Components of a Roof Series – Underneath the Shingles – Underlayment

Among roofing contractors, there is a lot of debate about the purpose of underlayment and what type you should use.  Some roofing contractors will tell you that underlayment’s only purpose is to “dry-in” the structure before the roof shingles are installed and that once the roof shingles are installed that the underlayment serves no purpose as the shingles themselves provide a waterproof roof.  This sounds good, but I recently inspected a roof on a five (5) year old house (basically brand new) that had roof leaks EVERYWHERE.  Upon inspection, it was clear that the roofer didn’t install underlayment.  This basically “new” roof required a complete tear off and new roof including replacement of large portions of the roof deck itself.  The general contractor that built this house skimped on a relatively cheap underlayment layer, and left the homeowners with a very large repair bill.  I know the roofing contractor that got that job (not me), and they will get a proper installation this time.

For the roofing contractors that identify that underlayment isn’t necessary, kindly explain to me why every shingle manufacturer includes underlayment in their installation instructions?  There isn’t a shingle manufacturer in existence that doesn’t include underlayment in installation requirements, and not one of them will honor a warranty claim on their shingles without it.

I fall into that underlayment is another component of the waterproofing system that protects the roof deck material itself.  Underlayment provides a second line of defense against wind driven rain.  In addition, have you ever seen a roof with a shingle or two blown off?  If you lose a shingle due to wind and you have no underlayment you have a guaranteed roof leak.  With underlayment you may not have a roof leak even if you lose a couple of shingles in a storm.  However, wind driven rain gets everywhere, every nook and cranny, even causing water to flow uphill for short periods, and redundancy matters.

My experience is that underlayment also serves a secondary purpose of establishing a smooth surface for you roofing shingles.  In my previous series on roof decks, I identified that asphalt roofing shingles telegraph surface imperfections; a good quality underlayment is layer that ensures a perfectly smooth roof surface, thus eliminating potential to telegraph imperfections.  For this reason, I always install 30-lb felt as underlayment as it provides a thicker surface that the cheaper 15-lb felt.  15-lb felt tears too easily around the nails or staples, tears too easily when walking on it to install the shingles, and is difficult to lay down without wrinkles.  All of these tears lead to imperfections in the surface, and potentially lead to telegraphing through asphalt shingles.  30-lb roofing felt minimizes or eliminates these shortcomings. 

On a typical 30-square roof, 3,000 square feet of roof itself, underlayment material costs for 30-lb felt are ~$300.  As your house is probably you largest investment, do you want to skimp $300 on the price of a new roof?  Those homeowners I talked about earlier would have gladly paid $300 more for their new house to prevent the magnitude of roof repair they had to undertake on their “almost new” house.  Seriously, that general contractor should be barred from obtaining any more building permits.  If you need help with an issue such as this, contact a local Lees Summit roofing contractor for assistance.

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