You’ll more successfully choose and deal with roofers if you’re aware of all that can go wrong.


  • Where water has leaked and sheathing has rotted, shingles are simply nailed onto the rotten sheathing. The nails will pull out and shingles will come loose.
  • Unseasoned materials are used to replace sheathing. The materials will shrink, causing shingles to buckle.
  • The attic is not properly insulated or ventilated. The central part of the roof will be warmer than the overhang, causing snow and water to flow to the overhang and then freeze. The resulting “ice dam” may cause water to back up under the shingles and leak into the house.


  • At points where two planes come together, flashing is not put into place or woven valleys of shingles are not used. Leaks will occur at the joints.
  • Old flashings are reused although they are corroded, eroded, or punctured. They will leak.
  • No counterflashing is used, or what is used is not properly embedded in the mortar or adequately attached. Counterflashing should be installed over flashing materials to keep flashings dry; without it, water will leak past the step flashing.
  • Valley flashings are too narrow. In a heavy downpour, water will wash up under the shingles and turn back down under the flashings.
  • Flashings are not made of heavy enough material. They will crack, puncture, or erode away, causing leaks.
  • Instead of a series of short- to medium-length pieces stepped over one another, flashings consist of long pieces, nailed at various places along their length. With expansion and contraction caused by temperature change, the metal will fatigue and split or nails will pull out.


  • Nails are too short. If nails are not long enough, they may pull loose.
  • Nails do not have barbed (or otherwise deformed) shanks. They may pull loose.
  • Nail heads are too small. They may puncture the surface of the shingles, hastening deterioration and leaks.
  • Nails are driven into knotholes or spaces between sheathing boards. They will work their way up, forming lumps in shingles.
  • Too few nails are used. Shingles may come loose in wind or with repeated temperature changes.


  • Successive courses of shingles are not overlapped, as required for the slope of the roof. Water will back up under shingles or go through nail holes.
  • Cutouts or edges of shingles on successive courses are not adequately spaced, or openings of one course are too near nail holes on the course below. Water may weave from an opening on one course down through a nearby opening or nail hole in the course below, causing leaks.
  • Too little space is allowed at joints of wood shakes or shingles. They may swell and buckle in damp weather.
  • Shingles or slates are too heavy for the framing. Sagging of the roof may damage shingles or create leaks where shingles do not lie properly.
  • Damaged or cracked shingles are used. They may leak.
  • In a new roof that is applied on top of an old one, tops of shingles are improperly placed against butt ends of shingles on the roof. The roof’s surface will appear uneven.
  • The courses slant or the cutouts and edges are improperly aligned (neither random nor regular). Appearance suffers.
  • Bulges and warps in shingles of a previous roof are not flattened. Appearance suffers and leaks may occur.


  • Gutters sag, are loose, do not slope adequately, or are installed too low. If gutters overflow, or if water flowing from the roof overshoots gutters, it may cause water to enter the house.
  • A built-up roof is applied over a metal roof. The metal will expand and contract, tearing the felt.
  • Bituminous paint is used on a metal roof. It may form bubbles, allowing the metal to rust underneath.