Yes. Underlayment provides a vapor barrier between the roof deck and the shingles, and is a secondary waterproofing element of the roof, and prevents the wood deck from leaching oils from the asphalt shingles.
If a roof has more than one layer of roofing, the roof should be torn off. In most cases this is a code requirement. Codes rarely permit more than two layers of roofing. To determine this, all you need to do is contact your local building inspections department.
Below, are many other reasons you may need to consider completely tear-off the existing roof system(s).
Bad decking – if spongy areas are noticed when walking on a roof, or if you see sags between the rafters or trusses, there’s a chance that some of the roof deck may be deteriorating or be damaged. Deterioration can be attributed to dry rot or delamination of the plies in the plywood due to glue failure. Dry rot is wood rot caused by certain types of fungi and if it isn’t taken care of, it can spread. If there is suspicion of bad decking, then a full tear off should seriously be considered. Ice dams – ice dams can be a big problem. In areas where there the average January temperature can be below 30°F, and no ice and water protection membrane is present on a roof, then a full tear off should be considered. Twenty year old buildings with no ice and water protection and no prior problems, can suddenly experience thousands of dollars in damage when a freak cold front hits. Incompatible shingles – if a heavy weight architectural style shingle is used to cover a light weight strip shingle such as 3-tabs, then the roof will look good. However, if a light weight shingle is used to cover a heavy weight shingle, the light weight shingles have a tendency to show all the bumps and ridges (called telegraphing) and won’t look good. Existing roof is in poor condition – if the existing roof is in really poor shape, such as tabs being severely curled or if the rows are crooked, then complete tear off and replacment should be considered. Shorter Life Span – There is no known documented research, but most roofing professionals agree that with an overlay, the average lifespan of the shingles will be shortened by about 10%-20%.Ice & Water Shield offers premium leak protection for sloped roofs in climates subject to ice dams and wind-driven rain. It is applied to the roof deck prior to the application of the finished roof covering, which is most often shingles but can also be tile or metal. The membrane goes under shingles and seals around nails that hold the shingles in place, so water that doesn’t drain properly cannot penetrate the roof. It also creates a weather-tight barrier against wind-driven rains that cause shingles to lift and leak. Affordable and easy to install, it provides added peace of mind for many years, even in the most brutal winters.
Ice dams form when snow continually melts at the roof edge. When snow accumulates on a roof, the heat in the attic will cause it to thaw and the resulting water will run down to the eaves where there is no heat and it will refreeze. This can occur on a daily basis until large icicle form at the eaves. If no protection was installed when the roof was put on, the ice can eventually back up under the shingles and cause leaks. It can also get bad enough to rip the gutters right off of the building. Ice dams can allow moisture to damage attic insulation which reduces the R-value of the insulation and raises the energy bills, they allow water to penetrate wall cavities which can cause paint and plaster to peel and also rust nails, electrical boxes, or any other non-rustproof metal building material located in walls.
There are three good ways to help prevent ice dams or the damage caused by ice dams. Proper ventilation will help maintain the ambient air temperature at the roof level thereby not allowing the snow located on the roof above the living areas of the home to thaw. Heavy attic insulation will help insure that very little heat gets into the attic. The installation of an ice and water protection membrane to the eaves and valleys of the roof which will help prevent damage, but doesn’t treat the root of the problem, which is heat loss. Heat tapes are often used as a solution but rarely prove effective.
Attic ventilation is an important part of roofing. Proper attic ventilation extends the life of a roof and reduces problems because it minimizes the temperature differential between the attic and the air outside. Proper ventilation will remove moisture and heat from the attic. Trapped heat and moisture can raise energy costs, cause ice dams, and damage roof system components as well as structural and personal items located inside the attic where temperatures can easily reach 150°F (65°C). Condensation that forms inside attics can be caused by the use of washing machines, dish washers, bath tubs, showers, and tumble driers unless these items are properly ventilated through the roof. In some cases the condensation can be bad enough to be mistaken for a roof leak.