When you get into a situation where you are asked to do work outside your scope of work, don't get into a situation where you have to find a sub at the last minute. Get quotes from you subs before you bid a job. If you go to work without a locked in price, you'll be open market when it comes to what you pay (time and materials).
There are consultants in the market just bidding work and subbing all of the work out. Anything you can self-perform will be more profitable, but anything else should be subbed out. If you are a new company starting out, stick with the type of work you know and sub everything out.
Construction industry veteran talks about using subcontractors in the construction field on this episode of the ProfitDig Construction Show.