Base Sheet Fastening and Wrinkles Before Cap Sheet
A 16,000 ft² roof over new construction has a 5/8 in. exterior-glue plywood deck spanning 24 in. on center. The specified base sheet is a 40-mil SBS-modified bitumen sheet with a fiberglass mat, mechanically fastened, then a torch-applied cap sheet. The building is in a 115 mph basic wind speed area, and the manufacturer’s approved fastening schedule calls for 1 in. cap nails at 12 in. on center in the field, 6 in. on center within 3 ft of edges, with side laps 3 in. and end laps 6 in. During a site visit, the consultant finds that one crew has used 3/4 in. staples in two 30 ft by 60 ft areas, many cap nails are overdriven and cut into the base sheet, several wrinkles 1/4 to 1/2 in. high run across the field, and one side lap is only 2 in. wide. The crew has already installed about 4,000 ft² and wants to cover it with cap sheet before the end of the day.
1. Assess whether the installed base sheet can be accepted and what risks the observed conditions create.
Read the worked answer
Do not accept the observed areas and do not cover them. Each condition is a performance defect, not a cosmetic issue:
- Staples: unless specifically approved by the manufacturer for that assembly and wind rating, staples are not an acceptable substitute for cap nails. A 3/4 in. staple may not penetrate the plywood sufficiently, and staples can pull through the base sheet under uplift.
- Overdriven cap nails: these cut the sheet and reduce effective fastening; the cap no longer distributes load, and the hole becomes a water entry point.
- Wrinkles: 1/4 to 1/2 in. wrinkles mean the base sheet is not in full contact with the deck. They create voids, stress concentrations, and poor cap-sheet adhesion; in a 115 mph wind area they can contribute to membrane flutter and fastener fatigue.
- 2 in. side lap: this is less than the specified 3 in. lap, reducing seam width and reliability.
The assembly cannot be assumed to meet the wind-uplift design. The risk is blow-off, seam failure, and concealed moisture.
2. Describe the correction sequence before any cap sheet is installed.
Read the worked answer
Stop installation and establish a hold point. Remove or repair the nonconforming base sheet:
- Remove the stapled areas and replace with the approved base sheet fastened with the specified cap nails at the correct spacing and length.
- Replace fasteners that are overdriven or cut the sheet; add fasteners where spacing is deficient. Do not simply seal over damaged fasteners.
- Remove and replace wrinkled areas, or cut and splice only if the manufacturer approves a specific repair method. Adding fasteners through a wrinkle generally leaves a void and is not equivalent to a flat, fully contacted sheet.
- Correct the 2 in. side lap by replacing or adding a properly lapped ply; do not rely on sealant to widen a deficient lap.
- Re-inspect the corrected work before cap sheet is installed. Verify fastener type, length, spacing, embedment, lap widths, and flatness. Document the hold-point release.
3. What inspection evidence should be required to release the corrected area?
Read the worked answer
Require a 100% visual inspection of the corrected areas for fastener type and spacing, overdriven caps, sheet damage, wrinkles, and lap width. Perform random pull tests on fasteners to confirm embedment and cap performance, and probe or measure laps. Confirm the fastening schedule against the manufacturer’s approved wind-uplift design for the 115 mph zone, including the enhanced edge zone. Photograph corrected areas and record the hold-point release. The cap sheet should not be installed until the consultant and, if required, the manufacturer’s representative accept the base sheet.