An Alternative to a Very Flat Roof Extension Reader BOB in SANDPOINT writes: “Hello we want to add a patio / work on car / whatever, extension off the front of an existing shop with a metal roof – joining the existing metal roof with the metal roof of the extension with a sort of pitch break. The challenge is the 10′ height of the existing roof and only having 18″ drop to the garage entrance. Ideally we would extend out about 14′ but I am thinking the extension roof would not be steep enough. I was planning on installing a pitch break joint where the two roofs meet (but not having an actual difference in pitch – or as little as possible).also, being in Montana we do have significant snow. I know with the pitch issue of only having the 18″ drop – and the target of 14′ I will have to compromise but trying to figure how much and where.”
Mike the Pole Barn Guru says:
Well Bob, you do have some challenges and are not the first person to have them. Most steel roofing companies will not warranty their product on roof slopes of under a three in 12 slope (3/12). At very least, doing a single sloping roof this flat, in snow country, poses a myriad of challenges – some of them you have already considered. Rafters and purlins for such an extension will require them to be engineered to support the weight of snow sliding off steeper main roof. Ponding is a real concern, as ice and snow will build up and not slide off. You should probably look at using 26 gauge steel with ribs higher than what is on your existing building (meaning steel profiles will not match) and perhaps installing over solid sheathing – either OSB or plywood, with 90# asphalt impregnated (felt) paper or a synthetic water and ice shield below. If you are thinking this is sounding expensive or perhaps impossible, there is an alternative option. Your extension roof could be done as a reverse gable – meaning you would not have height issues in front of your existing overhead door opening. This would also allow for snow to slide off to right and left of your extension, rather than plopping off directly into your line of travel in and out. We use reverse gables frequently, in new construction, to protect door openings from rain water or snow slide offs. Here is some extended reading for you: https://www.hansenpolebuildings.com/2015/07/reverse-gable-porch/
Engineered post frame building construction allows for nearly any situation a client can imagine to be achieved structurally. As some of you long-time loyal readers may have read – “You are only limited by your imagination, budget and available space”. Hansen Pole Buildings’ Designer Doug has a client who contracted with a third-party to create floor plans and elevation drawings. Sadly, Doug’s client paid $900 for this work, when it might have been done for $695 or even free with this service: https://www.hansenpolebuildings.com/post-frame-floor-plans/
As drawn, this design would have a fairly low sloped ‘shed’ style roof spanning 20 feet from building face to outside with a trussed roof system. These two reverse gables would be framed in on top of shed roof purlins. I can see some potential challenges occurring here. Shed roof slope appears to be less than a 3:12 roof slope. This voids steel roofing paint warrantees provided by most roll formers. It also means every side lap has to have a butyl sealant between overlap and underlap per R905.10.2 of the International Residential Code: “1. The minimum slope for lapped, nonsoldered-seam metal roofs without applied lap sealant shall be three units vertical in 12 units horizontal (25-percent slope).” While I was not privy to distance along the wall length of this shed roof, it appears to be a great enough distance so a fairly significant structural header will need to be placed from column-to-column to support the low heel of shed trusses. If this is in snow country, snow is going to build up between these two reverse gables and weight will need to be accounted for. While this design is totally doable, it will entail additional investment in materials, plus more than a fair amount of time to assemble everything and maintain water tightness. What would I have recommended? Instead of a shed roof design, use a reverse gable porch with a single gabled truss spanning from corner column to corner column. Roof slope could match the main building, being steep enough to maintain warranty and leak free integrity. Plus – much easier to construct!
Today’s article has been sparked by reader DARRELL in ATLANTA who writes:
“How to frame a reverse gable over a door on an existing pole barn. Thank you.”
Mike the Pole Barn Guru responds:
Ultimately this will be a question you will need to have answered by a RDP (Registered Design Professional – architect or engineer) who provided sealed plans for your original structure, as they will have to verify ability of existing structure to support loads imposed by this reverse gable porch. Most reverse gable porches are attached to a wall below the existing roofline. Structural design of new roof system will need to be able to accommodate weight of snow drifting against existing wall or sliding off from roof (depending upon whether reverse gable will be added to endwall or sidewall). Siding will need to be removed from existing wall in area of reverse gable. A truss will need to be placed against this wall – usually it will be easiest for span of truss (and width of reverse gable) to be from one existing roof supporting column to another. If reverse gable will have overhangs, then this truss against existing wall needs tails 1-1/2 inches longer (measured horizontally) than width of overhang, in order to attach fascia boards. 2×4 (wide face to wind) siding backing needs to be added to this wall, approximately two inches above truss and following same pitch. Your RDP can specify connection of truss to existing columns. It may be necessary to add a bearing block below the truss heels, in order to adequately support roof loads. Two new columns will support new gable parallel to the wall including your door. For sake of preventing things from running into them, they should be no smaller than 6×6, and be fully concreted into the ground. A single truss will be notched into face of these columns 1-1/2″ opposite from existing main wall. If reverse gable has no overhang, neither will this truss. If endwall overhangs, this end truss attachment will be lowered (in comparison to main wall truss) by thickness of purlins, adjusted for roof slope. A 2×4 siding backing should be nailed to face of both top and bottom chords of this truss. Depending upon span between these two trusses and roof loads 2×4, 2×6 or even larger purlins should be placed edgewise. They will butt into the side of truss against existing building and be attached with hangers. If no endwall overhang, attachment to opposite truss will be the same. If an endwall overhang will be included, then purlins run over second truss and are attached to top of it with Simpson H1 brackets. With an endwall overhang, solid 2x blocking will be placed between purlins, to prevent rotation. Solid blocking should be held 3/4″ out past 2×4 siding backing. With enclosed overhangs, soffit will attach to this solid blocking. With open overhangs, endwall J Channel will butt up against blocking. Fascia boards and fly rafters of same dimension as purlins will need to be installed, if there are overhangs.