# Loft Conversion Floor Joist Sizeing

Discussion in 'Builders' Talk' started by Drc200, Oct 26, 2006.

1. ### Drc200Member

I have a typical 1950's ex council house. I am thinking of converting the loft and need to increase the existing ceiling joists to support a floor. The existing joist seem to run in one length from front to rear of the house. There is a wall running across the house in the centre but am not sure if this is used to support the joists.

Can you tell me when calculating the joist size wheteher the existing joists affect the calculation or do I just need to calculate as a new floor?

Is it possible to run joists straight through like the existing , is there a maximum standard length of joist?

Thanks
Dave

2. ### jonah.New Member

You can get long ones but they are not stock items so you will end up paying more. What is the overall span you require

Jonah

3. ### foomanNew Member

well when you tell building control about it they will be able to tell you the sizes needed

4. ### jonah.New Member

From memory the longest I have used are 7.2m and these I think are limited to 225 x 50 sort of size at C16 grade (old SC3 ish). A 225 x 50 at 400 centres will only actually span just over 4.5m though.

Cheers - jonah

5. ### Willy DuwittNew Member

I have a typical 1950's ex council house. I am
thinking of converting the loft and need to increase
the existing ceiling joists to support a floor. The
existing joist seem to run in one length from front
to rear of the house. There is a wall running across
the house in the centre but am not sure if this is
used to support the joists.

Can you tell me when calculating the joist size
wheteher the existing joists affect the calculation
or do I just need to calculate as a new floor?

Is it possible to run joists straight through like
the existing , is there a maximum standard length of
joist?

Thanks
Dave

Firstly what you currently have are rafters ( 4x2's? )and not joists. They are there to form the ceiling and not intended to support a floor of any kind.

To do what you want, you will need to run new joists parallel to the rafters. These are likely to be 7x2 or greater, depending on your span. Building Control will tell you which you need.

The wall running down the middle of the house is likely to be a supporting wall and as such will be supporting the rafters. Assuming it is a structural wall, the new joists could also rest on it. You would therefore need 2 joists to span from front to back ie 1 from front wall to centre and 1 from centre to back wall. But do you really want the joists to go out to the outer wall? Have you got the height at the eaves to rest a new joist on the wallplate? Or would you be better off putting a beam across the loft and hanging your joists off that?

Is this going to be a proper loft conversion or just a boarded storage area in the loft?

Speak to your local BCO about what you intend to do and he should be able to point you in the right direction.

6. ### Â­New Member

> Firstly what you currently have are rafters ( 4x2's?)and not joists. They are there to form the ceiling and not intended to support a floor of any kind.

ROFLMAO

Rafters support the ceiling now do they!

7. ### trenchNew Member

Hello drc
Just a couple of pointers.
1. Your BCO is not there to design the job for you. He'll give you all the advice he can but you will soon hack him off if you hand him a blank piece of paper and a pen and say "Tell me what i need"

2. As a general principle, leave your ceiling joists alone and install a new floor above them, but see 3 below

3. If you are seriously considering a loft conversion get the thing designed by a someone who has designed loft conversions in the past and is up to date with the regs and building practices concerned. You'll end up in a mess if you seek abstract advice about the odd section of the works, here and there.

4. You'll need to consider escape windows, smoke detection, stair design, insulation & electrics and also any other structural alterations necessary to create enough space in the loft for your new room/rooms.

I hope I didn't sound patronising, that was the last thing i intended, but a loft conversion is a lot more awkward to design and build than a normal extension and you need to know exactly what is expected of you before you start.

8. ### Willy DuwittNew Member

Firstly what you currently have are rafters (
4x2's?)and not joists. They are there to form the
ceiling and not intended to support a floor of any
kind.[/b]

ROFLMAO

Rafters support the ceiling now do they!

That's me told!! Dont know what the fek I was thinking when I wrote that. Think I need a holiday!

9. ### jonah.New Member

Trench

Excellent post - good points well put sir

Jonah

10. ### Drc200Member

The centre wall is a supporting wall, The largest span is just under 4.3M.

I haven't checked the space at the wall plates, as to whether the joists will fit. I am not keen on the beams along the length and joists between system.

Will consider options further, may have to relent and get somebody to design it properly.

Thanks

Dave

11. ### nearnwalesMember

we do alot of loft conversions and the floors have to be totally dependent of your existing joist with a 25mm gap.

when the new joists are put in place we have to put chicken wire up and around every joist so the 100mm insulation doesn't bridge the 25mm air gap required.

12. ### yorkshireboyMember

I have never used this method.What is the purpose of this 25mm air gap ?

"we do alot of loft conversions and the floors have to be totally in-dependent of your existing joist with a 25mm gap."

Prevents sound transmission, and possible damage to ceilings below. The upstair's new floor has to be completely independent as said in post above.

Should be ok to fit the new joists in-between the existing, but not touching, and clear from ceiling below.

14. ### yorkshireboyMember

There is no reason for a gap between floor and ceiling joists in a domestic situation .Why is it OK to have a joist with plasterboard below and floorboard above at first floor but if you have a second floor a strange chicken wire and airgap situation is needed

15. ### nearnwalesMember

because attic joists ant think enough to carry a floor.

i think because bc want insulation between the joist for sound transmission they want it to breath (not sweat) hence the air gap.

and because the new sound regulations things have changed like you have to double plaster board ceilings and some walls.

16. ### trenchNew Member

Reason for chicken wire is probably to retain rockwool RW2/flexislab between joists to provide fire resistance to floor.

Its pointless leaving a breathing gap of any type if you don't ventilate it, but you won't get condendsation in a floor seperating 2 heated areas anyway.