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A GreenBuildingAdvisor question describes a homeowner’s proposed shed roof with 4 inches of Rockwool insulation, wood battens and a metal roof over an existing 5/8-inch ZIP sheathing layer. The post asks whether horizontal purlins and corrugated metal can avoid a second sheathing layer, but it does not provide a verified design or a confirmed answer.
A homeowner seeking to finish a Zone 4C shed roof has asked GreenBuildingAdvisor whether metal roofing can be installed over 4 inches of Rockwool exterior insulation using wood battens, without adding a second layer of sheathing. The question lays out a proposed assembly, but the source does not provide an approved design or a confirmed answer on the roof’s structure, fasteners or water management.
The homeowner says the house framing is complete and that a 5/8-inch ZIP sheathing layer is already installed on the roof. The proposed next steps are to place 4 inches of Rockwool above it, hold the insulation down with 2-by-4s, and attach those members with 8-inch structural screws. The homeowner also proposes using the 2-by-4s as an air barrier and to form the eaves, then installing metal roofing above them.
The main design question arose after one standing-seam metal-roofing company reportedly said an OSB deck was needed on top of the 2-by-4s. The homeowner wants to avoid another sheathing layer because of the added work, cost and weight, and asks whether horizontal 1-by-4 purlins over the 2-by-4s could instead support corrugated metal roofing.
The post also raises a separate concern: controlling water flow beneath the metal on a shed-style roof. The homeowner says the goal is to complete the roof before winter. No engineering calculations, product specifications, permit review or final installation details are supplied in the material.
Roof Support and Water Control
The question concerns more than choosing between OSB and purlins. The roof assembly must transfer the weight of the metal roofing and other loads through the battens and long screws into the roof structure. Whether the proposed members, fasteners and spacing can do that depends on project-specific details that are not provided in the post.
Water management is also a central issue. The homeowner specifically asks how to limit water moving beneath the metal on a shed roof, but the source does not establish what underlayment, drainage path, edge detailing or roof pitch the design would use. Those choices affect whether water can drain safely and whether the layers below the metal are protected. The proposed assembly should not be treated as a tested or approved construction detail based on the question alone.
For readers planning a similar roof, the practical takeaway is that insulation thickness alone does not determine roof suitability. The metal-roofing manufacturer’s requirements, structural design, local code and complete water-control details need to be considered together before materials are installed.
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The Proposed Roof Assembly
The GreenBuildingAdvisor item is a homeowner’s request for guidance, not a report of a completed installation or a formal building recommendation. It describes a monopitch-style home, an existing ZIP sheathing layer and a planned layer of mineral-wool insulation beneath a metal roof. The homeowner references an online “over roofing” example that uses another layer of ZIP sheathing over 2-by-4s, but says that approach would add labor, weight and cost.
The source identifies the home as being in Climate Zone 4C and says the homeowner is mostly working alone, with a goal of getting the roof on before winter. It does not state the roof pitch, building dimensions, design loads, screw pattern, metal-roof product or local approval status. Those omissions limit how specifically the proposed details can be evaluated.
“Can I use 1×4 purlins running horizontally over the 2x4s and corrugated metal?”
— Homeowner, in the GreenBuildingAdvisor question
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Key Design Details Still Missing
The source does not say whether a qualified professional or the roofing manufacturer has reviewed the complete assembly. It also gives no structural calculations or confirmation that 8-inch screws are suitable for the actual layers, loads, spacing and supporting framing. The reported recommendation for an OSB deck is attributed to one roofing company; the material does not include the company’s written specification or explain whether it applies to every product or roof condition.
Other unresolved details include the roof pitch, purlin spacing and attachment, metal-panel profile, underlayment, eave and ridge or upper-edge treatment, and how water would drain if it passes the metal panels. The question does not include an answer establishing whether corrugated metal over horizontal purlins is acceptable for this particular building. No conclusion about code compliance or permit approval can be drawn from the supplied information.
roof water drainage under metal roof
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Review Before Installing Roofing
The next step is to obtain a project-specific roof detail from the metal-roof manufacturer and have the support and fastening design reviewed by a qualified building professional or structural engineer. The review should address the complete load path, insulation and batten arrangement, fastener length and spacing, roof pitch, underlayment and water drainage, as well as applicable local code and permit requirements.
The homeowner’s stated aim is to complete the roof before winter, but the source does not report a decision, construction outcome or follow-up guidance. Until those details are confirmed, the proposed purlin-and-corrugated-metal approach remains a question rather than an established solution.
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Key Questions
Does the source confirm that purlins can replace the second OSB deck?
No. The homeowner asks whether 1-by-4 purlins can support corrugated metal over the proposed 2-by-4s, but the supplied material contains no confirmed answer.
What roof layers does the homeowner propose?
The post describes existing 5/8-inch ZIP sheathing, planned 4-inch Rockwool insulation, 2-by-4s attached with proposed 8-inch structural screws, and metal roofing above. The exact final assembly is unresolved.
Why is another sheathing layer being considered?
A standing-seam roofing company reportedly told the homeowner that an OSB deck was needed above the 2-by-4s. The post does not provide the company’s written specification or establish whether it applies to other roofing products.
What is not known about water control?
The homeowner is concerned about water flowing beneath metal on a shed-style roof, but the source gives no roof pitch, underlayment choice, edge detail or drainage design and does not report a solution.
Source: rss
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