Loft floor joists for long span

[Logidex]

Hello,

I've seen that there are a lot of threads regarding floor joists for a loft conversion. But I couldn't find one that discussed the issue I am concerned about.

I'm going through the process of designing a loft conversion which will have two very large rooms.

One of the rooms will require floor joist to span a distance of 5.4m with both ends resting on load bearing walls. The other room is close to 5m.

The architect originally specified doubled up 75x200mm joists making 150x200 joists. But I'm worried that the joists will be working at their practical limits and the last thing I want is for the loft to be built with floors like spring boards by which time the architect and builder will have had their money.

I've asked the architect to look into it for me but he seems to have lost interest.

So I have been trying to find cost effective solutions for ensuring the floor will be solid.

I've been in touch with Telebeam who estimated their aluminium floor joists would cost £11k plus VAT although it wasn't clear whether this was for one or both rooms. I don't need the telescopic property of their joists so this was for just the middle sections. This strikes me as being quite expensive but I haven't been able to find alternative suppliers for aluminium joists.

I'm not after specific measurements at this stage. I just want to know what alternatives there are to using just wooden joists.

Easi-joists are no good for me because the max depth I can use is 200mm and they would need to be deeper than that.

I thought there must be a solution that can use steel box sections with wooden battens on top but apparently this is too radical an idea.

The only other thing I can think of is to use flitch beams instead of simply doubling up the joists.

Does anyone have any suggestions?

 

[tony1851]

I've asked the architect to look into it for me but he seems to have lost interest.

Not surprising, really, it's an SEs job to size joists where loading/deflection might be critical.
Are you building a dormer, or just working within an existing roof void? The reason is that if you are within an existing void, then presumably you won't be loading the floor to full domestic loading right into the eaves.
This can be allowed for and reduce stress/deflection of the beams.

 

[Blagard]

Sounds as if height is a problem if 200mm joists are being considered (even doubled up).
Going for deeper joists may solve the problem.

Really this seems like a situation where an Architect and Engineer need to get their heads together to come up with a practical solution.

 

[Logidex]

Thanks for replies.

Architect and SE are one and the same.

There will be a large flat roof dormer above the two rooms. The face of the dormer will be built off the rear wall of the house. The cheeks will be built off flitch beam (parallelt to floor joists) in the larger room and tripple joist in the small one again parallel to floor joists.

I will have 2400 floor to ceiling. I can in theory eat into this but not keen to do so if it can be avoided.

 

[RonnyRaygun]

You would need to have a genuine 100x200mm C24 section to limit your deflection to 12.1mm.

Regularised sections are smaller on both axes - if they have been regularised to 47mm you will end up with a 200x94 section and your deflection will go up by 1mm.

NHBC guidelines recommend limiting the deflection of floors to 12mm, so it's a bit tight.

You could use a series of flitch beams. Even a 3mm plate in between the timbers would double the stiffness of the section.

 

[tony1851]

Regularised sections are smaller on both axes - if they have been regularised to 47mm you will end up with a 200x94 section and your deflection will go up by 1mm.

If you have 2 identical sections side-by-side, IIRC you can increase 'E' by 1.14,
so maybe the deflection would be a little less than having a solid 100x200?
Just wondering.

 

[RonnyRaygun]

Regularised sections are smaller on both axes - if they have been regularised to 47mm you will end up with a 200x94 section and your deflection will go up by 1mm.

If you have 2 identical sections side-by-side, IIRC you can increase 'E' by 1.14,
so maybe the deflection would be a little less than having a solid 100x200?
Just wondering.

good point, make that 10.6mm. I think that's fine for 5.4m, especially when around 25% will already have been taken up by the dead load.

 

[Logidex]

Again, thanks for replies

Architect/SE specified 75x200 C24 doubled up, not 50x200. Does this mean that what the architect has specified is well up to the task?

I'm liking the idea of flitch beams with 3mm plates. Would it also means using less thick timber e.g. 50x200 either side of the steel (subject to checking of course)?

 

[tony1851]

Would have thought that buying lengths of 3mm x 200mm steel, plus drilling and bolting, would have been more expensive and time-consuming than simply upping the width of the timber and nailing them together.
Plus you would have the SE's fee for doing the calcs and detailing, unless you get Ronny to do it, in which case I'm sure he won't charge!

 

[freddiemercurystwin]

Or just bang some steels in and use some standard size joists, like 98% of other loft conversions, even with a few hundred quid spent on a proper SE to do the job rather than the OP's Bellend of a designer.

 

[jockscott]

Are we assuming the rooms below the proposed loft conversion are of a similar size? If not then surely the dividing walls below could potentially offer some support and decrease the actual span ??

 

[Logidex]

I've only paid architect/SE for the planning app drawings so far so I think he will be happy to check calcs for floor joists.

There is another block wall in room below about a third of the way in from one end but it's not load bearing.

 

[freddiemercurystwin]

One hopes it actually required planning approval.

 

[excited]

google telebeam

 

[tony1851]

google telebeam

and then google 'bankruptcy'.