Loft storage

[bulboy]

Hi, got a new house - 3 bed semi and before I move I want to create a bit of storage space to the loft. The roof structure is almost classic in having 50x100 joists running front to back, two binders 75x175 running gable to party wall. two purlins propped off the binders???

In addition I have a short beam 75x175 running parallel to one of the binders on top of the wall bearing walls created by the chimney stack and airing cupboard. this is where the cold water tank used to be.

TO make the things more challenging the loft is insulated with old cellulose insualtion between the joists and new wool insulation laid across to total thickness of about 250-350 mm.
Span between the binders is ~1750mm. I need to check on the joists but the loft hatch is a 62 cm square.

I am planning to introduce a second 75x175 beam in the middle between the binders and board across.
Or use wall hangers and install cross beams between the binders? But then I will have to cut the insulation. Also I do not want to go too much towards the unsupported binder in the master bedroom as most likely the ceiling will crack.
What would be the maximum span for 18mm chipboard?

Any additional ideas or comments?

It's generally light stuff to be stored like Christmas deco and scale models with occasional box of books (I will take care to put it close to the chimney stack.

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[dhutch]

Having just raised my loft insulation from 2inchs to 10inchs (4inch joists) I raised the joists using timber placed on top of them, tied in at the end with the 'binders' (timber on the joists, linked to the purlins, to ensure were talking about the same bit) I used 3*2, making short blocks out of the ofcuts placed between them.

So you have 4inchs of joist, 3inchs of block, 3inchs of timber, floorboards.
All held together three inch screws, down counter bored holes (cheaper then 4.5inch screws). Two of us did the work in a day, second day to get the flooring in over the top, but that was before the insluation went in to a clean sheet.

I will post some photos if i get chance.

Daniel