The word "underlayment" can be confusing to some. While it isn't a commonly used word, you might hear it pop up if you are shopping for a new roof or if your existing roof needs to be repaired for any reason. In fact, it is very common that the outer (visible) roof material is in perfectly good shape, but the layer underneath (the underlayment) has damage that needs to be repaired. This is where most leaking becomes problematic. What keeps a roof waterproof is a combination of the outer material, the underlayment, and the weatherproof sheeting. This sheeting goes in between the two and is made out of a sort of viscose-like plastic. These three elements are key to the overall water resistance and weatherproofing of any roof.
When it comes to roof repair or replacement, the underlayment is the first thing you need to consider for a number of reasons. First of all, if the underlayment is damaged, it can affect the structural integrity of your roof. Severe rot or decay can be very expensive to repair if it penetrates the underlayment and reaches the rafters. Having a new underlayment is ultimately going to prolong the lifespan of your roof and protect your structure. In fact, some people will have their underlayment replaced where needed and then just have the same outer material reinstalled. On most roofs, you don't need to replace 100% of the underlayment. Instead, you just need to replace the boards that are actually damaged.
The Problem With Underlayment Repair
The problem with repairing or replacing underlayment is that it is nearly impossible to predict how many boards will need to be replaced until all of the outer material and sheeting is removed. For example, if you have a roof with 100 4'x8' boards, the roofer might suggest that you need to replace 20 of them. However, they might find—after exposing the underlayment—that over 50 of the sheets are going to need to be replaced. The end result is that you have to pay a lot more for the materials and labor or removing and replacing the boards. Of course, once the roofing material is removed and your underlayment is exposed, you're going to need to replace all of the boards that roof inspector suggests, or else your building could fall foul of the building codes.
In the end, even though roof replacement could end up costing more than you originally budget for, it is always going to be worth it when you take into account the long-term safety and structural integrity of your building.