Cutting ends of new loft floor joists

[antonyc]

Why can't you cut it like this?

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• tony1851
• 6 May 2015
• 1

Hang on a sec. I've not drawn the image correctly.

This is more like it. I forgot to draw the birdsmouth.

 

[antonyc]

Why can't you cut it like this?

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• tony1851
• 6 May 2015
• 1

Hang on a sec. I've not drawn the image correctly.

This is more like it. I forgot to draw the birdsmouth.

Hopefully, you cansee what i mean now!

 

[tony1851]

If you can shove the joist as far in as it can go, and cut to the angle of the roof, then assuming you still have about 4" of joist left vertically above the inner face of the wall plate, then that would be OK.

Remember that the shear stress at the ends of domestic floor joists is usually small. Also, because it is in a loft, your joists will not be carrying the full domestic loading of 30lbs/sq ft over the whole span. The logic of this is that you can't get heavy loads such as wardrobes etc down into the restricted space at the eaves. I would assume normal loft loading of around 6 lbs.sq ft for about 2 - 3 feet out from the eaves. This further reduces your load, and hence shear stress, considerably.

 

[antonyc]

If you can shove the joist as far in as it can go, and cut to the angle of the roof, then assuming you still have about 4" of joist left vertically above the inner face of the wall plate, then that would be OK.

Remember that the shear stress at the ends of domestic floor joists is usually small. Also, because it is in a loft, your joists will not be carrying the full domestic loading of 30lbs/sq ft over the whole span. The logic of this is that you can't get heavy loads such as wardrobes etc down into the restricted space at the eaves. I would assume normal loft loading of around 6 lbs.sq ft for about 2 - 3 feet out from the eaves. This further reduces your load, and hence shear stress, considerably.

Thanks Tony, do you have any documentation or anything which can support that because I want to have some sort of explaination to show if the council get involved at some point. Antony

 

[RonnyRaygun]

Antony,

If you've got a shear area of 100x50mm (vertical height X width), you've got an area of 5000 square millimetres for shear.

Taking C16 giving an allowable stress of 0.67N/mm2 and multiplying by 1.1 load sharing factor to give 0.737N/mm2, this gives you an allowable shear load of 5000x0.737x10^-3 = 3.685kN per joist.

Assuming a dead load of 0.6kN/m2 and an imposed of 1.5, conservatively along the full length of the joist, and assuming 450mm spacings, this gives an applied load of: (0.6+1.5)*0.45 = 0.945kN/m.

3.685 / 0.945 = 3.9m. This means you can have a joist 7.8m long (3.9m of load to each end of the joist) and you still won't exceed the allowable shear load. Obviously your joists will be significantly shorter than this.

In summary, it's unlikely that any sensible BCO would question the detail, but if he does, the calc you need is above

 

[tony1851]

@OP - damn, he's beaten me to it as usual

Go with those figures and you won't go wrong - just keep fingers crossed you don't get a dim inspector!

 

[antonyc]

Antony,

If you've got a shear area of 100x50mm (vertical height X width), you've got an area of 5000 square millimetres for shear.

Taking C16 giving an allowable stress of 0.67N/mm2 and multiplying by 1.1 load sharing factor to give 0.737N/mm2, this gives you an allowable shear load of 5000x0.737x10^-3 = 3.685kN per joist.

Assuming a dead load of 0.6kN/m2 and an imposed of 1.5, conservatively along the full length of the joist, and assuming 450mm spacings, this gives an applied load of: (0.6+1.5)*0.45 = 0.945kN/m.

3.685 / 0.945 = 3.9m. This means you can have a joist 7.8m long (3.9m of load to each end of the joist) and you still won't exceed the allowable shear load. Obviously your joists will be significantly shorter than this.

In summary, it's unlikely that any sensible BCO would question the detail, but if he does, the calc you need is above

Thanks!!!

So I may not need to drop the ceiling then or even need joist hangers?

I'm going to make a proper plan to scale..

 

[tony1851]

So I may not need to drop the ceiling then or even need joist hangers?

We kept trying to tell you that!

 

[antonyc]

So I may not need to drop the ceiling then or even need joist hangers?

We kept trying to tell you that!

I know you did but I'm hesistant as builders umm and arrr and the council are the same... I'm going to get a proper to scale plan and I'll see... No use messing about without having the exact measurements, etc. I can see that this is more precise than I originally thought.

 

[antonyc]

I'm armed and ready!

Here are my plans, I created them using AutoCAD.

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• antonyc
• 7 May 2015
• 1

 

[antonyc]

Hmmm, they aren't large enough on here... I'll upload them to my server.

You'll be able to see them here:

http://accbs.co.uk/misc/closeup.bmp
http://accbs.co.uk/misc/current.bmp
http://accbs.co.uk/misc/proposal1.bmp
http://accbs.co.uk/misc/proposal2.bmp

Thanks everyone!

 

[RonnyRaygun]

The problem you have with your proposal is that you are planning to take load from the roof onto the floor joists, which means the shear load will increase significantly.

A better option may be to beef up the central purlins to take the load from the rafters once the struts have been removed.

To be honest, it's not something you should be doing without input from an SE (and I don't just mean on a forum), and building control will expect to see calcs that prove the new structure works.