My Roof is Sagging

Until homeowners or a home buyer observes the ridge of their roof sagging or a big dip in the plane of the roof, they care little about how their roof is framed.  When they observe a sag or dip in their roof they begin to wonder why this has appeared after all these years.

So what causes a roof to sag?

Roof framing is commonly comprised of rafters, which are the sloped members that the roof deck is nailed to, and ceiling joists or rafter ties, which are the horizontal members. When you are standing or sitting in an attic the rafters are seen above you and the ceiling joists below you. These members are commonly configured such that they form a triangle with two sloped top sides and one flat bottom side. The triangle is a very interesting shape because no matter which direction you apply a load or a pressure, the other two members will provide support.

For example, when rafters support the weight of the roof , they push against the opposing rafters (at the ridge board) and push (thrust) outward at the supporting wall (Figure 1). The ceiling joists not only provide vertical support for the weight of the ceiling as a beam, but they also resist the outward horizontal thrust of the rafters at the top of the wall. That is, the ceiling joists (or rafter ties) restrain the end of the rafters and the top of the wall from moving outward. The restraint by the ceiling joist at one side of the dwelling is countered by the outward thrust of the rafter and the restraint of the ceiling joist at the opposite end (Figure 2).

Some of the findings in our inspection recently.

Generally there are three causes of saggy roof:

  1. Weakened roof material
    Water damage can be a major cause of roofing problems. It can cause sections of your roof to rot and weaken. There are lots of ways snow and rain can infiltrate your roof. Moisture can find weaknesses in your roof and settle there. As it sits, it causes the roofing material to deteriorate. It can even cause mold and mildew to develop, further weakening it. Your roof has some protection from the elements – shingles and flashing, or example – but that doesn’t mean that it’s impenetrable. Make sure your roof is properly draining rainwater off of it. Have a professional do periodic checks for signs of water damage. Poor ventilation is another way your roof material can weaken. Poor ventilation means that air isn’t circulating which means that moisture is sitting stagnant on your roof.
  2. Faulty Roof Material
    The quality of your roofing material plays a big role in determining how long it will last. A roof constructed using asphalt shingles may be less expensive, but it won’t last as long as one that uses, say slate, which can sometimes last 100 years. This applies to all of the elements that come together to form the roof – beams, nails, etc. Also, the best roofing materials in the world won’t matter if they are not installed properly. Make sure that whatever roof work you have done is done by a professional.
  3. Too Much Weight
    Snow, ice and rain can cause big problems if they’re allowed to settle and sit on your roof for a long time. Your roof is designed to support a certain amount of weight. It can withstand the occasional snow or rain storm, but prolonged stress isn’t good for it. According to information from the University of Arkansas, it’s important to weigh the risks when a homeowner is deciding whether or not to remove snow from a roof. “For example,” a report for the school reads, “if you leave the ice and snow on a roof, the roof may risk collapse. If you remove the snow, brittle shingles are likely to be damaged, or the person removing the snow may get injured. The expense and liability of having someone else remove the snow needs to be considered before taking action.” Try to learn how much weight your roof can take by getting the original plans for your house or by consulting a professional. A professional could also tell you whether the amount of snow or ice you are dealing with is safe or not.

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