RV Doctor – How Do I Brace a Sinking RV Roof?

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April 23, 2009

Dear RV Doctor,
I have a roof air conditioner that is causing the RV roof to sink in. It’s on an older RV and water is pooling around the base and causing a leak. What can I do to re-brace the roof and raise it back to normal? – Tom Scot, (Shinglehouse, PA)


Tom, the constant jostling and the weight of a roof top air conditioner can take its toll on many of the older RVs still on the road today. It is quite common to find the roof area around the air conditioner sunken or sagging, especially on those RVs with “soft” roof construction. The typical result is a good-sized pool of water around the unit that often culminates in water leaking into the roof area.

It will first be necessary to determine the rafter spacing. On most soft roofs the rafter positions are very evident. Measure the distance between the centers of each rafter. It will usually be either sixteen or twenty-four inches. Next, completely remove the air conditioner. Uhh, be sure all the electricity is properly turned off and locked out first!

Keep in mind, the rubber seal will, in all likelihood, be ruined when removing the unit, so be sure to have a replacement on hand before starting. Take care when lifting the unit off the roof. Old sealants may cause the unit to stick to the roofing material often creating tears in the metal. Be sure to clean off the old gasket and any sealant from around the 14-inch opening and remove any staples through the roofing material around the opening.

Next, cut a piece of ¾-inch plywood – twenty-eight inches wide by however long is necessary to span two adjacent rafters. Usually there is one rafter fairly close to the opening and another, sixteen or twenty-four inches away.

Position the plywood flat on the roof, centered directly over the opening, making sure it straddles one rafter in front of the opening and one rafter aft of the opening. Next, from inside the RV, mark around the opening. Cut and remove that 14-inch square opening out of the plywood. You will now have approximately a 28” X 34” piece of plywood with an approximate 14-inch square cutout somewhere in the middle.

Now, mark and cut the plywood in half longitudinally down a centerline resulting in two, 14” X 34” pieces of plywood with a “U” shaped opening in each piece. Insert each piece into the roof cavity through the 14-inch hole, underneath the sheet metal yet on top of the existing rafters. Slide each piece into position on either side of the 14” opening taking care not to disrupt the roof insulation in the process. After both pieces are in position under the metal roof and on top of the rafters, secure the roof metal to the reinforcement material around the 14-inch opening only.

Next, apply new sealant and a new gasket, and then reinstall the air conditioner. This repair will not only effectively raise that sunken mounting position, but will also provide a much better supporting foundation for the weight of the air conditioner and possibly eliminate future water leaks.

(Please feel free to comment, however, please also note that due to the volume of communications I receive from multiple channels I cannot guarantee a personal response in every instance. However, questions of an overall general interest may be considered and published in an upcoming RV Doctor column.)

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1 comment

  1. John Shelton

    Before removing the AC unit from the roof, would you not recommend that one inspect the ceiling area around the AC (from inside obviously) for irregularities that would indicate rotted roof joists caused by even slight leaking over a period of time? Also inspect the joists carefully after AC removal. I had a travel trailer several years ago that correcting for a sagging AC led to a complete new roof, including ceiling, roof framework, and outside skin. Some framework repair on one side was also necessary including removal and reinstallation of much of the outer skin and windows on that side.. All of this was due to unattended small roof leaks. (This story should be a “word to the wise”)