I need some help figuring out how to hang joists for an attic in a Victorian end-terrace

[timber]

I'm looking to put a floor down in my attic to form a storage room/office. It only has 3"x2" roof ties spanning 3.7m which are barely enough to hold up the ceiling and so I was hoping to put some more substantial joists in between them onto which I'll lay some OSB flooring above and hang the ceiling from below.

Although I'm not necessarily going to build this to regulations, it looks as though 8"x2" at 400mm might just about meet regs for spanning 3.7m, but I'm not sure of the best way to hang them.

 

On one side there's an internal 4.5" brick wall and at the other there's a 9" external brick wall which is slightly interesting as the top course of bricks at one end are set back ~2.5" to form an aesthetic detail on the outside. Also at the midpoint of the wall (far right of the photos), the bricks aren't flush with the wall plate - they protrude slightly by about 1/2". What do you think would be the best way at each end?

For reference, the roof ties are 3" high, the wall plate is 3.5" and the shallow gap below it 4".

 

 

Edited August 20 by timber
remove incorrect info

[Redbeard]

I'd use a structural engineer, 'cause you're only wanting to do it once. Where will the insulation be? Even if you use PIR will there be enough height left once you have come down a minimum of 162mm? (162mm assumes 25 PIR between rafters, 125 under, 25 battens and 12mm pl'bd - all taped to within an inch of its life, of course. I'd strongly encourage you not to skimp on the insulation even if you are

19 minutes ago, timber said:

not necessarily going to build this to regulations

 

I am assuming the insulation will be at sloping ceiling level, otherwise you'll be very cold in there.

[timber]

Thanks, I'll cross the insulation bridge when I come to it - if the attic ends up either too cold, or too low depending on the insulation then I'll just use it for storage. 

My main priority right now is to put a ceiling up for the room below but I need to consider it's thermal and acoustic insulation at the same time (the neighbours have an attic room and the sound comes right through into my attic). I could throw up some plasterboard and then roll out some insulation out on top, but it would
a) get absolutely filthy from all the mortar dust from the tiles

b) not really offer any kind of sound insulation from the neighbours attic. 

I thought that if I at least just installed some joists now before ceiling goes up below, I could put some OSB flooring down above, and install some heavy wool slabs between the joists. At least that way the dust from the tiles would be contained and easily swept up, I'd add some decent storage space and it would help to sound insulate the bedroom below from next door's attic. 

 

 

[timber]

I think what I'm going to do is simply double up the roof ties instead with some additional 3x2 timbers sistered to them above with glue and screws, and then board on top of that. 

That will give provide 6" of cavity for heavy wool insulation between the bedroom below and roof space, some basic cold/dusty storage in the roof space and doesn't involve lowering the nice high ceilings in the bedroom which I'd like to keep. 

As is typical for standard terraced houses with staircase across the middle, the middle of the roof space's floor is supported by two 4.5" brick walls that go right down to the foundations, so plenty of support in the middle which is where the most head room is in case I ever fit insulation below the tiles and add a desk etc.    

[scottishjohn]

4 hours ago, timber said:

I think what I'm going to do is simply double up the roof ties instead with some additional 3x2 timbers sistered to them above with glue and screws, and then board on top of that. 

That will give provide 6" of cavity for heavy wool insulation between the bedroom below and roof space, some basic cold/dusty storage in the roof space and doesn't involve lowering the nice high ceilings in the bedroom which I'd like to keep. 

As is typical for standard terraced houses with staircase across the middle, the middle of the roof space's floor is supported by two 4.5" brick walls that go right down to the foundations, so plenty of support in the middle which is where the most head room is in case I ever fit insulation below the tiles and add a desk etc.    

are you putting in stud walls up there as well ?

noreasonwhy you should not use theties already ther to attach your foam backed plasterboard to them and additons as needed to match up the PB sheet lengths

your roof is adold one so make sure it has plenty of ventilation getting at eaves  so you don,t cause a moisture problem 

[timber]

15 hours ago, scottishjohn said:

are you putting in stud walls up there as well ?

noreasonwhy you should not use theties already ther to attach your foam backed plasterboard to them and additons as needed to match up the PB sheet lengths

your roof is adold one so make sure it has plenty of ventilation getting at eaves  so you don,t cause a moisture problem 

Well spotted. The batten's for a stud wall in the bedroom below, I pushed it up there to keep it out of the way.

Good idea using PIR plasterboard for the ceiling - I never thought of that. Thanks. 

[timber]

In case anyone's interested, in the end I decided to install some new 8"x2" joists @ 18" centres (3.6m span) between the existing roof ties and hang them from the wall plate with heavy duty hangers. Because the nailing area was limited (I couldn't get in above the wallplate) I chose some hangers that allowed me to fit 14 nails along with 4x M10 80mm coach screws. 

[scottishjohn]

so you using 6x3 palsterboard then  

Idid not know they made pir pklastergoard in 6x3

[timber]

You're probably right about the PIR . I matched the existing spacing just to make my life easier avoiding existing ties/rafters. I guess I'll just have to buy bigger boards and cut them down..

[scottishjohn]

29 minutes ago, timber said:

You're probably right about the PIR . I matched the existing spacing just to make my life easier avoiding existing ties/rafters. I guess I'll just have to buy bigger boards and cut them down..

or  add  extrs smaller joists  joists  in some places to make sheets work or

run sheets other way  with bits nailed  in between joist for edge support

 

 a bit of thinking at start should cut down amount of extra PB edge supports needed

 

like maybe cutting some boards into stripes to get you back on  the joists 

 

eg cut first to 36" then then an 18" strip  to get 54"

then same again 

 

lots of ways 

 

just a bit of buggering about   on paper to see which is best  way