floor joists

[gedg]

can cut out a 4' hole from a main joist for a waste pipe \if so how do i re strengthen joist

[Lardybloke]

I take it you are just cutting a slot out from one side. Attach strong battens alongside the remaining joist on all sides if possible. And if it is possible I would recommend putting in some noggins to span across to ajoining joists.