Removing celing jiosts

[LauraH]

Im having a air source heat pump fitted in the loft and need a suction opening of 1.5 meters. To fit this in the roof space it needs to be sucking from the hall way which is were my eves are so i cannot fit the unit running parallel with the joists and need to remove three with an opening of 450mm to make it fit. house length is 8M by 6M brick with a tradition roof with two purlins half way down the roof joist measuring (71/2' by 2') The opening is surrounded by brick walls under neath and I will brace like a loft hatch with additional ceiling to roof ties.
Roof Pitch is 45degrees
see picture at:

http://postimg.org/image/8pl2vuejl/
http://s8.postimg.org/mw0tr2pet/Loft.jpg