In the loft of my bungalow I have two perlin beams 9" X 3" ( main roof support) which are 9ft 3" apart. To create further space, I am looking at fitting wooden joists on steel joist hangers between the two perlins. It's my intention then to relocate the water storage tank ( capacity 40 galls). Can some one tell me what size joists I will need for this.

Many thanks,

Benno