I'm planning a DIY loft conversion project. I plan to strengthen the (ceiling) joists by "sistering" with wooden joists. The ceiling joists are 4.5" by 1.5" whilst the floor joists in the house are 7" by 2" (actual measurements). From my knowledge of beams (joists), the stiffness varies with the width and the cube of the height. On this basis I calculate that theoretically I would need to sister five 4.5" x 1.5" together to achieve the same effect as a 7" by 2" . Given that I plan to use the space for storage only, I consider that it would be sufficient to sister two 4.5" x 1.5" on to each existing ceiling joist. I do not have sufficient space to install full length joists, so I plan to concentrate on the central half of the span only (the section most likely to sag, and the section carrying most of the additional weight).I plan to glue and bolt the joists together using large washers to avoid damaging the wood. I would welcome any comments on this plan.