One of the most common additional cost items customers run into when having a roof replacement quoted is the need for new plywood underlayment. It can be confusing to explain, and expensive, so this article will hopefully help you understand why it is needed in the first place.
In “olde timey” days there was no such thing as plywood. When people wanted to make a solid surface they took milled planks of 1” thick wood (usually 8-12 inches wide) and nailed them onto some supports. This method was effective in that it allowed for a nailable surface and made use of available materials, but it was in no way perfect.
The boards themselves were prone to warping and cracking. And over time, as they dried out, they shrank, creating gaps as large as an inch between planks.
You probably never even thought about that. After all, your roof has been up there this whole time and has never even mentioned this to you! Now, here you are, some 70 or more, years after your home was originally built, looking to replace your roof and up pops the news that you need to install new plywood sheathing in order to properly install your new roof.
Well, here is why… When new shingles are installed, it is impossible to predict where exactly the nails will need to be placed in order to properly attach them. If the boards are 10” wide and the gaps are 1”, there is around a 10% chance that any given nail will hit a gap in the sheathing.
What this means is that several problems are likely to occur:
1. The nail potentially will work free because it’s not secured into any firm backing. This may leave a hole in the shingle and substrate through which water can infiltrate.
2. The nailed shingle has a higher probability of raising or being impacted by high winds. This could also lead to leaking and aesthetic problems. Depending on where the shingle is on the roof, it could even cause the shingle to be susceptible to being pulled from the roof entirely in very high winds.
3. Even if the nail stays in place, the lack of tension and downward pressure it would normally apply to the shingle and underlayment could itself cause leaking. Water has a way of finding the weakest link in any barrier and a loose nail is almost guaranteed to act as a funnel for water eventually.
So you may be asking yourself why wasn’t this a problem in the past? Well, there are several likely reasons. First, you could just have gotten lucky. More likely however is that you had multiple layers of shingles on your roof meaning that one of the newer shingles was helping to hold down and shed water over one of the older ones.
Regardless of whether it was luck or multiple layers that had you avoiding problems in the past, now is the time to take preventative measures. Contractors (good ones) are not in the business of creating future problems for customers, or themselves, and will likely insist on new sheathing, if it is needed.