C24 Joist span weights

[Dylan T]

Hi all - doing an order for timber for a suspended decking frame I'm doing for myself, and haven't got a huge amount of height to work with so planned to use 2x4's (47x100mm). The merchants have got treated c24 lengths. To check this was strong enough I've used this c24 calculator which gives max spans to BS 5268-7.1.

Do these max spans cover any risk of breaking, or just movement. For example if I don't exceed these am I ruling out failure but the deck could bounce around a bit, or is it safe to assume it won't move?

I've also had a go at working out the loads but feel I need a degree for that; if anyone can point me to a tool or easy formula for this that would be great. I'm using composite decking.

 

[cdbe]

If you use an online decking calculator produced by a decking company you should be able to size the joists, they will be based on C16 so increase span by maybe 10% for C24. Obviously it's easy to reduce the span using intermediate supports.

I'm not sure how relevent domestic load span tables would be - on the one hand a wild hot tub party might exceed what you'd ever have in your house, on the other hand you don't have to worry about a bit of bounce cracking the ceiling below. The span tables are more concerned with limiting deflection than snapping - wood will bend a long way before it breaks.

Have you looked at the composite joists? They will (hopefully) have been designed specifically for a typical decking load/span.

Don't forget your fall, especially if you're being tight with the joists - you don't want puddles in the middle!

 

[tony1851]

If it's in a domestic setting, your 4x2s will span around 2.1m @ 400 centres. Go much more than that and your deck won't collapse (unless the load was very high!), but will have a tendency to bounce when walking across it.

 

[Dylan T]

If you use an online decking calculator produced by a decking company you should be able to size the joists, they will be based on C16 so increase span by maybe 10% for C24. Obviously it's easy to reduce the span using intermediate supports.

I'm not sure how relevent domestic load span tables would be - on the one hand a wild hot tub party might exceed what you'd ever have in your house, on the other hand you don't have to worry about a bit of bounce cracking the ceiling below. The span tables are more concerned with limiting deflection than snapping - wood will bend a long way before it breaks.

Have you looked at the composite joists? They will (hopefully) have been designed specifically for a typical decking load/span.

Don't forget your fall, especially if you're being tight with the joists - you don't want puddles in the middle!

Thanks. There's little in the way of info on the joists themselves from my decking supplier; they do sell composite frames but its cost prohibitive and from what I can see it requires loads of their plastic supports so doesn't seem a great option. I've followed their advice of max 400mm centers and yes they advise a fall a of .05% every m which I will be following.

I do want to avoid bounce; I only really have room to from 47x100 to 47x125 which is obviously a jump in cost. Would you think that would make a huge difference?

 

[Dylan T]

If it's in a domestic setting, your 4x2s will span around 2.1m @ 400 centres. Go much more than that and your deck won't collapse (unless the load was very high!), but will have a tendency to bounce when walking across it.

Thanks; I am using 400 centers and and my unsupported spans will be less than that (i can always stick a few more supports in). The other consideration is the thinner the joist the more likely some wont be true...hmmm

 

[Dereekoo]

Just a heads up on the TRADA tables are related to timbers in a dry inside environment (Service conditions 1 and 2) your decking will be open to the elements (Service condition 3 ) which tends to reduce the allowable stresses of the timber used in calculations by about 80%.
Just to give you a feel for the things based on Tonys 2.1m span and standard floor loading of 150kg/m2 (2 men standing on every square metre of decking)
Service (1/2) Deflection 50 x 100 = 3.6mm Service 3 = 4.5mm
Service 3 Deflection 50 x 125 = 1.9mm Service 3 = 2.4mm

Of course this is all theoretical, loading/span will make a difference

 

[^woody^]

Just a heads up on the TRADA tables are related to timbers in a dry inside environment

And are ultra conservative.

 

[Dereekoo]

And are ultra conservative.

As all things structurally tend to be with FoS everywhere

 

[jks7492]

How big is the joist span you're actually proposing?

 

[Dylan T]

How big is the joist span you're actually proposing?

For the most part nothing exceeds 1.5m but theres a few cases of 2m; currently thinking of changing those instances with an extra support

 

[John D v2.0]

The merchants have got treated c24 lengths

How long are you hoping the deck to last? And how quickly will the joists dry out after rain/dew?

If not specified the treated timber are likely to be designed for indoor unheated spaces such as roof voids, but not outdoor. Depending how long you want your decking to last you might want to get some better treated joists, wickes do them for delivery in 6 by 2 but I'm sure other places do 4 by 2. https://www.wickes.co.uk/Wickes-Incised-Exterior-Grade-Timber-Joist-47-x-150mm-x-3-6m/p/256401

https://www.tdca.org.uk/timber-decking/timber-preservation/