Loft flooring questions

[zeth]

I have a circa-1900 terraced house with a nice empty loft. There are no trusses running through the middle like in modern houses. Ignoring the wasted space under the eaves, the main section is about 3.5 by 6 meters (covering two bedrooms with a brick wall between) and there is another smaller connected part (maybe 3 by 4 meters) over the back of the house, which I am not so interested in right now.

I want to insulate and board most of the main section for storage, minus the eaves where it is too low to walk under.

I will add a loft ladder and more conveniently-sited hatch for access. I don't need a habitable room but I want the floor to be strong enough so Imelda Marcos can safely wander around putting things in and out of boxes.

At the moment it has some very tatty rockwool insulation between the joists which is (at least was originally) 100mm according to the bags which are left there with a few other random bits of wood and a few spare roof tiles. There is no water tank in the loft at the moment but there are a couple of power cables going over the joists. I have ferreted around up there, standing on the joists, and it seems relatively sturdy.

My initial thought was to put crossjoists at right angles to the joists, and screw some loft chipboard panels to the top of the counterjoists. Then I could have one layer of insulation between the joists and another between the counterjoists. The existing joists are 41 cm apart.

Since I want to be able to walk normally rather than the normal loft ferreting, should the counter joists be screwed into the brick walls? Should I then be thinking about steel and structural engineers or a proper joiner or something? Or is that complete overkill?

If the crossjoist idea is a goer, is there any particular size of timber I should use?

Can I put insulation between the rafters instead and would that simplify making a floor?

Thanks for reading.

 

[Chri5]

Do you want to insulate the downstairs rooms, or are you more concerned with insulating the 'new' loft room ?

A new thick insulation on the floor will keep down stairs warmer, but not the loft.

Why not change the existing insulation for some high density modern, more efficient product, then simply fix the new floor without the cross joisting ?

With the £££'s saved on wood then insulate and board the roof structure, keeping existing air vents clear.

You then end up with warmer rooms down stairs, and a loft which won't get too damp and cold in winter.

 

[zeth]

Thanks for the reply Chri5, without the cross-joisting, how do I know if the joists will support a person walking around?

 

[Chri5]

A loft structure is about 30% of the strength of a suspended floor.

Boarding it will give further rigidity and spread any foot traffic over a wider area.

Since you suggest the loft is for storage and not a daily used habitable room, then what I have suggested is fine for 90% of loft spaces.

If I twist your question 180 degrees, what makes you think the existing structure + boarding wouldn't be suitable for what you intend ?