Ask The RV Doctor – Sinking Roof

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July 8, 2013

by Gary Bunzer

Q: 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

A: 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 16″ or 24″. Next, be sure all the electricity is properly turned off and locked out, then completely remove the air conditioner. Keep in mind that the rubber seal will likely 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″ opening and remove any staples through the roofing material around the opening. Next, cut a piece of 3⁄4″ plywood – 28″ wide by however long is necessary to span two adjacent rafters. Usually there is one rafter fairly close to the opening and another 16″ or 24″ 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″ square opening out of the plywood. You will now have approximately a 28″ x 34″ piece of plywood with an approximate 14″ 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″ 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″ 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.

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