to the other (approx 12.6m)

My thoughts are to use H4 TP 90mmx90mm joists on stirrups with H3 TP 90mmx45mm joists supported on joist hangers between bearers.

as the deck will be very close to ground level - possibly even touching the ground in some places, i also intend to coat all timber in black paint on H4 treatment stuff. (probably not the technical name )

actual decking will be mixed Aus hardwoods.

I'm also looking to source the bearers in 5.4m lengths and cut them down to size so they are a continuous beam with no joins

post assembly, all joints between bearers and joists to be coated with more paint on h4 treatment.

I've drawn the whole lot up in sketchup and attached some screenshots with dimensions in the hope that those more knowledgeable than me might point out any glaring mistakes or, hopefully, tell me that the design will work.

Red parts are the 90x90 bearers and the three different coloured blues are different length 90x45 joists

Stirrup spacings


Joist and bearer layout (the two square holes are to go round trees)


Joist spacings


If anybody needs anymore info let me know.
I await you critiques (be gentle)

Regards,
Paul

May I just ask, is there a possibility to take away some of the soil so you do get a more reasonable clearance beneath the deck? Not sure what stage the site is at but if it's early stages and there is excavation still happening, could be the right time to do this.

I agree with Shauk, you really should keep the timber out of the ground. And excavation will also provide the opportunity to ventilate the deck which is also very important.

Instead of 90x90 bearers I recommend 2/90x45. This is a better option as 90x90 is sometimes not fully dried after treatment and if it bows or twists even a small amount as it dries you will really struggle to make it work.

Originally Posted by shauck

May I just ask, is there a possibility to take away some of the soil so you do get a more reasonable clearance beneath the deck? .

there may be in which case I will use a traditional joist over bearer construction with a 'deeper'' profile to the bearer. However, it will come down to seeing if it can be done by whomever is subcontracted to clear the site as otherwise it will be considered an alteration to the build contract.

Originally Posted by Phil B

I agree with Shauk, you really should keep the timber out of the ground. And excavation will also provide the opportunity to ventilate the deck which is also very important.

Instead of 90x90 bearers I recommend 2/90x45. This is a better option as 90x90 is sometimes not fully dried after treatment and if it bows or twists even a small amount as it dries you will really struggle to make it work.

Fair enough. My thoughts for using the 90x90 is that it was H4 treated but I'm guessing if i used to 90x45 pieces painted in black goo that would have the same effect. I think the whole lot will clear the ground without excavation but only by an inch or two



Thanks for the help guys

Discuss it with your builder. You may even be able to hire someone separately to do this, although your builder may be able to do it cheaper if he already has someone on site with the equipment. If you get the clearance right, you won't have to use H4.

Quick question: If the pour has not yet happened, and it is that close to teh ground, is it worth extending the pour and having a concrete patio as opposed to a wooden deck?

Lots cheaper, and heaps less effort?

I think, just having had a quick look, that you should be able to get away with something much cheaper than a 90x90 as a bearer.
I have written a guide to how to properly calculate your internal timbers for your deck on our site's blog. It has the correct sizes for low level decks outlined, and explains the concepts behind the calculation of floor load widths etc Hopefully it might be of assistance/save you some cash
How to Build a Deck – Step One

I would change the layout. Run 3 bearers - outside , centre and against the house then run the joists between these on hangers. In the current layout you will need so many joist hangers it aint funny, 260 at a quick count. If you can get the clearance by excavating a bit, then do joist over bearers with triple grips.

Originally Posted by Micmac

Quick question: If the pour has not yet happened, and it is that close to teh ground, is it worth extending the pour and having a concrete patio as opposed to a wooden deck?

Lots cheaper, and heaps less effort?

Ahh yes... but the boss says she wants a deck

Originally Posted by Softwoods

I think, just having had a quick look, that you should be able to get away with something much cheaper than a 90x90 as a bearer.
I have written a guide to how to properly calculate your internal timbers for your deck on our site's blog. It has the correct sizes for low level decks outlined, and explains the concepts behind the calculation of floor load widths etc Hopefully it might be of assistance/save you some cash
How to Build a Deck – Step One

Thanks softwoods - I'll have to sit down and read but the link looks promising.

Originally Posted by ringtail

I would change the layout. Run 3 bearers - outside , centre and against the house then run the joists between these on hangers. In the current layout you will need so many joist hangers it aint funny, 260 at a quick count. If you can get the clearance by excavating a bit, then do joist over bearers with triple grips.

Wouldn't that mean a 2.6m joist?? and I've still got to run joists at 400 - 450mm between centres. So I'll still have a lot of joists just running the other way. Or have I missed the point