According to an article from Bob Vila’s website, roofing repairs need careful deliberation before hiring your contractor:

Making good decisions is the key to minimizing near- and long-term- costs related to any home improvement. This is especially true for large complex jobs like reroofing. In this particular case, some of the most important decisions should be made before you hire a contractor or choose a shingle manufacturer.

The first decision is whether to simply patch leaks and damaged areas or whether partial or complete reroofing is in order. If you choose the latter, you’ll also have to decide whether to roof over your existing roof or whether to remove it. There are cost consequences either way.

The roofing system, alongside the gutter system, is the primary line of defense of your home against the elements. This is the reason why many roofing experts agree you should inspect your roof at least twice a year. Here are a few signs that should alert you that it’s time to call a reputable roofer in Palo Alto:

Presence of Molds or Rotting
Your roof and mold don’t mix. More often than not, mold begin to grow when your shingles are damaged so that moisture collects underneath the shingles. This speeds up the deterioration of your roof. In addition, molds can spread quickly, affecting an entire roofing system if left unattended.

Worn-out Shingles
Finding large amounts of “sand” in your gutter system is a warning sign that shouldn’t be overlooked. Although the granules look like sand, these particles are a sign that your shingles have started to erode.

An Inside Job
Inspecting the exterior surface of your roof isn’t enough. After checking the condition of your roof, head to the attic and check for any signs of water damage, such as leaks, dark spots, or sagging portions.

If your roofing system exhibits any of the signs above, it may be time to consider calling a roofing contractor. Many experienced roofers in Menlo Park, such as Shelton Roofing, will be more than happy to assess the condition of your roof and to advise whether your roof can get by with repairs—or may need complete replacement altogether.

(from Should You Replace or Repair Your Roof?, Bob Villa)