Creating larger loft space

First timer, so be kind please. I have just bought a small 2 bed terraced and want to convert the loft to create a small 3rd bedroom. I am pretty handy at DIY and have done two previous refurbs on terraces. I have also done a loft conversion in a 1920 3 bed semi which was passed off by the building inspector last year and so have some knowledge and experience.
The available floor space in the terrace house is not very big and is reduced further by two large beams (purlins I think the pro's call them) approximately half way up the pitch of the roof on either side, which is where I would need to put the velux. The height looks similar to the images shown in Duzzers forum. To create more usable floor space I would like to remove the purlins and lower the loft floor by about 2ft. I plan to position two 8"x 4" timbers across the span of the upstairs and then hang 6" x 2" across from one to the other. Once in place, I will add 3" x 2" uprights which will be fastened to the roof to provide support so the current supporting beams can then be removed. There is sufficient space in the bedrooms and bathroom below to do this.
My questions are:
Can a loft floor space be lowered as I have proposed.
What is the best way to remove the beams (purlins I think) and still provide adequate support for the roof and can I support the roof as proposed.
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I plan to position two 8"x 4" timbers across the span of the upstairs and then hang 6" x 2" across from one to the other.

Is this to create a floor? You haven't given dimensions but I think you'll be looking at hanging 8x2s supported between steel. If you remove the purlins then the new floor and/or supporting steels will end up doing the purlins job. Which is why steel is often used. [/i]

Deluks said:

Is this to create a floor? You haven't given dimensions but I think you'll be looking at hanging 8x2s supported between steel. If you remove the purlins then the new floor and/or supporting steels will end up doing the purlins job. Which is why steel is often used.

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Hi Deluks, thanks for your reply. You are correct this is to create a new floor for the attic. Therefore I plan to lower the upstairs ceiling by 2ft also. My problem is the new attic floor will be approx 2ft lower and so whether I use timber or steel this will also be lower and so not in contact with the roof timbers. The span from side to side of the attic is approx 5mtrs. There are two other beams close to the eaves which will stay in position.
I have attempted to draw a diagram of how the attic is now and what I propose. Its a little crude, but I have limited software to use.
I am not sure you must use steel as the building inspector was happy with the use of timber for the last conversion, which had a longer span.

It would be easier/cheaper to add a dormer to get headroom in the roof. You are even more likely to need steel as you are removing the roof 'triangle' by removing the joists and lowering them to between walls.

Anything is possible though, and if you are stuck on timber, then it can be done in timber. Don't ask me how, it will need to be designed by someone with letters after their name. Like Jean-Claude Van Damme (A.W.O.L)