1. A New Roof
A new roof is a huge undertaking. You have to pull permits, remove the old roof, have it hauled away in an environmentally friendly way, know what to buy, get a good price on the materials, plan out a time to do the project (hopefully when it won't be raining) and then you have to actually do the work. For most novice diyers, this is simply too big of an undertaking. We have seen too many people hurt themselves falling off a latter or a roof. And we have seen too many homeowners have to admit defeat part way through the project and call in a professional to fix what they have started. In our community, we all believe in the guys at Orange County Roofing Contractors (http://orangecountyroofingcontractors.org) as they do great work and they stand behind it. You want to find a roofing company that has a good reputation, is licensed, bonded and insured, and will stand behind the quality of their work.
Again, if you have advanced skills and you can solder and work a level and know how to make sure that a pipe is draining properly, have at it. For the rest of us, plumbing is a high risk venture. The problem with plumbing is that after you finish the work you close up walls and floors and you can't see the work any more. If you develop even a small leak, it can be catastrophic and you may not see it for months - when so much of your house has been destroyed. Changing a faucet, sure go ahead and do it yourself. Running copper pipe throughout your house - get it done by a professional. We swear by the good folks at Bill Metzger Plumbing in Irvine (http://billmetzgerplumbing.com/). They have always done great work and they can usually fit us in pretty quickly.
3. Cinder Block Wall
This was a tough one for me. I have seen some great diy walls over the years. But the reason I caution against it isn't the fact that I have also seen some straight up disasters (although I have). The reason that I caution against it is because it is soul sapping, mind numbing work that ALWAYS takes twice as long as you think it will. I have seen too many homeowners quit part way through a cinder block wall much to the annoyance of their wives and so I caution against it. If you are a insanely detail oriented person that loves taking the time to do a job right no matter how long it takes - then go ahead. But for the rest of us normal people, hire a contractor. It isn't as expensive as you think it will be and in my opinion the time that it will take you to do it yourself (string line, anyone) isn't worth what it costs.
DIY Disaster Video
As a treat, I'll leave you with this DIY Disaster video. Hopefully it will inspire you to think before you DIY!