Best Approach for Roofing Over a Concrete Deck? Dealing with Moisture Concerns

Moisture testing is step one

The most reliable method to assess moisture in an existing concrete roof deck is in-situ relative humidity (RH) testing, as defined in ASTM F2170. This process involves drilling test holes into the concrete — typically to 40% of the slab's depth — then inserting calibrated humidity probes to measure the internal relative humidity within the slab.

This method is far more accurate than surface-based moisture meters, especially for roofing applications, because it captures moisture levels below the surface, where vapor can continue migrating for weeks or months. A reading of 75% RH or lower is generally acceptable for roofing installations, unless the manufacturer has provided specific alternate requirements.

According to industry best practices, such as those recommended by Wagner Meters, you should perform at least three RH tests for the first 1,000 square feet, plus one additional test for every subsequent 1,000 square feet. This ensures that your readings reflect the moisture profile across different deck sections, which may vary due to sunlight exposure, drainage conditions or previous repairs.

Proper RH testing upfront eliminates guesswork and reduces the risk of adhesive failure, blistering and long-term moisture entrapment beneath your roofing system.

Use a vapor retarder — strategically

A vapor retarder is often essential for concrete decks, especially in warm, humid climates or when the interior is climate-controlled. However, it must be installed correctly:

• Installation: Directly over the concrete deck, beneath the insulation.
• Type: Use a self-adhered or hot-applied vapor barrier with high perm resistance (Class I or II). Some systems also integrate vapor control layers into the adhesive membrane.

That said, don't install a vapor barrier unless you're 100% certain it's needed — in the wrong climate or application, it can create a condensation trap. Climate zone, building use (e.g., high humidity interiors like a gym or pool), and dew point calculations should guide this decision.

Insulation: Go continuous and moisture-resistant

For insulation, close-cell polyisocyanurate boards with glass facers or extruded polystyrene (XPS) offer good moisture resistance. Avoid materials like expanded polystyrene (EPS) unless you use venting strategies or a hybrid system.

• Attach insulation cautiously: Mechanical fasteners can create thermal bridging and leak paths over concrete. Using fasteners, consider cover boards or staggered layers to minimize bridging. Insulation adhesive should be considered.
• Use cover boards: High-density polyiso or gypsum cover boards help protect the insulation and provide a better substrate for adhesion.

Roofing system: Fully adhered versus mechanically fastened

Both can work over concrete, but here's the breakdown:

• Fully adhered systems are typically preferred over concrete decks. They help minimize uplift pressures, avoid fastener penetrations that can cause leaks or thermal bridging, and bond better to the substrate when the proper primer is used.
• Mechanically fastened systems are costly and difficult to install over concrete.

Recommendation: Go with a fully adhered single-ply (TPO or PVC), or consider a hybrid or two-ply modified bitumen system if redundancy is a priority.

Additional best practices

Slope the deck (minimum 1/4" per foot) or use tapered insulation to promote drainage. Ponding water over concrete exacerbates moisture intrusion and the breakdown of membrane adhesion. Use venting strategies only if moisture is present and full removal isn't feasible, but this is less common today due to system advancements. Ensure compatible adhesives and primers are used with concrete. Surface prep (clean, dry, smooth) is critical for adhesion.

Summary of recommendations

• Perform a moisture analysis of the deck before proceeding.
• Use a vapor retarder if conditions require it (high interior humidity or certain climate zones).
• Closed-cell insulation with high R-value and low moisture absorption is used.
• Favor fully adhered systems over mechanically fastened.  
• Always include a cover board for durability and adhesion performance.
• Ensure proper slope and drainage.