Extending joists

1 Dec 2019
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United Kingdom
I am having steels installed next week so have been looking at the joist set up on the 1st floor. I need a joist depth of 250mm so 10x2 timbers to accomodate a soil pipe within the floor void below the steel.

Existing joists are 7x2 so they need to be 3" deeper. Can I extend the existing after they are cut into the steel or should I just remove them and install new 10x2 joists?
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If you need the extra strength of a 10 x 3 you cannot just add 3in depth to a joist - there is simply no way to guarantee that such a combination would give you the required strength or stiffness. If on the other hand there is no added strength/stiffness requirement you can simply screw 3 x 2in softwood to the undersides of the existing joists, although this will require something like 125 to 150mm screws to do the job properly.

What has the S/E stipulated?
Don't need the extra strength as they are existing joists just being cut into the new steel. Its just to add depth to accomodate the soil pipe depth below the steel.

Just thinking by time I buy and add a 3x2 to the bottom had I better just install new 10x2 joists. What do you think.
Or I could just have a local bulkhead if this is adding to much extra work
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In that case if you own an impact driver capable of driving the big screws you'll need, then I'd go with 3 x 2s. Tha amount of work depends on what kit you have.

An alternative might be to carry the new ceiling on MF. This us lighter and faster to install
MF ceiling is certainly an option and probably easier than adding the timbers.

Would you do all this to keep a flat ceiling or have a 800mm wide bulkhead?
Either. TBH bulkheads down one side of a room often look to me like you are trying to hide something ugly - but then I'm trade, so I know you probably are! You could possibly get creative and disguise it by building a dropped ceiling/bulkhead round all 4 sides of the room, forming a clerestory in the middle (this could also be used to hide concealed lighting if you like).

MF is a lot less work than timber and is generally faster, but ideally it does need a laser level. Other than that the only specialist tools are tin snips, 2 or 3 small Mole type C-clamps and maybe some string lines
Thanks for the help. I hadn't even considered MF until you mentioned it. This is clearly the way forward. I wanted to avoid bulkheads if possible as I think they look like a bit of a bodge.

Got all the tools already so think it's a no brainer.

If I plan to hang a single skin of 9mm board what centers do the top hats need to be set at? I understand the primary channels are 1200 centres as well as the fixing angles. Is this correct.
You should set out on 600mm centres for 9.5mm boards in a ceiling. This will work for 2400 x 1200 and 1800 x 900 boards. Your primary centres are correct

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