Best Practices for Supporting Type B Roof Deck on Purlins
Space the purlins so the 1.5" Type B deck stays within its catalog construction span and its design load tables, give every rib enough bearing, and fasten the deck at each purlin to the pattern the engineer of record sets for South Florida wind uplift. We supply both the Type B deck and the C and Z purlins, cut to length.
Match purlin spacing to the deck
Purlin spacing is the deck span. The catalog construction spans below are the upper limit for erection loads only; the design span comes from the engineer of record using the downward, upward and deflection tables. Lighter gauges usually need closer purlins, so compare a lighter deck on tighter purlins against a heavier deck on wider spacing.
Bearing and fastening on cold-formed purlins
- Check bearing width: the catalog lists web crippling for 1.5", 2" and 3" bearing at end and interior supports.
- On thin purlin flanges, self-drilling screws are common; follow the engineer's fastener type and pattern (36/7, 36/5, 36/4 or 36/3).
- Sidelaps: generally not required under 5'-0" spans; over 5'-0", fasten at midspan or every 36", whichever is smaller.
- Use closer patterns in roof perimeter and corner zones where the drawings call for them.
Order deck and purlins together
Our C purlins run 12 to 20 gauge and Z purlins 12 to 18 gauge, both 55 ksi with G60 or G90 galvanizing. Send the framing plan and we will quote the purlins, deck and Tek screws as one package.
1.5" Type B construction spans (200 lb load on a 1 ft width, ANSI/SDI RD-2017 and SD-2022)
| Gauge | Single span | Double or triple span | Cantilever |
|---|---|---|---|
| 22 | 8'-03" | 10'-02" | 2'-02" |
| 20 | 11'-00" | 13'-07" | 2'-09" |
| 18 | 14'-11" | 18'-05" | 3'-10" |
| 16 | 19'-03" | 23'-08" | 4'-10" |
Related Questions
Should I use C or Z purlins under Type B deck?
Both work as deck supports. Z purlins can be nested and lapped over supports for continuous runs, which is common on long building lengths. C purlins suit simple spans, openings and framing that needs a flat back. The engineer of record picks the shape and gauge from the loads and span; we supply either, cut to length.
Can Type B deck be welded to light-gauge purlins?
Welding to thin cold-formed flanges is difficult to do well and can burn through, so self-drilling screws are the usual choice on purlins. The connection type, size and pattern still come from the engineer of record, who sizes them for the wind uplift on your roof. Tek screws are available to ship with the deck.