How is it best to board up my loft if I want to use it as a 'useable' area in my home, also can I install my own roof window?

Basically what you are describing is a loft conversion.By adding a window and strengthening the floor etc to make it "usable" you are actually making it "habitable".You would have to involve building control and there are loads of regulations that would need to be met making the project MUCH more expensive than what you maybe had in mind.You would also need an architect to design the room for you in a way that meets all of the current regs including insulation,ventilation,floor and roof strengthening and fire escape.Under no circumstances should you remove ANY of the roof structure without expert advice.

If you want to use the loft as a usable area, and fit roof windows, you need planning permission, building regs and a Structural Engineer. There is a lot more involved than just boarding a loft, steels will probably be required, larger floor joists, stairs, fire doors, head heights to be considered, insulation. the list goes on.

consult an engineer, will have to replace these with verticle and lateral ties, building controll will have to be involved if you plan on selling any time soon, as for the window if the house is listed or in a conservation area you will need to consult your local planning department, may not need planning but better to ask first

We have an aluminium solution that negates the use of steels in loft conversions and is much less disruptive, the easylofts system works in any trussed loft where the timbers criss cross across the loft space. This allows the utilisation of the entire area.