A warranty is “a company’s written promise to repair or replace a product that you buy from them if it breaks or does not work, usually for a specific period of time after you buy it” according the the Macmillan Dictionary. When it comes to construction, there are two different types of warranties that are generally provided. 
The first is called a “materials warranty”. This is a promise that the manufacturer gives with regard their materials. It is important to find out what the materials warranty is on the roofing materials your roofer utilizes in fixing or replacing your roof. Materials like asphalt shingles will have a much shorter warranty period than materials like metal roofing.
With a materials warranty, if the materials were installed correctly, you should not have a problem for the length of time the manufacturer notes. If you do have a problem with the materials, they may provide you with new material as stated on the warranty.
The second type of warranty commonly found in construction is a warranty for the work done. This warranty is different company to company, and not all construction companies will provide them. However, the idea is that if the materials are fine, and the installation was at fault, the company will repair their installation for a certain period of time after the work was completed.
A warranty shows that the company stands behind the work they will do, and that they expect to be around and available for a good length of time. When you look for a roofer in Portland, be sure to check what types of warranties they provide, and discuss with your roofer which materials will be best by comparing quality, warranty, and price.