Converting loft for storage [was "floor joist Calculation"]

Following on from my last posting, I'll definitely be using 2" x 8" beams for the new floor. This will meet the BCO regs, and isn't going to be anywhere near as expensive as I'd previously thought. (I think the quote I must have been given previously was for mahogany beams, not softwood :-)

My roof is made from King post trusses and the new floor is going to be entirely separate, as it I'll be raised an extra 1" off the wallplates. The trusses are on a 600mm spacing, and each one has a king post in the middle. It'd be really nice to not have them there, but not essential as the roof space is quite large as it is.

So, the new questions are :

1) Is it possible to modify trusses? I have searched around t'internet and some sites say "absolutely not", some say "yes, if a structural engineer does the calcs for the modifications"

2) Assuming "yes" to the above, is it the sort of work I can do myself (I am a pretty competent DIYer)

3) Is there generally enough redundancy in the trusses so that one can be modified at a time While the trusses on either side take the temporary additional load

A reply to my previous post suggested that the king post is simply a hanger to reduce the effective span of the truss (I guess to stop the horizontal member form sagging). If this is the case, is it likely that I'd be able to run a wooden beam from the gable, under the apex of a number of trusses with the other end supported by an vertical post that is supported by an additional joist that spans the wallplates ( I know, I need to do some sketches). With the above in place the kingposts could be removed (having created a new hangers from a noggins between the new floor joists to prevent the truss horizontal member from sagging)

Of course I'd get a structural engineer to draw up plans and to the calcs, but before spend £££ I'd like to get a consensus if what I'd like to do is even feasible.

Thanks

Chris

p.s. my next posting will have a link to some piccies and sketches!

You can place the new joists adjacent to the existing and bolt them through so that they no longer need support in the middle. Before removing the king posts, get a structural engineer to agree that they were only holding up the old joists, not the roof!

The main disadvantage of this method is that it isn't quite as good on sound insulation. This is usually only significant in flat construction, though, rather than houses.

You'll need to examine the roof construction to see if any additional strengthening is required to the rafters or purlins, especially if you are intending to add any weight to it, or intend to remove any additional struts such as any diagonal ones to the purlins.

Anything can be modified.

Only you can know your own competence.

Yes. However, if you bolt the new and old joists together, any pure king post becomes immediately redundant, so this isn't actually relevent.

Christian.