Floor Joists between I Beams in Loft

7 Feb 2009
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United Kingdom

I'm scratching my head at the moment and wondered if I might get a little advice. I live in a semi detached bungalow and the ceiling is suspended so I have no ceiling joists in the loft. I have 3 No. I beams in the roof space sitting above the plaster ceiling and just a brick wall at either end. The beams support roof trusses with purlins running along the length.

I would like to create a little bit of storage just in the middle section between 2 of the beams. I thought of spanning some floor joists between these beams and I have thought of 2 ways to do this:-

I Beams measure 165x 260 deep
235mm inside height measurement
65mm depth to the vertical on each side.

Span between Beam centres 3065mm

I propose 5 joists in total


Run a length of 38x200 within each opposing I beam section, perhaps bolting this to the beam. Attach joist hangers to these and then fix 50x200 joists between these 2 beams at 600 centres.

Please see link of joist hanger. Sorry I had trouble uploading pictures. Beginners eh?


I've gone for 600 to cater for my existing loft insulation of 570mm width. I can then fix some struts between the middle of these 5 beams and fix some 1220x607x18mm chip/ply boards on top.

My concern here is with the depth of the 38mmx200 along the length, the joist will only be bearing 27mm on the beam(this could be resolved by option 2) although they are supported by the joist hangers.

I think these beams are over sized but I wanted to keep the depth in case I decided at a later date to build a floor in the other bays of the loft which have greater spans(up to 4m). This would keep consistency of floor height.

50x200 seems more available than say 38x220. I originally went for this size(38x220) because the finished floor height would be very close to the top of the beam. I thought this was neater but after working out the total cost I thought better to use smaller sections. 200mm depth will not fill the inside of the beam but apart from aesthetics can anyone see any drawback in using a smaller section? I also guestimated the sizing and don't know whether I need C16 or C24 grade. I'm also confused by the Building Regs options listing dead loads i.e. "not more than 0.25kn, 0.5kn" etc.

Option 2

Run 50x200 joists butted tight between the I sections @600 centres then fix strutting between the joists at each end and in the middle. Then fix 1220x607x18 boards on top as before. This way could be easier to install initially but if I use joist hangers to fix all the strutting it would mean more work and more hangers=more £s. There will be a greater bearing onto the I beam but it might be difficult to fix to the beam to provide lateral stability. There is more point loading going on but this shouldn't be a problem with a big steel beam or should it?

To ssummarize-

Do these sizes look OK?

Would you go for Option 1 or 2?

Does the long section in option 1 need bolting to steel beam?

Would you try and match height to top of steel beam i.e(going for 38mmx220) ?

Which grade of timber should I go for C24 or C16 ?

Thanks! I hope I've made it clear.

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