Guest User
4/28/2015
I will DIY installing a standing seam roof on my converted cabin, with a front and back porch pitch change from 6:12 heated space to 3:12 unheated space. Ice damming has been an issue. The majority of my online research has shown most manufacturer's installation guides use Z type flashing at pitch changes between panels. This is a concern with the ice damming because it may allow ice dammed water to backup, as with the current asphalt shingles. I've heard small pitch changes (not exceeding 3:12 from drain plane to drain plane) are possible with lighter, 26 gauge metal through bending the panel, but haven't found a manufacturer's install guide to confirm this. I've seen this configuration in my town, but haven't gone knocking on neighbors' doors asking about it. What are your thoughts on this? Do you know of any manufacturer that approves this configuration?
Dick Bus
4/28/2015
I think the configuration you are referring to is probably an exposed fastened panel that is overlapped at the pitch break. That is not possible with a standing seam nor will it eliminate the problem caused by ice damming. you can greatly reduce the amount of ice damming with adequate insulation of the living space and properly balanced ventilation under the roof panel. Also installing a high temperature peel and stick underlayment from the eave till four feet beyond the inside of the exterior wall (I hope that makes sense) will eliminate leaks from ice damming. I would also recommend that you use a metal panel that is manufactured by a member of the Metal Roofing Alliance as they all have agreed to supply the best standard for residential applications.
Guest User
4/28/2015
Hi Dick, Thank you for your response. I am familiar with the peel and stick water and ice shield underlayment, and it's IRC requirements in my area. It brings up a few questions, if you don't mind replying. Can water and ice shield adhere to asphalt shingle? I have a single layer of asphalt shingle and am hoping to avoid a tear off. Some MRA manufacturers do list conditions in which standing seams can be installed over asphalt shingle, provided a sufficient underlayment is installed first, without purlins. I've heard purlins, if new decking is not installed, help under panel ventilation, but may add to condensation on the underside of the panel. What are your thoughts? Again, thanks
Dick Bus
4/29/2015
Matt, adhering ice and water shield to one layer will not be a problem. The may reason I recommend removing the existing shingles is that way you can better inspect the plywood for any deterioration that may have occurred. Purlins running parallel to the eave does not provide any ventilation. you would need to create a grid pattern with purlins running vertically (best attached to the trusses) and then running horizontally. Ventilation, insulation as well as a vapor barrier will prevent condensation. Take a look at Cool-vent insulation board by Hunter Panel with 5/8 plywood that can provide ventilation and insulation in one step. Make sure that you have a balanced intake and exhaust for the ventilation to work properly.

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