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A homeowner asked GreenBuildingAdvisor for guidance on a shed-roof assembly using 4 inches of exterior Rockwool insulation beneath a metal roof. The report records the proposed layers and the homeowner’s questions, but it does not provide a verified design or establish that purlins and corrugated metal are suitable for this roof.
A homeowner building a Zone 4C shed-roof house asked GreenBuildingAdvisor how to install a metal roof over 4 inches of exterior Rockwool without adding a second layer of roof sheathing. The reader’s proposed assembly and questions are documented in the report, but the supplied material does not include a confirmed design recommendation or establish that the proposed purlin-and-metal approach is safe for the building.
The reader said the house framing was complete and that a 5/8-inch ZIP sheathing layer had already been installed on the roof. The proposed next step was to place 4 inches of Rockwool above it and hold the insulation down with 2-by-4 members. The homeowner also described those members as serving as an air barrier and forming the eaves, with 8-inch structural screws intended to secure the assembly.
Above the 2-by-4s, the reader proposed installing a metal roof. According to the post, one standing-seam roofing company said an OSB deck above the 2-by-4s was needed. The homeowner also cited an online example of “over roofing” that used another layer of ZIP sheathing, but wanted to avoid that added deck because of labor, cost and weight.
The specific alternative under consideration was horizontal 1-by-4 purlins over the 2-by-4s, topped with corrugated metal. The reader also asked how to limit water flowing beneath the metal on a shed roof and said the goal was to get the roof completed before winter. The supplied report contains the question, not a documented answer resolving those issues.
Roof Attachment and Water Control
The question concerns more than choosing between sheathing and purlins. The proposed layers have to work together as a roof assembly: the insulation and framing need a reliable attachment path, and the metal roofing system must be installed in a way that addresses water movement and the roof’s geometry. The reader’s concern about water beneath the metal is especially relevant to the design they described, but the source does not provide a tested detail or confirm how that concern should be handled.
For homeowners, the report is a useful record of a common design dilemma: reducing added material and installation work while still meeting the requirements of a particular roofing product and building. It is not a construction specification. The roofing company’s advice, as reported by the reader, applies to the company’s proposed standing-seam approach; it does not by itself answer whether a different metal profile over purlins would work with this exact assembly.
That distinction matters before materials are installed. A roof design depends on project-specific details, including how the roof covering is attached and how loads pass through the insulation and framing. The supplied material gives no engineering review, product-specific installation instructions or confirmation of local code requirements. Readers should not treat the question alone as approval of the proposed screw lengths, purlin layout or roofing method.
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The Proposed Roof Layers
The homeowner described a monoply-style house, a term used in the original question, with a shed roof in Climate Zone 4C. The reported starting point was 5/8-inch ZIP roof sheathing. The planned build-up added Rockwool above that layer, then 2-by-4s, followed by a metal roof. The reader’s design therefore differs from a conventional roof in which the metal covering is installed over a continuous deck without the stated exterior insulation assembly.
The report identifies two approaches the homeowner had encountered: a roofing company’s stated requirement for OSB above the 2-by-4s, and an online “over roofing” example that used an additional ZIP layer. The reader’s preference was to avoid a second continuous deck and instead use horizontal 1-by-4 purlins with corrugated metal. Those examples are background to the question, not confirmation that either assembly—or the proposed alternative—has been approved for this house.
The account provides no roof dimensions, pitch, local wind or snow design values, specific metal-roof product, fastener schedule or complete details for the eaves and water-management layers. It also does not explain what the reader meant by the 2-by-4s acting as an air barrier. Those missing details limit what can be concluded from the report.
“Can I use 1×4 purlins running horizontally over the 2x4s and corrugated metal?”
— The homeowner, in the question posted to GreenBuildingAdvisor
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Key Assembly Details Remain Open
The supplied source does not say whether the reader received a response, what final design was selected, or whether the roof was completed before winter. It also does not confirm that horizontal 1-by-4 purlins and corrugated metal can replace a sheathing deck in this particular project.
Important project-specific information is absent, including the roofing manufacturer’s installation requirements, roof pitch, framing and attachment calculations, and details for managing water at the roof edges and penetrations. The report does not establish whether the proposed 8-inch screws are appropriate for the full assembly or whether the listed materials meet applicable code requirements. The reader’s description of the 2-by-4s as an air barrier is not accompanied by construction details demonstrating how that function would be achieved.
As a result, the article supports only a limited conclusion: a homeowner asked for help comparing roof-deck and purlin approaches. It does not verify a buildable assembly or resolve the water-control question. Any specific recommendation would need to be checked against the selected roofing product and the house’s plans and site conditions.
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Verify the Design Before Roofing
The next step described by the reader was to obtain guidance and complete the roof before winter. The supplied source does not report a later update or a final decision. Before proceeding, the homeowner would need to confirm the complete assembly with the metal-roof manufacturer and the relevant design professionals, including how the roofing will be attached over the insulation and what deck or purlin arrangement the product permits.
Project-specific review should also address the roof’s water-management details and applicable building requirements. Until those checks are documented, the proposed purlin-and-corrugated-metal arrangement should be treated as an open question—not as a confirmed alternative to the OSB or ZIP deck approaches mentioned in the report.
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Key Questions
What roof assembly did the homeowner propose?
The reader described 5/8-inch ZIP sheathing, 4 inches of Rockwool, 2-by-4s secured with planned 8-inch structural screws, and a metal roof. The exact final assembly was not confirmed in the supplied report.
Did the report confirm that 1-by-4 purlins would work?
No. The homeowner asked whether horizontal 1-by-4 purlins could support corrugated metal over the 2-by-4s, but the supplied material does not include an answer or an approved design.
Why was a second sheathing layer being considered?
The homeowner reported that a standing-seam roofing company said an OSB deck was needed above the 2-by-4s. The reader also cited an online example using another ZIP layer, while seeking to avoid added labor, cost and weight.
Does the source explain how to control water under the metal?
No. It records the homeowner’s concern about water beneath the metal on a shed roof but does not provide a verified water-management detail.
What remains to be checked before construction?
The selected roofing product’s installation requirements, the project’s attachment and framing details, water-management provisions and applicable code requirements remain unverified in the supplied report.
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