Substrates and Underlayment
Purlin systems and solid deck substrates for metal roofing, underlayment types including synthetic and ice-and-water shield, ventilation requirements, and condensation prevention.
- Compare open purlin and solid deck substrates and state when each is appropriate
- Select the correct underlayment type (synthetic, felt, ice-and-water) for metal roof applications
- Design ventilation systems to prevent condensation on the underside of metal panels
- Identify purlin gauge, spacing, and alignment requirements for metal panel support
Lección 1
Purlin Systems
Open Framing vs. Solid Deck
Metal roof panels can be installed on two types of substrates:
Open purlin framing uses steel or wood members (purlins) spaced at intervals to support the panels. The space between purlins is open to the building interior (or to insulation in the cavity). This is the standard substrate for commercial and industrial metal buildings, pre-engineered metal buildings (PEMBs), and agricultural structures.
Solid deck uses plywood, OSB, or steel deck to create a continuous surface beneath the panels. This is common in residential, re-roofing, and architectural applications where an underlayment is required between the deck and the panels.
Open Purlin Framing
Materials: Steel C/Z purlins or wood 2x members
Common use: Commercial, industrial, PEMBs, agricultural
Underlayment: Not required (panels bear on purlins only)
Ventilation: Open space provides natural airflow
Solid Deck
Materials: Plywood (min 15/32 in), OSB (min 7/16 in), steel deck
Common use: Residential, re-roofing, architectural
Underlayment: Required between deck and panels
Ventilation: Must be designed into assembly (intake/exhaust)
Steel Purlin Specifications
Steel purlins for metal roofing are cold-formed C-sections or Z-sections. Common specifications:
| Purlin Size | Gauge | Flange Width | Max Span (typical) |
|---|---|---|---|
| 4 in C | 16 ga | 1-5/8 in | 10-12 ft |
| 6 in C | 16 ga | 1-5/8 in | 14-18 ft |
| 8 in Z | 14 ga | 2-1/2 in | 20-25 ft |
| 10 in Z | 14 ga | 3 in | 25-30 ft |
Purlins must be straight, level, and aligned. NRCA recommends a maximum deviation of 1/8 inch in 10 feet for purlin top surfaces. Misaligned purlins cause panel waviness (oil-canning) and uneven fastener loading.
Wood Purlins
Wood purlins (typically 2x4 or 2x6 lumber) are used in residential and light commercial applications. They must be straight, dry (moisture content below 19%), and free of excessive knots or splits. Wood purlins are fastened to the rafters or trusses below.
Steel purlins must be aligned within 1/8 inch in 10 feet to prevent panel oil-canning. Open purlin framing is standard for commercial metal buildings; solid deck with underlayment is standard for residential applications.