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Loft Flooring Question Concerning Loft Conversion

Postby joeflaherty » Tue Jun 25, 2019 8:07 pm

Hi all,

I have a bit of a conundrum regarding our loft conversion and we're getting conflicting advice from different builders who are quoting for the job so I hope that you fine folk can help!

We're planning on converting our loft and have had the structural plans and calculations done. The plans show the new loft floor, comprising three steel beams and new wooden floor joists. The new floor will be going on top of the existing loft joists, rather than replacing them.

The disagreement is whether, in order to put in the new steels and joists, the first floor ceiling plasterboards, which are attached to the existing joists, needs to be removed. One is saying that the ceiling needs to be removed, the other is saying that it doesn't.

Naturally, this will have a major impact on whether we can actually live at home while the works are underway which will be a huge factor in who we proceed with.

I've attached the structural plans showing the location of the new floor joists and spec (175x50) as well as the cross-section showing the new floor going on top of the existing one (rather than replacing it).

So who's right? Does the first floor ceiling need to be removed or not? Both are adamant that they're correct!! Any help gratefully received - many thanks in advance.
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Screenshot 2019-06-25 at 21.09.35.png
Screenshot 2019-06-25 at 21.08.17.png
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Postby htg engineer » Wed Jun 26, 2019 5:43 am

The joists would normally go between the steel, over the top of the existing ceilings/joists but not in contact with them.
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Postby joeflaherty » Wed Jun 26, 2019 8:09 am

htg engineer wrote:The joists would normally go between the steel, over the top of the existing ceilings/joists but not in contact with them.


Thanks HTG. The plans show a minimum gap of 15mm between the new steels and joists and the existing floor. I assume that this is so that the new floor is structurally independent of what's already there.

So are you saying that there's no need to remove the first floor ceilings which are attached to the existing joists?
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Postby htg engineer » Wed Jun 26, 2019 11:16 am

I'm in the middle of doing my loft, the new joists come off the wall plate, and didn't require any steel.

The new joists are above the existing ceiling joists, I've never seen a loft conversion where the old joists and ceiling were removed, unless there's is a requirement to - to get the required 2.4m head height?
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Postby joeflaherty » Wed Jun 26, 2019 11:32 am

A guy on another forum has come up with a potential solution which would be to notch the new 175mm floor joists at the bottom to fit into the steels, which would then give c. 65mm of clearance above the existing floor. The new joists would run alongside the old ones.

The height is potentially an issue, but from my rough calculations, we should end up with c.1952mm of floor to ceiling height. It's not ideal, but it's better than nothing and on a cost/benefit analysis, I'm not sure that it makes financial sense.

My rough numbers show that removing the old joists (and first floor ceilings) would give us a floor to ceiling height of c. 2114mm including the new floor so about 160mm in it.
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Postby htg engineer » Wed Jun 26, 2019 4:05 pm

Is that head height with your 50mm insulation over the rafters, plasterboard/slim.

I would just run by building control, before starting. We needed 2m on the stairs and he wouldn't pass 1.9m so had to alter them to get the height
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Postby joeflaherty » Wed Jun 26, 2019 5:16 pm

I think I've actually made a small error on the plans.

I've checked the surveyed plans for the existing loft and it shows a floor to ceiling height of 2,286mm.

Having spoken with the SE - there needs to be a 15mm void above the existing floor. The 152mm RSJ then goes on top with the 175mm joists notched with an 18mm floorboard on top.

So if my understanding is correct:
15mm void +
152.4mm RSJ +
11.2m void above the RSJ (18mm floorboard, minus 6.8mm height of the RSJ flange) as the floorboard is going on top of the joist not the RSJ
This gives a total new height of the new floor including voids of 178.6mm, say 180mm.

This would leave 2,106mm headroom in the loft which *should* be fine.
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