Decking joists

[Logan]

I’m looking to build a 3mx5.7m deck and wondering what is the minimum height of C16 joists I can get away with that will safely support the structure? 

 

The reason being, I’d like to keep it as low as possible due to it being the highest point of the garden already.

 

Thanks in advance!

[ProDave]

Unless it's high off the ground, which you don't want, you can give the joists as many intermediate supports as you like, and 4*2 would do nicely.

[Conor]

As Dave says, 4x2s, supported every 1.2m or so with bits of block /pavers.

[JohnMo]

Really depends on how solid you want it to feel and how they are supported and the span they have stretch across. Outs is off the ground so attached to ground in a limited way via pillars.

 

Most of ours are in in either 47x225 or 47x175 on 400mm centres. The 175mm can span about 3m or so and the 225mm 4m+.

 

If on the ground mounted as other responders have noted.

[Logan]

Thanks all for the responses. 
 

1. Does anyone know the clear span lengths for C16 2x4” (45x95mm) and 2x5” (45x120mm)? Needing to work out how many posts I require for support.

 

2. How should I secure bearers to the corner posts as the bolts/screws from one bearer will be in the way when trying to secure the second?

 

3. How should the bearers be joined/secured at a post if they need to be extended?

 

Thanks in advance!

[Mr Punter]

Just use https://www.timberbeamcalculator.co.uk/en-gb/span-table/floor-joists?load=1.5&class=C24

[Gus Potter]

There are two aspects to this. Generally if it is more than 600mm above the ground then it becomes a "structure" and thus it needs to be "structurally" designed. One reason for this is that beyond this height it can also sway sideways.

 

In my day job as an SE I often design decks that are above 600mm in height. To roughly size the timbers I use an old rule of thumb for joists at 600 mm centres which is:

 

Joist span 3000 mm.

 

Divide by 25mm to convert to inches.. 3000 / 25 = 120 inches.

 

Divide this by 12 to convert to feet = 120 /12 = 10 feet.

 

Half of 10 feet = 5.. if it is a domestic application and I want to be safe.. and make sure I can later justify the fixings for the handrails etc I'll add 2 to that making 7 inches so I need a 170 x 50 joist.

 

This is a thing that folk often miss.. make you floor deck too thin and you'll have problems later with the hand rails and other stuff. Skimping on the decking joist depth can be false economy.

 

 

 

 

 

 

 

 

[ProDave]

  On 29/07/2024 at 21:47, Gus Potter said:

Half of 10 feet = 5.. if it is a domestic application and I want to be safe.. and make sure I can later justify the fixings for the handrails etc I'll add 2 to that so I need a 150 x 50 joist.

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Explain this bit please, I am clearly having a senior moment?

 

I see 2 + 5 =7 but how does that get to a 150 x 50 joist?

[Gus Potter]

  On 30/07/2024 at 07:59, ProDave said:

Explain this bit please, I am clearly having a senior moment?

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No your not Dave!

 

That is an error, 2 + 5 = 7 inches ~  170mm deep. Can you edit at your end?

 

 

[ProDave]

  On 30/07/2024 at 09:57, Gus Potter said:

No your not Dave!

 

That is an error, 2 + 5 = 7 inches ~  170mm deep. Can you edit at your end?

 

 

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Done.

[Dave Jones]

4x2 with 100x100 posts conced in ground every 1200 will be rock solid. you can allways put a length of 4x2 both sides of the posts if you went mega rigid for the little extra cost.

 

Dont bodge it by sitting on bits of block.slabs etc.

[Gus Potter]

  On 30/07/2024 at 10:23, ProDave said:

Done.

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Big thanks from me Dave for sorting that out.