Can You Install a Slate Roof On A Formerly Shingle Roof?

 

Replacing an asphalt shingle roof with a slate roof is one of the most significant roofing upgrades a homeowner can make. In Pittsburgh, where many homes were originally built with slate and later converted to shingles, this type of project is especially common—but it is also commonly misunderstood.

Slate roofing is not simply a higher-end shingle. It is a completely different roofing system with unique structural, material, and installation requirements. Homeowners searching for slate roof installation Pittsburgh or slate roof installers near me often discover that their existing roof must be carefully evaluated and upgraded before slate can be installed correctly.

This article explains what is actually required to convert a former shingle roof into a properly installed slate roof, and why experience matters at every stage.

Why a Former Shingle Roof Needs Special Preparation for Slate

Asphalt shingles are lightweight and forgiving. Slate is heavy, rigid, and designed to last for generations. Because of that, a roof system that performed adequately under shingles may not meet the standards required for slate.

Many of the problems seen with slate roofs—cracked tiles, leaks, or early failure—can be traced back to improper preparation when converting from shingles. A successful slate roof installation starts long before the first piece of slate is installed.

Reverting Back to Slate on Homes Originally Built With Slate

In Pittsburgh, it is very common for homes to have originally been built with slate roofs, only to be switched to asphalt shingles decades later due to cost or convenience. When current homeowners decide they want slate installed again, the process often differs slightly from installing slate on a home that never had it.

In many cases, the home’s original structure was designed to support slate, which can work in the homeowner’s favor. However, this still cannot be assumed. Over time, framing may have been altered, decking may have been replaced with thinner materials, and previous roofers may have removed slate-specific components such as solid decking or copper flashing. Even when reverting to slate, a full structural evaluation and decking inspection are required to confirm the home still meets slate requirements. Restoring a home to its original slate roof can be an excellent decision, but it must be done with the same care and preparation as any slate installation.

Replacing A Slate Roof With Shingles

In most cases we will recommend against this since slate roof’s are designed to last longer and can be repaired in sections starting with the worst. We will always recommend extending the natural duration of a slate roof if possible, rather than replacing a slate roofing system designed to endure 100 years with a shingle roof that is designed to last up to 30.

Structural Evaluation Comes First

One of the most important requirements when replacing shingles with slate is confirming that the home can safely support the additional weight. Asphalt shingles typically weigh only a few pounds per square foot, while natural slate can weigh three to five times more depending on thickness.

In Pittsburgh, many older homes were originally framed for slate and can still support it, but that cannot be assumed. Over decades, framing may have been altered, repairs may have been made, or materials may have weakened. A professional slate roofing contractor will evaluate the rafters, spacing, lumber size, ridge beams, and overall load path of the roof.

If reinforcement is needed, it must be addressed before installation begins. Installing slate on an under-supported structure can lead to sagging, cracked slate, or long-term structural damage.

Full Removal of the Existing Shingle Roof Is Required

Slate cannot be installed over asphalt shingles. A complete tear-off of the existing roof system is mandatory. This includes removing the shingles, old underlayment, flashing, and edge metals.

This step is not only necessary for installation, but also allows the contractor to inspect the roof deck and framing for hidden damage. Any issues discovered at this stage can be corrected before slate is installed, rather than becoming costly problems later.

Roof Decking Must Meet Slate Standards

Many shingle roofs are installed over decking that is not suitable for slate. Thin plywood, spaced boards, or decking with soft or uneven areas can cause slate tiles to crack over time.

A slate roof requires a strong, rigid, and continuous deck. In many conversions, decking must be upgraded to thicker plywood or repaired to eliminate deflection and movement. This ensures the slate remains stable and properly supported for decades.

This step is especially important on older Pittsburgh homes, where decking materials and installation standards may vary significantly from modern requirements.

Underlayment Must Be Upgraded for Long-Term Protection

Slate roofs are designed to shed water, not seal it out the way asphalt shingles do. That makes the underlayment system especially important, particularly in Pittsburgh’s climate with snow, ice, and freeze-thaw cycles.

When converting from shingles to slate, a high-performance underlayment system is typically installed across the entire roof. This often includes full-coverage ice and water protection along with a heavy-duty slate-compatible underlayment. These layers provide redundancy and long-term moisture protection beneath the stone.

Flashing Must Be Completely Replaced

One of the most critical requirements when installing slate over a former shingle roof is replacing all flashing with materials that match the lifespan of slate.

Asphalt shingle roofs commonly use aluminum or galvanized steel flashing, which may last only a few decades. Slate roofs can last a century or more. If inferior flashing is reused or installed, it will fail long before the slate does, leading to leaks that compromise the roof system.

Proper slate installations use copper or stainless steel flashing at valleys, chimneys, walls, and penetrations. This ensures the entire roof system ages together rather than failing in sections.

Slate Requires Specialized Installation Techniques

Installing slate is a craft, not a production process. Unlike shingles, slate tiles are installed individually by hand. Each piece must be sorted by thickness, properly spaced, and fastened in a way that allows the stone to perform naturally without stress.

Nails must be driven precisely, never overdriven, and fasteners must be corrosion-resistant. Headlap and exposure are carefully calculated to ensure water is shed properly. Cutting slate requires skill to avoid cracking or weakening the stone.

This is why homeowners searching for slate roof installers near me should look beyond general roofing experience. Slate installation requires training, patience, and attention to detail that many shingle-focused crews simply do not have.

Shingle Roofing Materials Cannot Be Reused

Another common misconception is that some components of the old shingle roof can be reused. In reality, slate roofing requires its own system of fasteners and accessories.

Copper or stainless steel nails are required, along with slate-compatible ridge and hip details. These materials ensure corrosion resistance and allow for future repairs without damaging surrounding slate.

Using improper hardware can shorten the lifespan of an otherwise well-installed roof.

Permits and Inspections May Be Part of the Process

Because converting from shingles to slate often involves structural considerations, permits are commonly required. In some cases, especially with older or historic homes in Pittsburgh, additional approvals may be necessary.

An experienced slate roofing contractor understands these local requirements and ensures that the project complies with all applicable codes before and after installation.

Cost Considerations When Converting from Shingles to Slate

Slate roof installation is a premium investment, especially when replacing a former shingle roof. The cost reflects not only the material, but also the structural evaluation, decking upgrades, specialized labor, and long-lasting flashing systems.

While slate costs more upfront than asphalt shingles, it often replaces multiple future shingle roofs. For homeowners planning to stay in their home long-term, slate can be a cost-effective solution over the life of the property.

Why Experience Matters When Installing Slate Over Shingles

Many slate roof failures are not caused by the slate itself, but by shortcuts taken during conversion from shingles. Inadequate decking, reused flashing, improper fastening, or lack of structural evaluation can all undermine the performance of the roof.

Choosing a contractor with true slate experience ensures that every requirement is addressed, from preparation to final installation.

Why Pittsburgh Homeowners Trust Roberts Roofing for Slate Roof Installation

Roberts Roofing understands what it takes to properly install slate on homes that previously had shingle roofs, including homes being restored to their original slate design. With experience working on complex roofing systems and Pittsburgh’s diverse housing stock, their approach focuses on long-term performance rather than quick installation.

From structural evaluation to final detailing, Roberts Roofing ensures that slate roofs are installed the right way—so they can last as long as they are meant to.

Schedule a Slate Roof Evaluation With Roberts in Pittsburgh

If you are considering slate roof installation in Pittsburgh and want experienced slate roof installers near me, the first step is a professional evaluation of your existing shingle roof.

Visit https://www.robertsroofingpgh.com/
to schedule a consultation and learn what is required to properly convert your roof to slate.