We recently had the roof of our 90 year old home re-shingled. The roof decking is boards, not plywood or particleboard, consequently there are gaps between the boards. We expected tarpaper to be installed before the shingles, consistent with previous installations. The roofing company told us that tarpaper wasn’t installed, as it would void the manufacturers warranty. It seems to us that the tarpaper provides an additional level of softening/insulation and water-tightness between the shingles and the roof. The boards have gaps and are not perfectly smooth, which would also make the shingles less prone to breaking. This would especially be a concern when someone walks on them to put up storm windows.
Tarpaper on roofs, properly called asphalt-saturated felt, was a common product used for many years on older homes with natural wood roof sheathing. In some areas it is still widely used, while in others it is normally omitted, especially on newer roofs sheathed with plywood or OSB. There is some debate about the need for this underlayment, and in answering your question we will look at some of the points on this issue.
The roofing contractor that installed the shingles on your home was either misinformed or making a poor excuse for not installing underlayment on your roof. I have never heard of this product voiding a manufacturer’s warranty and many recommend its use. Two Canadian manufactures of asphalt roofing are IKO and EMCObp and their websites both address the use of roofing felt underlayment.
Most asphalt shingles have bases composed of either organic cellulose-based material or fibreglass-based material. Both have advantages and disadvantages and determining the type installed on your new roof will help answer your question. If the shingles are organic based, which is much more common in our cold climate, underlayment is not a requirement, on most roofs, but is recommended by both manufacturers cited. If your shingles are glass-based and your roof has a pitch less than 8/12, which is quite steep, EMCObp actually requires an underlayment be installed. If the pitch is less than 4/12, which is normally considered low slope, they require two layers of felt to meet their standards.
Asphalt shingle underlayment is typically dry felt that is impregnated or coated with an asphalt saturant. According to IKO (www.iko.com) the use of an underlayment is recommended to protect decking material from wind-driven rain. They also cite the Canadian Asphalt Shingle Manufacturer’s Association (CASMA) in stating that asphalt-saturated felt protects shingles from the resins that may be released from wood board decking. They go further to state that many manufacturers require the use of asphalt-saturated felts on wood decks to validate warranties. This is directly contradictory to what was stated by your roofer. The company may not have been negligent in leaving out the felt, but they are not being honest with you and should have given you the option to include this as an upgrade. I don’t think you have much recourse in blaming them for not including the tarpaper, as that is quite common, but they may deserve a good dressing down for not giving you the straight goods on the true reason for the omission.
Roofing underlayment should provide extra protection from water infiltration, but will add additional cost to the job. Its use may not be necessary, but is a sign of a high quality job. The roofing felt will help to provide more complete coverage of the gaps between the boards you noted. This will reduces the risk of water infiltration due to wind driven rain, or if shingles are torn or cracked. CASMA also notes that use of underlayment can help minimize “picture framing”. This is the noticeable outline of the roof boards, below the roofing, due to irregularities in roof thickness.
According to the CASMA website (www.casma.ca), “There are many reasons why the use of asphalt-saturated felt as an underlayment, prior to applying asphalt shingles, makes good roofing sense”. In many home repairs or renovations there may be steps that are not always necessary, which can be omitted to minimize cost, but will provide a better end result if included. Roofing underlayment falls into this category. If you talk to five different general contractors or roofers you will likely get five different opinions, but I think the choice to include roofing felt in your upgrade is summed up well on the EMCObp website (www.emcobp.com). “Considering the relatively low cost of felt underlayment, EMCO recommends it be used under the vast majority of roof applications”.