Page 13 - Summer 2017 GMC Vintage RVing

Technical Corner MAINTENANCE, PARTS, TIPS, INNOVATIONS AND MORE Why Does My Roof Leak?Jim Bounds | 1975 TransMode | 877-275-4462 | Cooperative Motor Works | www.gmccoop.comA real wet subject! In fact, it is not the roof leaking. Aircraft grade aluminum sheet does not leak. It is the seals on the perimeter and the penetrations in the roof sheet that leaks. Okay, how does that help me? Some have entirely covered their roof in the attempt to stop all leaks...and for a time that will work but the longevity of the job is related directly to the rubberized sealant and whenit deteriorates it is a real mess trying to scrape the entire roof. The trick is to stop the leaks and keep up with those sealed areas.First, on the perimeter you have dissimilar materials of overlapping seams at the front (A) and rear (B) of the main aluminum roof panel. Different flex, expansion, and shrink rates require this seam to be sealed with flexible, self-leveling material that will seep into the seam and seal uneven surfaces. The sealant needs to have great elastomer rate and be either clear or paintable.The side rails (C). General Motors stated the GMC Motorhome was designed for a 15 year useful life, this means the aircraft designed side rail seal “system” of a 1978 coach needed to be serviced in 1993. This rail is a cover for the highly pliable sealant (40-year-old butyl tape) under the rail. Gobbing this rail with sealant does little good. The lateral movement of that cap front-to-back rail is too great for most any visible sealants.The rail must be removed, the goopenpucky scraped out, the surfaces cleaned and new hi-elastomer rate sealant needs to be put in the rail, re-installed with its original type machine screws. (13 screws per rail in 1973, 25 screws in 1978) These two unique seams account for a large part of water leaks.Next are the “penetrations” in the roof. There are usually three square openings (14" x 14"), up to three small plumbing vents, a rectangular roof vent each having many screw holes in the roof to secure them. There are 20 screw holes for the ladder rail, three holes on each of the 10 top clearance lights, a TV antenna with no less than six screw holes with a 1" hole and roof radio antenna in a 1" hole – lets not even talk about the screw holes holding down the roof pod! I mean what could possibly causea water leak!The more holes, the more opportunities and the more vigilance needed. Holes do not seal forever and seal popping fromtwisting the body will start in the same place because that is the weakest seal. Like an aircraft fuselage, the GMC body is designed to flex. Modern chemistry has given us many sealants with unique properties, match the need with the sealant. Self-leveling, clear or paintable for open seams and wipe out excess. Hi-elastomer rate under the side rails and a good UV stable adhesive/sealant for screws and vent flanges.Finally, understand keeping leaks from a flexible body is a constant thing. Vigilance and careful inspection...and keep up with it.LadderMOTORHOME ILLUSTRATIONS BY BYRON SONGERWindow FrameLightsAVentAir ConditionerVentWindow FrameC Over Door CapWindow FrameACVentRailPlumbing VentsLightsBC ACVent or Air ConditionerBLightsWindsheild SeamBLightsWindsheild Seam Rear Cap SeamSUMMER 2017GMC MOTORHOMES INTERNATIONAL 13Seam