Uneven Roof Surface

By Peter OConnor, Project Manager | April 20, 2020

When you look up at your roof and notice deflection or uneven surfaces, what is the cause? The answer can be any number of issues.

Many roof problems start because older structures were not designed to carry the heavy snow loads in New Hampshire. Although each town is different and to generalize for example purposes, most towns in New Hampshire have a ground snow load of 70lbs per square foot. That’s about the equivalent of a 10-year-old child standing on your roof at every square foot of its footprint. Realistically, we don’t see that much snow load every year, but it’s not out of the realm. When it happens, our roof structure often deflects and/or creeps, and doesn’t always bounce back to what it once was, i.e. level.

In the photo below, the client asked us to inspect their roof because there were many areas of uneven roof surface. Our inspection found a house that had numerous additions put on at varying times. The roof framing for each addition had varying issues causing the uneven roof surfaces.

A high line, running perpendicular to the main house ridge was caused because the home had an addition constructed off this location. This high line was the original exterior wall that still extended up toward the roof and acted as a stiff support directly below it. The wall was framed as a structural wall, so it was not deflecting the same as undersized framing on either side of it.

Another addition over the kitchen and porch was not only under designed but the framing connections were not constructed correctly. Multiple connections had failed. Unfortunately, in this case, the best and most practical solution was to remove the entire roof structure and start over. Believe it or not, the client was happy to hear this because they had been wanting to increase the ceiling height in the house, so a new roof structure could simultaneously accomplish this renovation.

While there certainly can be structural concerns with deflecting roofs, some deformities can be cosmetic. This can be related to poor framing installation or defective roof finishing material, such as shingles or rolled rubber roofing. Poor installation of the roof sheathing can also cause uneven roof surfaces that get confused for a bigger problem.

When plywood roof sheathing is installed, manufacturer recommendations say to leave a 1/8” gap between the sheets of plywood. This is to allow for expansion of the plywood panels as their moisture content increases from humidity or rain, before the finished roofing material is installed. As the sheets expand, and don’t have enough room between adjacent panels, they will begin to bow. This can give the illusion of a problem with your roof but is typically only a cosmetic issue.

When Team Engineering inspects roof framing, we typically find repairs that were done incorrectly or addressed the wrong problem. These may be repairs done by homeowners, or contractors that don’t first consult with a licensed Professional Engineer. For example, a damaged roof rafter can’t be patched with only a 2”x piece of lumber that is shorter or shallower than the original rafter. A sagging or deflected rafter can’t be supported by the attic floor framing if that framing isn’t designed to carry that load.

Team Engineering can inspect to determine the cause and design a solution to repair the problem(s).  If you have a concern with your roof or have questions about a roof repair you are looking to install, call Team Engineering to get our expert advice!

Leave a Comment

Your email address will not be published.