🏠 Underlayment Installation Over Existing Self-Adhered Roofing – What Happens When It Can’t Be Removed?

Roofers and homeowners often face a tricky situation during re-roof projects involving self-adhered underlayments—like Polystick TU Plus—that are nearly impossible to remove. So what happens when the original underlayment can’t be taken off? Can a new layer be applied on top? Will inspectors approve it?

At Oasis Engineering, we recently helped a client resolve this exact issue during a concrete tile roof replacement. Below, we break down the problem, code considerations, and how our Engineering Letter of Certification service can help you move forward—even if inspections were missed or the dry-in stage has already been completed.

❗ The Problem: Existing Polystick TU Plus Underlayment Wouldn’t Come Off

During tear-off, the roofing crew found the existing underlayment (Polystick TU Plus) was fully adhered to the plywood decking. It was bonded so strongly that removal risked damaging the structural deck.

To avoid delay, the crew:

• Nailed down the existing underlayment every 6 inches using 1” metal cap nails.
• Installed a new layer of Polystick TU Plus over the original.
• Sealed all laps with modified roof cement.

This created a two-ply system—but raised concerns when it came time for inspection.

🛑 Why Inspectors Flag This

Most roofing approvals assume direct application of the underlayment to bare wood decking. Overlaying one layer over another isn’t a standard install in product documentation—so inspectors may hesitate to approve it without additional engineering justification.

If the dry-in was completed without calling for inspection—or if the tile has already been installed—some jurisdictions may require a certified engineering letter to verify that the installation still meets Florida Building Code (FBC) requirements.

✅ How We Help: Engineering Letter for Dry-In Approval

At Oasis Engineering, we provide certified engineering consultation and dry-in letters for cases just like this. Our process includes:

1. Photo Review and Site Visit
   We review detailed site photos—and perform an on-site assessment—to verify fastening, overlaps, product details, and general installation quality.
2. Code + Product Approval Review
   We reference the Florida Product Approval (e.g. FL# 5259.2-R45) to ensure all fastening methods (e.g. 6″ on center with 1″ cap nails) align with code requirements, even in a modified install.
3. Engineering Letter of Certification
   We issue a signed and sealed engineer letter certifying the underlayment install is structurally sound and meets applicable roofing and wind uplift standards under the FBC.

This allows homeowners and contractors to pass inspection, even when conditions on site vary from standard installation manuals.

💬 Common Scenarios Where This Applies

• Self-adhered underlayment can’t be removed and a new layer is installed on top.
• Dry-in inspection was missed, and tile or metal roofing is already being installed.
• Contractor or homeowner needs engineering backup to satisfy inspector or close permit.
• Inspector requests engineer review or special inspection documentation post-facto.

📩 Need Help? Here’s How to Start

If you’re dealing with a similar situation, we can help. Here’s what to do:

1. Take clear photos of the roof deck, fasteners, lap seams, and labels if visible.
2. Confirm the product used (e.g. Polystick TU Plus) and send the Florida Product Approval number if available.
3. Tell us the inspection status — whether the tile is already installed or dry-in is still visible.

Send it all to us via our form:
👉 Submit your roofing request here

Or reach out directly via email for urgent help.

🧠 Final Word

Overlaying underlayment isn’t ideal—but it can be code-compliant when done correctly and with the right engineering review. Don’t let an inspection delay derail your roofing project.

With Oasis Engineering, you get:

• Fast 1–2 business day turnaround
• Florida-licensed engineer review
• Direct coordination with homeowners or contractors

Let us help you close that permit and move forward with confidence.