Advice needed on joist strengthening for loft conversion

[loftmonkey]

Hi all,

I'm in the planning process of starting a basic loft conversion in my flat which will be for used for storage and as a work area. The plan is just to put down some 18mm chipboard, insulate the roof and board off the roof and eaves.

I have consulted a structural engineer and he has advised me that the loft joists are not strong enough to handle it and will need strengthening.

He advised me to double up the joists by placing new 112x35mm joists either alongside the existing joists (attached to them) or between them. Also since I have a lot of electrical cabling and gas/water pipes up there running over the existing joists, he advised it might be a good idea to use 125x25mm joists so that the flooring is elevated half an inch above the existing joists and clear of the cabling and pipes.

All sounds fairly straightforward so far?

Now I'm practically a novice when it comes to DIY (although I have been doing some small jobs lately and my confidence is growing) so some might say leave it to a professional, but I'm willing to
give this a go as I like a challenge.

So with that said, I have a few questions about the best way to go about strengthening these joists. Bearing in mind the advice given to me by the structural engineer which he has determined by making all the necessary load bearing calculations.

1. There are some horizontal timbers going across (above) the joists which would make it impossible to place new HIGHER (full length) joists in the loft. So should I...

a) Try to find full length timbers the same height as existing joists that can fit under the horizontal timbers, then cut additional timbers to go perpendicular on top of half an inch to provide the floor clearance?

b) Use joist timbers of higher height, and cut notches in them for the horizontal timbers.

3. I'm having a problem finding a source for timber that meets the recommended measurements for the joists. Is it likely I would have to have it specially cut to order? Or should I buy standard stock size (125mm x 35mm or 100mm x 35mm)

4. How practical/difficult is it likely to be working with timber joists the full length (26ft) of the loft? Bearing in mind, I live in a second floor flat and have to get this up the stairs and through the loft hatch (81x52cm) and will be working alone! Roughly how heavy are they likely to be?

Thanks for any advice.. I will probably have a few more questions when I get to the next stage..

Paul