Sistering of loft joists over lath and plaster ceilings

[GazS]

Hi All!

I'd like to put 19mm, 800mm wide tongue and groove chipboard flooring across the joists in my loft.

The existing joists are roughly 90x45 hardwood at 450 centres. The span each side of a central structural wall is 4m with 8 joists on each side of the house.

My concern is that the existing lath and plaster (which is otherwise in good condition) is pretty thick, and is already very heavy. I reckon about 30kg/sqm based on what I took out for the access hatch).

I don't think this would leave much scope for additional floor load, so I'm thinking of sistering 90x45 or 130x35 LVLs to the existing joists. I'd need to raise them slightly to clear the upper side of the plaster. Thus, I'd need to remove the bracing, which runs diagonally and midway across the span to make room for the LVLs.

My thought is that it should be ok to remove the bracing (highlighted in yellow) given that the new floor will add significant rigidity once screwed down.
The line of the structural wall is marked in blue. It is symmetrical each side of that line.

Any thoughts on the (in)sanity of my plan would be very much appreciated!
Cheers!
Gaz

 

[blup]

Some of the purlin struts appear to be sitting on the binders which aren't themselves on the structural wall, so factor that in if removing them. My 4x 2 joists (1930's house) span 4 metres and support a lath and plaster ceiling, so unless it is going to be used for more than storage it may not be an issue, especially as the loft board will add some rigidity and some strength.

Blup

 

[Notch7]

I don't think this would leave much scope for additional floor load, so I'm thinking of sistering 90x45 or 130x35 LVLs to the existing joists. I'd need to raise them slightly to clear the upper side of the plaster. Thus, I'd need to remove the bracing, which runs diagonally and midway across the span to make room for the LVLs

it looks to me that those studs screwed across the tops of the joists are there for the timbers being used to spread the load of the bracing which stiffens the hip, purlin and one looks like it might go to the ridge. I would guess they are there to add restraint against racking caused by wind load

Do you need to stiffen the joists? -how bouncy is the ceiling in the bedrooms? -you could instead fit an 8 x 2 across the joists, at say mid point and screw it in place with timber loks -that would strengthen the ceiling and is less timber.

 

[GazS]

Thanks for the feedback, much appreciated!

@blub, it is just for storage and the joists are not showing any signs of movement, but i'm concerned that with the weight of loft board and then stored items it might start to cause sagging and thus crack plaster etc... so my goal is really just to strengthen the joists to allow for more storage and peace of mind.

@Notch7, There's no bounce at the moment... fixing an 8x2 across the midpoint might be an option, but I can't easily access the top plate on the right side as you look at the photo. The far beam that's been notched into, actually sits just inside the external double brick wall. I don't know the terminology for this type of cantilevered arrangement of the eaves, but I'm not sure that beam would be suitable to put a hanger on as some of the load could be transferred into an upward force on the rafters or downward force on edge of the ceiling... it's basically a fat ceiling joist...

 

[blup]

I’ve over boarded quite a few ceilings and it’s had no material effect but has added a lot of weight. Is it feasible to leave a gap where the binders are.

Blup