How to Roof Over Existing Shingles: Step-by-Step Guide for Homeowners
Skip costly tear-offs by properly layering new materials for longer-lasting protection.
How to Roof Over Existing Shingles: A Comprehensive Homeowner Guide
Adding a new layer of shingles over your old roof, commonly called a “roof-over,” can save time and money compared to a full tear-off. This process may be permitted by local codes under certain conditions and is best suited for homes with only one existing layer of shingles. This guide covers everything you need to know about roofing over existing shingles—from required inspections and preparation to step-by-step installation and post-project care.
Is Roofing Over Shingles Right for Your Home?
- Local code compliance: Most regions allow only two layers of roofing material. Building codes typically do not permit adding a third layer, which means a full tear-off is mandatory if two layers are already present.
- Roof condition: The existing shingles must be relatively flat, securely fastened, and free of major damage. Curled, warped, or missing shingles must be repaired before installation.
- Inspection for leaks and rot: If the underlying deck is deteriorated due to water damage, a roofover is not suitable—removal and deck repair are required.
- Roof material restrictions: Not all roofs are suitable for overlay. Do not install over: wood shakes, tile, metal, asbestos, roll roofing, or insulation.
- Warranty/insurance limitations: Check shingle manufacturer requirements and insurance policies. Many warranties do not cover roofovers or restrict coverage.
Preparation: Assess and Ready Your Existing Roof
Proper preparation is essential for the longevity and performance of your new shingles. A poorly prepped base will quickly undermine your new roof.
- Inspect thoroughly for rot, leaks, mold, and soft spots. Repair or replace damaged sections of deck and underlayment.
- Remove loose nails and fasteners.
- Secure any lifted, curled, or loose shingles. Re-nail or replace missing shingles. Split and flatten warped wood shingles where required.
- Check existing flashing (valleys, vents, rakes, and chimneys). Replace or add new flashing as necessary.
- Clean roof surface to remove debris, moss, and dirt for a smooth base.
Check Local Building Codes and Permits
Contact your local building department:
- Verify how many layers are permissible by code.
- Ask if permits or inspections are required.
- Review specific shingle installation standards for overlays.
Materials and Tools You’ll Need
- New shingles: Asphalt shingles suitable for overlay. Calculate quantities using area measurements (length × width for rectangles; for triangles, multiply both short sides and divide by two).
- Underlayment: High-quality underlayment or moisture barrier is recommended, especially in regions prone to leaks or heavy rainfall.
- Flashing: Drip-edge (for eaves and rakes), valley flashing, and boots for pipes/vents. Use new or overlay where old is worn.
- Starter strips and ridge shingles: For edges and ridges.
- Roofing nails: Typically 1½-inch length or as needed to penetrate both shingle layers and reach sheathing.
- Roofing tools: Roofing gun (nailer), hammer, aviation snips (for flashing), utility knife w/hook blade, tape measure.
Step-by-Step: Installing Shingles Over Existing Shingles
Professional roofers recommend tearing off the old roof for best results, but a roof-over is possible with careful preparation and proper technique.
- Step 1: Prepare the Surface
- Inspect and repair the existing roof as described above.
- Remove protruding nails. Secure any loose shingles.
- Install or repair underlying flashing where required.
- Step 2: Level the Eaves
- To create a flat base for new shingles, rip down two courses (rows) of old shingles along the eaves (the lower edge of the roof).
- Cut the first strip (starter course) to 5 inches high; lay flush against the butts of the original second course.
- Cut the second strip to 10 inches high; lay flush against old third course butts.
- This levels the edge, preventing bumps and unevenness at the start.
- Step 3: Add Starter Strip and Drip Edge
- Install a new drip-edge over the butt ends of old shingles on eaves.
- On rakes (sloped roof edges), use J-channel drip-edge that wraps around old rake flashing and starter courses.
- Step 4: Install Valley Flashing
- Leave old valley flashing in place if sound; otherwise add new over the existing.
- Step 5: Add Underlayment (Recommended)
- Lay high-quality underlayment or peel-and-stick membrane over the repaired shingles. Overlap rows and tape or fasten seams.
- This reduces the risk of leaks between layers and extends roof life.
- Step 6: Install Furring Strips (OPTIONAL)
- If original shingles are significantly uneven or damaged, horizontal furring strips can help level the surface and provide a consistent base.
- Furring also improves airflow, preventing trapped moisture.
- Step 7: Apply New Shingles
- Begin with starter strips, then lay the first full course of shingles, butting each course upward to an original course above.
- Use enough nails to penetrate both layers and reach the deck—typically 1½ inches.
- Continue up the roof, overlapping courses properly.
- Step 8: Flash and Seal All Roof Penetrations
- Repair or replace vent, pipe, and chimney flashing. Use boots and collar for pipes.
- Seal all flashing with roofing cement for watertight protection.
- Step 9: Finish With Ridge Shingles
- Install special ridge shingles at the peaks and hips for a professionally finished look and added weatherproofing.
Post-Installation Tips
- Inspect for exposed nail heads and seal with roofing cement.
- Clean up debris, old shingles, and unused materials from the site.
- Check for leaks and monitor roof performance following first rains.
- Maintain gutters and inspect flashing annually.
Pros and Cons of Roofing Over Existing Shingles
| Pros | Cons |
|---|---|
| Lower cost than tear-off (less labor, debris) | Shorter lifespan than full replacement |
| Faster installation | May void manufacturer warranty |
| Less construction disturbance; less risk of sudden leaks | Not suitable if underlying roof or deck is damaged |
| Reduces landfill waste | Can add weight and stress to structure |
| Allowed by code and manufacturers in some cases | May decrease home value or complicate future repairs |
Common Mistakes to Avoid
- Skipping roof inspection, especially for hidden rot or leaks.
- Ignoring local codes and building permit requirements.
- Overlaying more than one layer (never allowed by code).
- Poor flashing repair, which can lead to major leaks.
- Choosing low-grade materials or incorrect fasteners.
Frequently Asked Questions (FAQs)
Can I roof over my existing shingles?
Yes, if local codes permit and there’s only one existing layer in good condition. Roofovers are not allowed if two layers are present.
How do I know if my roof is suitable for a roof-over?
Inspect for major damage, rot, or uneven shingle surfaces. All defects must be repaired. If the deck is compromised, replacement is required instead.
Do I need new flashing?
Yes. Replace, repair, or overlay flashing as needed at all valleys, rakes, eaves, and penetrations.
Will a roof-over last as long as a full replacement?
No. New shingles installed over old ones generally have a shorter lifespan compared to roofs with a single layer installed on new sheathing.
Will my warranty or insurance cover a roof-over?
Check your shingle manufacturer’s warranty and your own policy—roof-overs often have restricted coverage or may not be covered at all.
Can I install metal roofing over shingles?
Yes, with proper underlayment and (optionally) furring strips for leveling. This can improve ventilation and moisture control.
How do I calculate the amount of materials needed?
Measure roof area in square feet (length × width; triangles count as base × height ÷ 2). 3 bundles of shingles typically cover 100 sqft.
What are the risks of skipping repairs before overlay?
Any unresolved damage, leaks, or warped shingles will cause the new roof to fail prematurely. Always repair base issues first.
Conclusion
Roofing over existing shingles is a viable option for many homeowners, offering savings and reduced hassle when the underlying roof is in good shape and local codes permit. Preparation, inspection, and proper installation are critical to ensure durability. When in doubt, consult local building officials or professional roofers to ensure a safe and legal job that protects your investment for years to come.
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References
- https://www.instructables.com/Shingle-a-Roof-Over-Existing-Shingles-Save-75-Give/
- https://indyroofandrestoration.com/how-to-install-metal-roofing-over-shingles/
- https://www.finehomebuilding.com/project-guides/roofing/installing-asphalt-shingles-over-an-existing-shingle-roof
- https://documents.gaf.com/technical-bulletins-&-notes/r-145-roofing-over-existing-shingles.pdf
- https://www.youtube.com/watch?v=d3bfaXW2-90
- https://www.youtube.com/watch?v=dIW97LsMH0U
- https://www.iko.eu/en/blog/reroofing-existing-shingle-roof
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