converting loft room to bedroom

[antony74]

Hi all,

I'm in need of some advise regarding my plans to officially convert our loft room to a bedroom in our new house. The room has been used as an unofficial bedroom by the previous owner but there are quite a few cracks in the ceiling below which I feel are due to the inadequate joist size.

The room is 4250x7000mm and the joists run front to back (so 7m) not across and they are supported at the center by a stud wall, the joists are 110x75mm at 380 centers. So I feel they are totally inadequate and won't meet building regs.

The ceiling height of the floor below is 2420mm and I don't want to lose headroom in the attic so its my plan to hopefully fit 2 steel beams below the joists to support them.

I need to know the max span of the existing joists so I know where I can place the beams, ideally they would be able to span 2700 so I can avoid the staircase at 1 end and avoid the shower at the other end of the house, I've looked on the 1992 building regs span table but they don't cover this size, can anybody help me with this...

I hope this makes sense.
cheers Antony

 

[freddiemercurystwin]

edit

 

[antony74]

Yeah but before I start spending money, I'd like to know if this way is possible and the more ideas and information the better..
cheers

 

[freddiemercurystwin]

edit

 

[antony74]

Yeah, I will check the steel out later regarding calcs, for now I'm just interested in what I can span with the existing timber joists, because I don't want to lose height in the loft and the floor is already t&g boarded and I don't want to disturb.
cheers

 

[hotrod]

for now I'm just interested in what I can span with the existing timber joists

110x75mm is not a standard ex-stock timber size and therefore is not listed in the TRADA Span tables. As a guide, the closest size listed for domestic floor joists - C16 @ 400mm centres is 120x75mm which gives a max permissible span of 3.07m. You will not be a million miles away from this figure.

 

[antony74]

Thats good cheers hotrod,

I see the TRADA span is 100mm more than the 1992 building regs part A table for the dead load cat of .25 - .5 kn/m2 infact its nearer the dead load cat of -0.25 kn/m2 which is 3.10, out of interest does TRADA have the specs for 97x75 joist so I can get a rough inbetween figure..

Cheers

 

[DevilDamo]

75 x 120 is the smallest of the 75mm range (on the TRADA tables).

 

[antony74]

bugger, ok cheers devildamo.

 

[DevilDamo]

That doesn't meant to say they aren't produced... just means they're not in the standard tables. Give that size to an engineer and they may be able to make it work or least produce a calculation.

 

[RonnyRaygun]

Antony,

I've just done a calc on your joists for a 2.7m span at 380mm centres using standard domestic floor loadings and C16 timber, and they will just about span that distance.

Your maximum span would be around 2.8m, although as the joists will probably be continuous over the steel, you might have a bit more room for manouvre.

 

[antony74]

Thats great RonnyRaygun, I'm glad I now know its within regs. The joist don't have stamps because they are old but they look sound but I suppose they will have to graded. Are you a struc eng and if so when I need some official calcs would you be willing to do them and if so how much would they cost..
thanks again
Antony

 

[RonnyRaygun]

Antony,

I work for a structural consultancy in North London but am not Chartered, so while I could do the calcs for you, I don't know if BC would accept them.

I'd try to find someone locally who could do them if I were you; they should have PI insurance and all the rest so you've got comeback if they make a cock-up

 

[Static]

I work for a structural consultancy in North London but am not Chartered, so while I could do the calcs for you, I don't know if BC would accept them.

BC would accept them, but will check them more if they are unbranded PJ calcs..

Please bear in mind that to span 2.7m with 110x75 joists you will need to screw down the floor boards at regular centres or they thing will bounce around like a trampoline..