6 Best Leveling Shims For Uneven Roof Surfaces Pros Trust

Leveling a roof surface is often the difference between a roof that lasts thirty years and one that fails in five. When rafters sag or decking bows, installers face the critical choice of cutting out structural members or shimming the plane to achieve a flat, uniform surface. Proper shimming distributes weight evenly, ensures proper water runoff, and prevents the premature cracking of shingles or the “oil-canning” of metal panels. Choosing the right shim isn’t just about thickness; it’s about durability, moisture resistance, and load-bearing capacity.

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Nelson Cedar Shims: The Go-To for Wood Decks

Cedar shims remain the industry standard for traditional wood-frame roofing projects. These wedges provide a natural, compressible fit that conforms slightly to the irregularities of aged roof sheathing.

Because they are made from rot-resistant cedar, they handle incidental moisture better than cheaper pine alternatives. They are the ideal choice when shimming under wood shakes or cedar shingles to maintain consistent material compatibility.

The primary trade-off is their sensitivity to extreme moisture cycles. If the roof deck is prone to persistent leaks or high humidity, cedar can eventually swell or decay. Use these primarily for minor leveling corrections on rafters or joists where a natural material is preferred for structural stability.

Wobble Wedges: Best for Precise Adjustments

When dealing with fine-tuning the height of a solar mount or leveling a specific fastener point, Wobble Wedges are unmatched. These plastic shims feature a textured, interlocking surface that prevents them from sliding once placed.

The benefit of these wedges lies in their stackability and variable taper. Unlike flat shims, the wedge shape allows for a graduated increase in height, which is essential when correcting a slight crown in a roof truss or a dip in a ridge beam.

They are made from durable, flexible plastic that won’t rot, shrink, or compress under heavy point loads. They are not intended for large-scale surface leveling across an entire roof plane, but they are indispensable for finishing touches on localized hardware installations.

EZ-Shim Composite Shims: Top for Fast Snapping

Efficiency is the currency of a professional roofing crew, and EZ-Shim composite shims deliver by providing a “snap-off” feature that eliminates the need for trimming with a knife. These shims are engineered to break at uniform intervals, allowing for quick, clean adjustments on the fly.

Composite materials offer the best of both worlds: they possess the rigidity of plastic but the structural integrity and fastener-holding capability closer to wood. They are virtually impervious to moisture, making them perfect for zones with high rainfall or frequent temperature fluctuations.

Because they resist rotting and insect damage, they can be safely left buried beneath layers of underlayment without fear of long-term degradation. They are the top choice for contractors who need a reliable, high-volume solution that won’t require call-backs due to material failure.

Paco-Shim Plastic Shims: Best Bulk Contractor Pack

For large projects involving widespread deck unevenness, buying by the unit is a recipe for budget bloat. Paco-Shim plastic shims are typically available in bulk contractor packs, making them the most cost-effective option for covering significant square footage.

These shims are usually color-coded by thickness, which allows a crew to identify the necessary elevation adjustment at a glance. This standardization reduces the risk of human error when a team of installers is working across different sections of the roof.

While they lack the textured grip of premium wedges, their sheer utility in high-volume applications is undeniable. They are best suited for structural shimming under plywood or OSB panels where consistent, reliable elevation is the priority.

Timber-Tuff Steel Shims: For Heavy Structural Loads

Standard plastic or wood shims often fail when faced with the immense point loads of heavy structural members or industrial-grade steel brackets. Timber-Tuff steel shims offer the extreme compression resistance required for heavy-duty load paths.

These are primarily used in commercial roofing applications, such as anchoring heavy HVAC units or steel support beams directly to the roof structure. They will not crush, deform, or shift over time, ensuring that the critical structural plane remains perfectly level under significant weight.

Always consider the galvanic reaction when placing steel against other metals. If using steel shims in a metal roof application, ensure they are compatible with the roofing material or properly isolated to prevent corrosion.

Go-Ez Horseshoe Shims: Ideal for Metal Fasteners

Horseshoe shims, often called U-shims, are specialized tools designed for situations where a fastener is already in place. Instead of sliding a full-width shim, these can be slipped around the shank of a bolt or screw to adjust the gap between two surfaces.

They are the gold standard for leveling metal panels, solar rack mounts, and flashing components. The U-shape provides a secure fit around the fastener, ensuring that the weight is distributed around the bolt hole rather than just on one side.

They come in various thicknesses and materials, including high-impact plastic and stainless steel. When installing roof-mounted equipment, always opt for the horseshoe style to ensure the leveling work doesn’t interfere with the integrity of the fastener’s seal.

Wood vs. Plastic vs. Composite Shims: Which to Use

The choice of material often dictates the lifespan of the repair. Wood shims are excellent for wood-to-wood contact and offer a degree of friction that plastic cannot match, but they are vulnerable to moisture and rot.

Plastic shims provide superior moisture resistance and are ideal for long-term concealment under synthetic underlayments. However, they can be slippery and may require additional adhesive if they are not being held in place by mechanical fasteners.

Composite shims effectively bridge this gap, offering the moisture resistance of plastic with a higher rigidity profile. Choose composite if the budget allows; it remains the most versatile option for modern roofing systems that demand high durability.

When to Shim a Roof vs. When to Re-Deck It

Shimming is a remedy for minor surface irregularities, usually defined as deviations of less than 3/4 of an inch. If the roof deck exhibits significant dips or severe structural sagging, shimming is merely a band-aid that ignores the underlying failure of the rafters or trusses.

A roof that requires shimming across its entire surface is a roof that needs to be stripped and re-decked. Attempting to build up a massive height differential with shims introduces a “teeter-totter” effect, where the fasteners become stressed and the overall assembly loses its rigidity.

Always evaluate the structural health of the underlying lumber before choosing to shim. If the deck is soft or the rafters are undersized for the current load, correcting the structure is the only way to ensure the long-term viability of the new roofing system.

How to Properly Fasten Shims for Roof Stability

Shims that are not secured have a high probability of migrating over time due to thermal expansion and contraction. Never rely on friction alone; always use mechanical fasteners or construction-grade adhesives to lock shims into their intended position.

When using nails or screws, ensure they are long enough to penetrate through the shim and deep into the structural framing. If the shim is quite thick, upgrade the fastener length accordingly to maintain the proper embedment depth required by local building codes.

For metal roof installations, be particularly careful that the fastener used to secure the shim does not create a new puncture point that could lead to leaks. Always back the fastener with an EPDM rubber washer or a bead of high-grade sealant to maintain a watertight seal.

Shimming’s Impact on Underlayment and Shingles

Any shim placed beneath the underlayment creates a potential “bridge” or high spot that can lead to premature wear. If the shim has sharp edges or corners, it will eventually poke through the underlayment and puncture the shingles from beneath.

Always round off the corners of wood or composite shims before installation to ensure a smooth transition. The goal is a gradual slope that the underlayment can comfortably contour to, rather than an abrupt “step” that creates a fold or a tear in the protective layer.

When using high-performance synthetic underlayments, remember that they are less forgiving of sharp protrusions than traditional 15lb or 30lb felt. Prioritize smooth, tapered transitions to protect the primary waterproofing layer of the roof.

The decision to shim a roof is a strategic one, aimed at correcting minor deviations to achieve a professional finish. By matching the right shim material to the specific structural and environmental demands of the project, the risk of future maintenance issues is significantly reduced. Always prioritize a solid, secure, and moisture-resistant approach to ensure that the leveling work supports the integrity of the entire roofing system.