Strengthening Loft Floor Joists

[jagillham]

As part of my “half conversion” of the loft, I have ordered some C16 (45x145mm) to bolster the original 50x100mm joists. The bedroom below ceiling is off, so I have easy access.

I have 2 options as far as I can see, as the new joists are taller.

1. Sit this on the wood above the bricks like the current ones (called headplate I think?) which raises the floor level.
2. Notch out 45mm from the new wood, to keep the floor same level but lower the ceiling.

I would probably rather option 2, as it keeps more headroom for the loft.

Firstly is this OK to do, and then secondly can I simply screw the new joists to the old to double them up?

Thanks,

James

 

[tony1851]

You can notch the new joists by 45mm at the bearing as this will not affect the bending stress in the joists, which is at maximum at mid-span.

Although sistering the joists will strengthen the assembly, you may still get excessive deflection depending on the span.

Just screw the joists together.

 

[catlad]

You could have built a new ceiling counter to what you have already and effectively having too ceilings more sound proofing/fireproofing.

 

[big-all]

you shouldn't really notch at the ends as you are in effect making that joist that size structurally along its length
the regs for notching and drilling exclude the first part off the joist and the middle also notching is not allowed under the timber as it weakens the timber

what is the actual unsupported span and the joist centers

 

[jagillham]

Thanks everybody for your input.

In answer to the last question... The span is more or less 3m, plus 10cm bearing at each end. The the current joists vary between 31cm and 32cm between them. I was planning on put these new ones beside the old.

 

[big-all]

Thanks everybody for your input.

In answer to the last question... The span is more or less 3m, plus 10cm bearing at each end. The the current joists vary between 31cm and 32cm between them. I was planning on put these new ones beside the old.

320mm is around 335 centers as its center off timber to center off timber
3m at 335mm is a good support ratio so unless its more than light storage the present support is enough

 

[tony1851]

you shouldn't really notch at the ends as you are in effect making that joist that size structurally along its length

Hi big-all;

You can notch floor joists at the ends - as long as its not too much - and 45mm off a 145 deep joist will not be a problem.

In domestic floor joists, the critical parameter is almost always deflection at the centre of the span, and this is not made worse by notching the ends.

However, if he notched 45mm out at the centre of the span, that would turn it effectively into a 100 deep joist.

 

[jagillham]

I've taken a closer look, and I'm going to have to notch either on top or below anyway.

You'll have to excuse my lack of terminology, but there is only 100mm between the timber the joists sit on, and the timber above supporting the rafters.

The plan for for the loft is all done up nicely for storage, but ready for a full conversion later if we need the space.

The unsupported span is 3130mm in answer the easier question.

 

[tony1851]

That's an unconventional way of forming the rafter/ceiling joist connection.
Still, just notch out the underside of your new joists just sufficiently to get the ends in the gap.

 

[jagillham]

I thought so, good job I didn't want to take the old joists out! The house is 1903, and I'm quite sure never saw a level or square in its construction!

Thanks for the help