Span table / calculations for 85mm joists

[Driver23]

Hi,

I am hoping to be able to use existing loft joists in a loft conversion without putting in a whole new load of joists. The middle 3 feet of the room is supported by staircase walls below, then if I put steels 6 feet further out on both sides (behind or below new roof supporting dwarf walls) and use hangers off them to support the existing joists (bit of damage to ceiling below but that needs underboarding anyway) that would give me a room 15 feet wide and lose no head height/ save on new/twinning joists.

The (Trada?) domestic span table I have goes as small as 97mm x 47mm joists at 400mm centres - the span given is 1.92 metres for C16. Mine are 85mm x 47mm at 360mm centres. (Timber is up to 150 years old, so am assuming it as equivalent to C16 not C22)

Does anyone know if there is a table for 85mm joists? Or a bit of software to do the calculation? Or is 85mm just too thin? It would save a fair bit on timber and work doing it my way, but most of all save valuable headspace.

Thanks in advance.

 

[noseall]

85mm joists!

You avin a larf!

You wouldn't even be able to drill 'em for cables or owt!

There is a reason why anything less than 100mm is not listed.......

 

[Driver23]

Drat. Couldn't just call it '97mm extra regularised'?

Cables could be run along the ends above supporting walls and then in between the joists.

I'll check what Building Control say but looks like it might be back to the drawing board and a few thousand extra for the sake of 12mm of thickness.

Sigh.

 

[DevilDamo]

Does anyone know if there is a table for 85mm joists? Or a bit of software to do the calculation?

That table/software is what is known as a structural engineer

 

[RonnyRaygun]

Just done a quick calc and from a stress and deflection point of view it looks like you might get them to work.

However, as noseall says, you need enough joist above and below to run cables etc. Not sure BC will allow you to keep them for this reason.

Either way you will need to get the calcs done on the steels so your engineer will do the calcs on the joists at the same time...and almost certainly specify something bigger.

 

[Driver23]

You're right! Just spoke to building control and they said there is no minimum, it all depends on spans and loadings. He said the TRADA tables are guidelines with a built in safety margin, they are not the only thing BC will consider. So if a structural engineer shows them the figures are OK then BC will allow it.

So it 'might' be that this is still possible... I'll post the outcome in due course in case it's of interest to any other DIYNOTers who don't weigh too much.

 

[RonnyRaygun]

We would never specify new joists of that size for a loft conversion, but if they work stress and deflection wise, and you can persuade your engineer and BC to let you keep them, then you'll be OK.

 

[DevilDamo]

We would never specify new joists of that size for a loft conversion

Is that because they're not listed in the TRADA tables But like you, I agree.

 

[RonnyRaygun]

Never done a loft conversion where we've had 6 foot spans to be honest!

But then, don't think anything less than 50x100 is practical. Usually it's at least 50x150 due to the depth of the steelwork.

 

[Deluks]

He hasn't mentioned steelwork as yet. Maybe he's thinking of supporting the roof off of the current joists using his 'dwarf walls'.

 

[geraint]

Never done a loft conversion where we've had 6 foot spans to be honest!

But then, don't think anything less than 50x100 is practical. Usually it's at least 50x150 due to the depth of the steelwork.

what has any timber size got to do with the steelwork...

 

[RonnyRaygun]

what has any timber size got to do with the steelwork...

Why use 100mm timbers when the steelwork is 152 UC sections? Most people want a flat ceiling so much easier to use joists at least that size and screw the floorboards and plasterboard straight on.
Also gives you a stiffer floor which most people prefer.

Sometimes headroom is an issue and the steels might be hidden behind the walls and smaller joists used, although for the sake of a couple of inches rarely worth worrying about.