Joist size for loft storage room?

[Ian H]

My daughters house has 85 year old saggy 2.75x 1.75 ceiling joists in the bedrooms.

We want to lower it and also put a platform (4 sheets of 18mm ply) up there for storage, there’s no intention for it ever to become a habitable room.

On one side the span is 4.0mtrs, on the other it’s 3.0mtrs. Could we replace the joists with 6x2 at 400mm centres and then copy the current 3x1 hangers from the purlins?

Also if I have a bigger overlap on the central wall and glue/screw/bolt them together, would this in a way reduce the span?

Would we have any risk of the ceilings cracking if we did it like that?

Thanks

 

[oldbutnotdead]

You'd be amazed how much load those 100 yr old seasoned slow-grown probably Douglas fir joists will take, especially if you leave the lath & plaster attached.
If the ceiling is ok (timbers not rotten/split/twisted, plaster not hanging off in lumps) then consider leaving it alone & putting a false ceiling below (3 x 2 framework @ 400 centres fully nogged at 400). If you have 350mm headroom to spare you can insulate in the false ceiling all the way to the walls (no need to allow eaves breathing space) & not have to mess on putting your deck on legs (t & g chipboard is usual in lofts).

Your 6 x 2 plan will work also
EDIT 3 x 2 not 2 x 2, fat fingers

 

[Ian H]

Thanks I’ll go with the 6x2, would you also use 6x2 for the noggins or could you use 4x2 so cables can thread along easier?

The old ceiling was in poor condition so we dropped the lath and plaster.

 

[oldbutnotdead]

Fair dos. Either for noggins, long as the nogs are 2/3 the depth of the joists you're fine. Given the price of timber at the mo you may find the metal herringbone struts are cheaper...

 

[Ian H]

In my head I would replace the 3x1 hangers with something bigger like this picture (random add that popped up on my phone):

The roof in her house actually has 2 sets of purlins.

 

[oldbutnotdead]

Doubt you'll need them all unless you're storing elephants up there. Did a similar job a while ago on 3.6 span, the ,6 x 2 on their own were a bit wobbly, once nogged the whole lot got very stiff.

 

[cdbe]

I used 8x2 nogged, with similar spans, partly because I had some, and because we keep a lot of stuff up there - I would actually say far more than you would load a domestic floor, also to put the insulation in (with an additional layer of celotex under/over). I purposely removed the ceiling hangers as I didn't want any load from the new "floor" being transferred to the old saggy purlins.

To avoid ceiling cracking don't put any board joints near the center line:

 

[Ian H]

I’ve ordered the 6x2 to come on Wednesday.

She doesn’t have anything to store up there yet with her being a first time buyer but no doubt, with being a woman, she will soon have a mountain of crap up there

I was also thinking should I screw or bolt to the rafters? The small stuff there now has a single nail into the rafters so I thought a couple of screws would do.

 

[catlad]

Don't forget if you are removing the old ceiling ties you need to consider replacing that structural element in someway.