Loft modification

I am looking to do a modification in my loft so that i can set up a desk to make a temporary office.

The slate roof was replaced with concrete tiles about 15 years ago, and additional beams were hammered in.

I have attached a couple of photo's. There are (4) horizontal beams at about 3-4 feet from the pitch of the roof. I will need to (cut these and )move these up by 15 to 18 inches. Will this provide enough support for the actual roof?

I was planning on putting in additionals (at least 8 extra) for more support.
For the flooring, there are rafters running the length of the house, supported by the end walls as well as the central staircase. On top of these (and parrallel with the staircase walls) are the main support beams for the roof.

Using 2 x 4's, am i better off using the main roof support beams as the basis for the floor (bearing in mind these are located on top of the two internal support walls), or straight onto the existing rafters? ( which run from the internal supoort walls to the external walls.)

Thanks in advance for any help

Unless you're an experience builder, which I guess you're not as you're asking for advice, then you'd be mad to alter your roof structure without proper advice.

This is definately not a diy job! Someone with experience needs to assess the roof and work out where all the loads are going, and I'm not just talking about simple dead loads, but wind loads too.

Someone should also give you proper advice on whether or not the structure can support the new floor loads you will be creating.

I am a structural engineer and would agree with comment above. Do not cut out the horizontal ties unless it has been checked out by a SE.

Agree also re the floor. Floor loading is 6 times ceiling loading and suitable mods will be required.

Steve