How Many Layers of Shingles Are Allowed on a Roof?

how many layers of shingles

Are you wondering how many layers of shingles you can put on your roof? Installing new shingles over existing ones is common practice, but there are limits to how many layers are allowed. This article will explain the issues around multiple layers of roof shingles and shed light on the regulations in California.

Having multiple layers of asphalt shingles on your roof can lead to problems down the road. The weight of too many layers can cause roof decking to deteriorate or even collapse. Shingles also lose their sealing ability over time, so excessive layers increase the risk of leaks. And when it’s eventually time to replace the roof, removing all those layers of shingles is difficult and time-consuming.

How Many Layers of Shingles Are Typically Allowed?

In most areas, building codes allow a maximum of two layers of asphalt shingles on a roof. Some jurisdictions only permit one layer, while a few will allow three. The limit aims to prevent overburdening the roof structure and allow proper water runoff.

California follows the International Building Code (IBC), which specifies no more than two layers of asphalt shingles are permitted. Some California cities and counties have adopted more restrictive policies, so be sure to check with your local building department before reroofing.

Why Are Multiple Layers of Shingles Problematic?

Adding a second layer of shingles may seem like an easy way to avoid tearing off the old roof, but there are good reasons most building codes restrict layers. Some key problems that can arise from excessive layers include:

  • Excess Weight: The more layers on a roof, the heavier the load becomes. All this extra weight strains the roof deck and structural framing.
  • Improper Drainage: Having layers of shingles atop one another impedes water drainage off the roof. Flashing and drainage planes become ineffective.
  • Decreased Wind Resistance: Too many layers of shingles make the roof more susceptible to wind uplift damage. The shingles’ bond to the roof weakens.
  • Increased Risk of Leaks: Over time, the sealing strips on shingles lose effectiveness. More layers mean more opportunities for leaks through aged shingles.
  • Ventilation Issues: Multiple shingle layers can block proper airflow to the attic space below the roof. This can lead to moisture issues.
  • Difficult Removal: Taking off multiple shingle layers is a big job when it’s finally time for full roof replacement. The labor costs go up significantly.

When Is Installing a Second Layer Acceptable?

If your roof only has one existing layer of asphalt shingles in fair condition, building codes do allow installing a second layer. There are some requirements, though:

  • The existing shingles must not already have too many layers underneath. Some areas prohibit second layers if there are already two or more present.
  • The roof deck and structure must be confirmed to be in good shape, without signs of deterioration or leaks below the shingles.
  • Complete replacement cannot be required by your jurisdiction due to weathering or condition of the existing shingles.
  • The existing shingles should be nailed down securely. Loose or missing shingles must be repaired.
  • Building permits are usually required for overlay projects. The building department will inspect to ensure compliance.
  • Proper underlayment and flashing must be installed to protect from leaks. Self-adhering membrane is often required.
  • The roof slope and pitch must be adequate for proper drainage with a second layer. Low slopes often cannot accommodate more layers.
  • Weight calculations should be done to ensure the roof structure can support the load of a second shingle layer. Trusses may need reinforcement.

If your roof already has two or more layers, complete shingle removal will be required before installing a new roofing system.

How Can You Tell How Many Layers Your Roof Has?

Sometimes homeowners are unsure how many shingle layers are on their existing roof. Here are some ways you can determine the number of layers:

  • Look at the roof edge for material build-up and clues about multiple layers.
  • Feel the roof surface thickness, which increases with more layers.
  • Inspect the attic underside for signs of multiple layers.
  • Ask your neighbors how many times the roof has been reshingled.
  • Look for old roofing permit records with your local building department.
  • Hire an experienced roofer to inspect and identify how many layers are present.
  • Remove a test square to inspect how many shingle layers there are from underneath.
  • Consider the roof’s age and assume at least one layer per 20 years.

Being able to identify all the existing roofing layers is crucial before reroofing. This ensures proper permitting and compliance with allowed limits. Contact a professional roofer if you need help determining the number of existing shingle layers.

Why Are Some Areas More Restrictive Than Others?

Some jurisdictions prohibit multiple layers entirely or restrict layers to just one. Climatic conditions and roofing installation practices help explain these stricter policies.

In colder regions with heavy snow loads, the concern is avoiding overburdening the roof structure. In windy regions like Florida, resistance to wind uplift damage becomes a bigger concern. And in fire-prone areas, limiting shingle layers can help reduce fire risk.

Building codes also evolve over time as roofing technologies improve. Modern synthetic underlayments do a better job preventing leaks than old roofing felts. So early building codes permitted more shingle layers, while current standards restrict them.

Can You Install New Shingles Over Old Ones?

Homeowners often ask if they can simply install new asphalt shingles right over top of their existing worn-out ones. While allowed in some cases, there are drawbacks to this shortcut method:

  • The roof’s lifespan will be reduced compared to a complete tear-off and redecking.
  • Attic ventilation issues may occur if vents are covered over.
  • Nailing the new shingles can be trickier.
  • Uneven layers make the roof surface look wavy.
  • Accessories like roof jacks and vents will need extension flashing.
  • Removal labor down the road will be multiplied.
  • Permits or inspections may be tougher to obtain/pass.
  • Leak risks increase long-term as the shingles age.

While legally permitted in many cases, installing new shingles over old ones is not really a recommended best practice for roofing. Tearing off existing shingles to expose the roof deck for proper reinstallation is the better approach for roof longevity, though it costs more upfront.

Key Takeaways on Allowed Shingle Layers

To summarize the key points on permitted layers of asphalt shingles:

  • Most building codes allow a maximum of 2 layers. Some areas restrict to 1 layer.
  • Additional weight from too many layers can damage the roof structure.
  • Proper attic ventilation and drainage are compromised by excessive layers.
  • It’s harder to spot leaks and other roof damage with multiple layers.
  • Complete roof tear-off provides for the best long-term roof performance.
  • Always check your local codes before assuming how many layers are allowed.
  • Identifying all existing layers is crucial before reroofing or adding more shingles.

Knowing regulations on allowable shingle layers will help you make wise roofing decisions. With proper installation, a second layer can buy some extra useful life for an existing roof before complete replacement is needed. But too many layers leads to problems, so check your area’s building codes.

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