Competence is very important when it comes to roof repair.  A Washington, DC roofing company that has no clue what they are doing will not do a good job. To check competence you will want to look at other jobs they have done.  Most roof companies will have a portfolio that showcases their previous work.  A competent company will be more than happy to share with you past projects they have completed.

Besides basic competency, you should also make sure that they have experience specifically in residential roofing.  Residential and commercial roofing varies quite a bit.  While a commercial roofer can probably handle a residential job they will not have as much experience and may not have all the necessary tools and knowledge to really go a great job. The same goes for residential roofers that have never done commercial jobs that are often flat roofing jobs.