Doubling up the Joists for Flooring


3 Jun 2004
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United Kingdom
In common with a lot of people on this site, it seems, I'd like to board a section of my loft. The house is 1950's wit a traditional roof. The existing ceiling joists are only 3" x 2" and the section that I'd like to use has a span of nearly 3.7m from internal shear wall to external wall. From the Joist span tables that I've looked at I need to use 50 x 170.

Can I put new 50 x 100 joists on the top of the existing ones with 170 deep noggins and gang nails or screw plates on the sides. I'd fix a new wallplate to the external wall and a corresponding timber on top of the shear wall with the new joists running between these, the ends being fixed with joist hangers.

The intention is to use the space for storage and occasional office use so I don't particularly want to go through the BCO route although I will probably put an escape velux in. Eventually I might get to build a proper staircase and board the rest of the loft at which time I will do the necessary building control stuff

Can anyone comment on my proposals? Before I start, I'll check my drawings and calcs with a tame Structural Engineer, but I just want to know if what I'm proposing is feasible.

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Best to put 170mm x 50mm along the side of the ex-joists(100mm x 50mm) and use the outer and internal wallplates for the support, may have to use some difference thickness of plywood for packing under the new joists up slightly if you have plaster/lath to clear it. This way will by-pass your ex-ceiling joists and no stress load problem. You could give the BCO a ring to see what their recommended size joist for loft conversion and use that size saving you have to rip up the old wrong size joists (170mm x 50mm) again if you come to do it again. I understand this is quick temporary solution but really should go through the BCO planning route as it could get complicated with your household insurance etc and give you more work in the long run.
Masona, thanks for the reply and your comments re. BCO are noted - I'll give them a call. I didn't explain properly in my first post but the reason that I don't want to put the new joists in side by side with the existing is that the outer end of the existing joists doesn't bear onto a wallplate because it is a gable end so the joists are built into the inner skin of the cavity wall. This will make getting a grounding for anything new nigh on impossible without knocking holes in walls.

This is why I have considered putting new 100 x 50 joists on top of the existing 50 x 75's. I really concerned with the validity of putting joists edge on edge to get the, hopefully, same result as a single piece of timber.
How about a wallplate rawlbolted to the wall and use joist hangers off that?
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Timber works better with two members side by side, placing the additional joists over of existing potentially may not make the overall member any stronger. Your far better off placing members alongside the existing. TRADA give some basic joist sizes for spans, with a span of 3.7m you'll need a stronger grade of timber (C24 which is expensive) if you can fit in 200x50s (grade C16) they are probably whats required for BC.

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