loft conversion

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soon i will be starting my loft conversion but i have a question for an alternative for the traditional 9x2 floor joist. The span of the new floor steel to steel will be six metres, my architect has suggested using 9x2 at 400c/c and lapping them over the central supporting wall thus making each joist either 3.3 or 3.6 "can't remember" now i have done a few loft conversions and know from experience when using joist in this manner that inveriably you end up with a floor that isn't strictly level. Now to avoid this i would like to use TJI joist because i know you can then span the entire six metres without having to cut them in half, however, as always cost is an issue therefore do you feel that the amount of time i would save by using TJI joists as appose to messing around with 9x2 would be beneficial from a financial and time perspective?
 
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chirpychippy said:
now i have done a few loft conversions and know from experience when using joist in this manner that inveriably you end up with a floor that isn't strictly level.
The BCO made use 9"x3" :!:

I used the wallplate and the centre wall for the 9"x3" joists overlapping, I made loads of difference thickness square plywood to pack the joists up for level with a string line. I couldn't use the RSJ as my footing wouldn't be strong enough on the 4 loading points, you're right this does take longer to do than the RSJ but then again the RSJ are very heavy & quicker to do but can be awkward to do depending on your layout of the roof. The advantage of found having joists to wallplate are smaller length joists and you will also have another loft floor area behind the studwall. I would rather do the job that take longer to do to make life easier, the choice is yours really.
 
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i am having to use 152x152x23 RSJ's because the house is victorian with bay windows at the front thus the wall plate at the front is buried in the eves and unaccessable, therefore i can't do wall plate to wall plate joisting even with the centre wall. the rsj's are going in at 10mm above the existing 4x2 joists so i don't have to do anything with them. As a result i will only be spanning the six metres between front and rear RSJ's appose to the full 9 metres,for the new dormer floor, i am not concerned about joisting behind the ashlar wall as i feel by increasing the height another nine inches will only serve to decrease the limited storage space i will have left. Therefore i am still delibarating whether to use TJI joists instead of 9x2! for spanning the six metres between the RSJ's?
 
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chirpychippy said:
Therefore i am still delibarating whether to use TJI joists instead of 9x2! for spanning the six metres between the RSJ's?
I don't know the calulation stress load size for the TJI joists, off my head I don't think the 9"x2" timber joists is big enough for 6m span, need at least 12" or you may have to use 9"x3". The TJI will be stronger and lighter, so best to have a word with the building inspector to see what they recommend when the drawing plan is submitted.
 
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6m span on TJI's youll be looking at quite a deep section. 356mm or 302mm @ 400 c/c which will give you an added complication of how to fix to the small steels and maybe headroom.
 
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Thank you once again i will stick to 9x2 at 400c/c this will work because they are spanning a load bearing wall at 3.0mtrs c/c, cheers
 
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