Floor joists within a loft conversion

14 Jan 2010
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United Kingdom
In the majority of loft conversions, the existing "ceiling" joists will not be capable of receiving the additional loads following a loft conversion to a habitable room, which is where upgrading of the existing joists is required. Now, when you design these new "floor" joists to go inbetween the existing ceiling joists, would you feel pretty confident in using the TRADA timber span tables to design those new floor joists or would you leave it for a structural engineer to advise/design?
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Yes, but I always go up a size anyway. I don't recall having ever done a lofty without an SE though so a moot point for me.
Thanks FMT :)

I've done a few in the past, but like you say... never without a SE.

I'm looking at a new job tomorrow and I was going to recommend getting a SE onboard but was just curious what Architects/Technicians would just use the information from the tables and be done with it... providing there are not any other structural alterations/implications.
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If it's straightforward timber spans then the span tables are usually the best option. The only time I can think I've calculated timber joists rather than use the tables is a couple of occasions when I needed to use odd size timbers. I can't think of many lofts I've done that didn't need steels somewhere though - or multiple timber beams. Personally I do my own calcs and steel design but I wouldn't advise that without suitable PI insurance.

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