Lee Calladine

I recently moved into a house that has a roof with raised tie trusses and no internal supporting walls. I wanted to use the loft as storage but the small dimensions of the roof ties are concerning for anything other than what nature intended. I have recently also had a survey done on the property to fit a heat pump, but would need the water cylinder to be located in the loft BUT I'm unsure as to what method can be applied to strengthen the roof ties as the wall plate is much lower and not accessible to bear weight and with no internal LB walls this is a 20ft span with nothing to bear against. How are raised tie roof trusses strengthened for a loft conversion? One carpenter mentioned about sistering larger joists and bolting them against the existing rafters with additional struts from rafter to joists along the edges, and another said about lowering all ceilings to implement a wall plate to wall plate span and using RSJs but that is a lot of work for a storage room. Any ideas?