Cross beam preventing sistering of joists

[OldKettle]

Hi,
My loft has joists which are approximately 6m long and are only 2x4

In some parts of the loft I have stacked new 2x4s on top, and then T&G laminate on top. However, there are a number of cross beams, so I have not spanned the whole 6m in one, but have stacked either side.

I am now doing a smaller section of the loft, and I would like to sister 2x8s alongside the existing 2x4, but am still prevented from doing so by the cross beams.

Below is a plan diagram of the area along with a photo of the cross beam.

What I would ideally like to do is sister all of the joists between the top outer wall and the internal brick wall (a length of approx 3.5 metres), such as the sistered joist shown in yellow.

I would like to know if that cross beam could be safety removed and then reattached to the top of the new 2x8s.

I am assuming that the purpose of the cross beam is to provide some lateral stability, but wonder if it can be temporarily removed.

If it cannot be removed I would have to notch to half the depth of the 2x8s to accommodate the cross beam, but wonder what that would do to the integrity of the 2x8.

Any thoughts would be much appreciated.

 

[OldKettle]

Having looked at the joists more closely, I have realised that another issue is that the joists have been mortared into the top row of bricks on the external and internals walls (so forming part of the top row of bricks rather than just sat on top of them).

I'm not sure if this is perhaps normal practice, but it leaves me with the other issue of the placement of the sistered joist on the wall.

I could notch the bottom of the sistered joist to accommodate the bricks, but would guess that it would be better to cut away a section from the top row of bricks to accommodate the sistered joist instead (as I've tried to show below).

Any thoughts on this and/or the cross beam would be much appreciated.

EDIT: I've finally realised that the cross beam are the ceiling binder, and it looks like I would need to prop before removing it.

If I simply sister a 2x8 against the 2x4, but don't rest it on the walls, would that:

1) provide any more strength than the current 2x4s alone
2) cause any potential instability if T&G flooring is laid on top of the 2x8 (possibly due to turning forces, on the unsupported 2x8)

 

[Alec_t]

turning forces, on the unsupported 2x8

Can't you support the 2 x 8 end on a wall bracket/joist hanger?