Shingle Roof Outsulation and Venting
We are located in Austin, TX. The home has a flat roof with a new PVC membrane as the overlay. For aesthetics and performance, we would like to add a sloped roof over the existing roof. Is it possible to install a sloped roof with insulation/outsulation with asphalt shingles being the roofing material. The PVC is to remain in place. Wishful thinking is the new roof can be an added layer of insulation and protection while allowing any leaks to dry to the outside of the shingle roof leaks if water finds its way to the pvc overlay. Removing the PVC roof is not in the budget. Moreover, we would like to use mineral wool as the insulator (comfortboard). Is, rafter + osb + outsulation + purlin + second layer of osb + membrane the only way to accomplish this? We are trying to minimize cost and weight. A metal roof would be one work around but may be out of the budget.
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Replies
Adding a sloped roof over an existing flat roof with a PVC membrane in Austin, TX, can indeed improve both aesthetics and performance. Your idea to incorporate insulation in the process while keeping the PVC membrane intact is feasible and can provide additional protection and thermal performance. However, it requires careful planning to ensure proper construction and moisture management. Here's how you might approach this:
1. Structural Considerations:
Firstly, consult with a structural engineer to assess the existing roof structure's capacity to support the additional weight of the sloped roof system, including rafters, sheathing, insulation, and shingles. The engineer can also advise on the design of the new sloped roof to ensure it meets local building codes and can withstand local weather conditions.
2. Designing the Sloped Roof:
Your proposed assembly, involving rafters, OSB, insulation (comfortboard), purlins, a second layer of OSB, and a roofing membrane, is a sound approach but might be more complex and heavier than necessary. Considering your goals, here are a few adjustments:
Insulation Placement: Using mineral wool (comfortboard) as the insulation material is an excellent choice for its thermal performance and moisture resistance. Placing the insulation above the first layer of OSB sheathing can provide the outsulation you desire. Ensure the comfortboard is adequately secured and covered to protect it from the elements.
Simplified Roof Assembly: A more straightforward and potentially lighter option might be rafter + OSB + comfortboard insulation + waterproof breathable membrane + asphalt shingles. This assembly assumes the insulation layer is sufficient to prevent thermal bridging and allows for any potential moisture between the layers to dry out.
Moisture Management: Ensure there's a strategy for managing moisture that might condense beneath the shingles or on the upper surface of the PVC membrane. The breathable membrane above the comfortboard can allow for outward drying, but you'll also want to ensure that any moisture reaching the PVC layer can dry or drain away to prevent trapping water.
3. Alternatives to Consider:
Metal Roof Alternative: While you mentioned a metal roof might be out of the budget, it's worth getting quotes. Metal roofs are lightweight, durable, and offer excellent performance in various climates. They can be installed over lightweight framing and might not require the second layer of OSB, potentially offsetting the cost with reduced complexity and labor.
4. Cost and Weight Minimization:
To minimize cost and weight, consider:
Reducing layers where possible by choosing materials that serve multiple functions (e.g., insulation boards that can also act as a nailable surface for shingles).
Exploring lightweight framing options for the sloped roof to reduce the overall load.
Conclusion:
Adding a sloped roof over your existing flat PVC membrane roof with integrated insulation is a viable project that can enhance your home's aesthetic and performance. Prioritize structural integrity, moisture management, and thermal performance in your design. Consult with professionals to ensure the project's feasibility, especially concerning structural support and moisture dynamics, to achieve a cost-effective and durable solution.