Loft Conversion - Sistering boards onto existing

[BarryDragon]

I am planning on doing a loft conversion over a few years. With the initial step being fancy storage for my home business, with the intent on putting stairs in a few years down the line as funds allow and it eventually becoming a habitable space.

My plan is to initially strengthen the loft joists by sistering 7x2" joists to the existing joists 3x2" and then lay down chipboard flooring. I will be getting a new lighting and plug circuit bought up from the consumer unit so there is light and plugs up there.

Further down the line I will reinforce the roof joists and make walls and insulate with Kingspan boards and multi-layer foil. And then stairs and finishing touches on the conversion, though this will be around 2 years down the line. All work will be carried out to building regs.

Will sistering the 7x2 joists to the existing 3x2 joists with purlins between each of the joists be sufficient for a loft conversion and secondly for me to keep stock up there.

The stock is not heavy.

The joists will be 4.2m long from outside wall to interior wall and then from interior wall to exterior wall. The spacing of existing joists is 16".

 

[freddiemercurystwin]

Is the internal wall structural then? How do you know it can cope with the additional loads? Generally there is some roof structure to be removed and its removal often involves a couple of steels. Got a pic of the inside of your loft? BC will likely insist on some SE calcs to back any structural alterations up.

 

[BarryDragon]

The loft is currently several inches deep in cellulose insulation so not sure pictures can add much in this case.

The attic is of gable construction and the roof is held up by a large wooden beam going from dividing wall to wall on the front and back of the house. The house is typical victorian construction built circa 1910.

There will not be any need for any roof structure to be removed in this case given its held up by the beam and that will remain. I intend on reinforcing the roof beams and adding rigidity to the roof with the walls being secured to the new roof beams and the floor joists.

I have done a bit more research since I originally posted this (still alot to do), I was hoping to be able to use 7x2 but I know that that is not advisable given the span of 12ft. At the moment my intentions are 8x2 C24 joists.

Using table F-1 on http://www.awc.org/pdf/STJR_2012.pdf and https://courses.cit.cornell.edu/arch264/calculators/example8.1/ it appears that 8x2 will be needed and altough I can just about get away with C16, I feel C24 would be a sensible option, as I dont want beams that are at the maximum span and load.

I have briefly contacted BC and will be booking an appointment with them once I have a firm idea of what my plans are and I have been advised to take some drawing of what work is planned.