Loft conversion flitch beams/floor joists

[Gemma_123]

Hello, hoping someone can help please.
I saw on another forum (old thread therefore not getting a response) about the following regarding floor joists in a loft conversion -

“I gain 4 inches of head height by using the existing 4 inch floor joists (instead of new 8 inch joists as is the normal specification) and doubling up each joist with C24 4x2s and adding a flitch in the middle as a sandwich. This gives you the strength required but gains you 100mm in head height. No need to lower the floors. All building control and engineer approved.

The flitches tend to be 8-10mmx100mm with staggered bolt holes using single sided TP connectors.

I've done this maybe 8-10 times, engineer designed, building control passed. Every one with a dormer.

I don't know of anyone else that uses this approach. It is designed to take the load of a dormer, in conjunction with using 3x 152x152 section steels and a ridge beam of course. The two outer steels have the dormer walls sitting on them so you don't lose head height because of them. The centre steel has the flitched beams sitting within the webbing and going 50cm into the spline wall below.”

Has anyone done anything like this? Am hoping to do the same to save head height, but I’m struggling to find any references to this being done?

Any help gratefully received!

 

[^woody^]

engineer designed

Could that be a clue?

If someone can justify the components and the loading then that's fine.

The ceiling will be wrecked, the sound transmission may be suspect, and installing services will be problematic.

 

[jks7492]

Its not common but yes you could do it, but will all need to be designed by a structural engineer. It might save some floor depth but it will end up costing you at least twice as much.

8 inch joists aren't the 'normal specification' for floor joists. The size of a floor joist is based on the span, not a standard size for all applications. Normally with a loft conversion you'd use a wider but shallower joist to maximise your head height.

 

[Gemma_123]

Could that be a clue?

If someone can justify the components and the loading then that's fine.

The ceiling will be wrecked, the sound transmission may be suspect, and installing services will be problematic.

Thank you. Appreciate the advice. I’m trying to save 100mm head height somewhere - I’m looking at long leg/speedy hangers, could these be put in on top of the wall plate and suspended so the top of the new joists line up with the old and go 100mm under? (Whole house is being renovated so mess not an issue) I have a bedroom wall inbetween the front and back of the house that could be used as support in some way. Or any other suggestions??

 

[^woody^]

Or any other suggestions??

The roof.

Rafter and insulation thickness or dormer height.

Raise the roof under the recent Permitted Development new rules.

 

[tony1851]

The roof.

Rafter and insulation thickness or dormer height.

Raise the roof under the recent Permitted Development new rules.

What new PD rules are those?

 

[^woody^]

What new PD rules are those?

The ones from 1st September 2020.

 

[Gemma_123]

The roof.

Rafter and insulation thickness or dormer height.

Raise the roof under the recent Permitted Development new rules.

Can’t change the roof height due to conservation area unfortunately. Thoughts on using hangers over the wall plate?

 

[^woody^]

These threads are great when bits of information are trickled in.

I always feel its best to have all the information first, all the constraints, all the objectives, all the reasons and then suggestions can be made without going through the "what about .....?" process.

 

[freddiemercurystwin]

What is ....... the existing ceiling joist to ridge height?

 

[Gemma_123]

These threads are great when bits of information are trickled in.

I always feel its best to have all the information first, all the constraints, all the objectives, all the reasons and then suggestions can be made without going through the "what about .....?" process.

Appreciate that - to be honest, I wasn't expecting such a speedy and detailed response! I'll start from the beginning..

Attached are my current plans/elevations and structural plans.
Now I've seen the head heights on the side sectionals, I'm hoping to save height in the loft by a different way of doing the floor and possibly the rafters (currently 75mm deep with a purlin and struts). I'm not changing that side of the roof but annoyingly the purlin is slightly in the way of where the Velux would go over the WC, so I think this has to be moved?

This is all so I can change the layout and to allow for the stairs to come up keeping the 1.9m head room, and therefore put the en suite in as attached in the updated plans.

I hope this all makes sense!!

So possibilities for the floor could be - notching out the end of floor joists, sitting them on the wall plate and supporting the bottom section with joist hangers - although I don't know if building regs would allow that?

Or installing steels above the ceiling rafters and using joist hangers to fix the new joists to them below the support/underslung.

Or indeed, another way of saving room??

Am in a conservation area, so not much wiggle room on ridge heights etc

 

[cdbe]

If headroom is tight the first thing I'd look at is ceiling height on the first floor - anything above 2.3m is fair game for the new loft joists, it's easy enough to achieve in practical terms, especially if the ceilings are coming down anyway. Hanging from steels and hangars off wall plates are accepted methods.