PLYWOOD

The following information is sourced from Fletcher Wood Panels Ltd and Carter Holt Harvey Ltd

Decking and Sarking

Plywood panels used for decking and sarking must be designed and specified to have the long axis of the panel (the face grain direction) perpendicular to the supporting joists.
Panels must be laid with joints staggered using a brick bond panel layout.
Sarking under roofing is provided in two situations;
  • Non- load bearing, where it is required to provide racking resistance to wind or seismic loads;to provide an element in the insulation system and limit condensation; to carry construction loads prior to and during application of the roofing.
  • Load bearing where traffic may be anticipated after roofing is completed and where a thin membrane roofing is applied and is exposed to view, thus requiring deflections to be minimal.
Both these situations can be developed into plywood diaphragms to transfer lateral or wind and earth quake forces efficiently into adjacent structural members.

Decks with Public Access

Regular Pedestrian Traffic

Decked areas where regular traffic is anticipated include roof level sun decks, lookout areas and balconies.
The roof deck is used as a floor and the criteria for selecting panel thickness and support spacings apply as for flooring. The thickness / span relationship is shown in Table 1 and is based on the following:
  • Distributed live loads of up to 4.5kPa carried without deflection exceeding 0.002 (span)
  • Pedestrian traffic will not cause unacceptable deflection or vibrations.
  • Face grain is at right angles to supports and plywood is continuous over two spans.
Long panel edges will be supported by nogging in the framing or will have tongue and groove edges. Allowable Spans for Plywood Decking –

Table 1 Flooring Criteria (Decks)

Plywood Thickness (mm)
Maximum Joist Spacings (mm)
15
480
17
600
21
800

Occasional Traffic

Decked areas required to support pedestrian loading in situations such as fire escapes, maintenance access, occasional domestic traffic etc., may tolerate a greater degree of deflection than is acceptable to a flooring criteria. The thickness / span relationship is shown in Table 2 and is based on the following:
  • Distributed live loads of up to 2kPa carried without deflection exceeding 0.003 (span)
  • Pedestrian traffic will not cause unacceptable deflection or vibrations.
  • Face grain is at right angles to supports and plywood is continuous over two spans.
  • Long panel edges will be supported by nogging in the framing or will have tongue and groove edges.
  • The specifier is satisfied that the flexibility of the decking and the roofing membrane are compatible.

Table 2 Allowable Spans for Plywood Decking (Roofs)

Low Traffic
Plywood Thickness (mm)
Maximum Joist Spacings (mm)
12
600
15
800
17
900
21
1200

No Access Substrate for Thin Membrane Roofing

Use Table 2 as a guideline. Where pedestrian traffic is not normal but should be anticipated during construction and where thin membrane roofing is to be applied. The designer must ensure that the flexibility of the decking and the roofing membrane are compatible.The thickness / span relationship as shown in Table 3 is based on the following:
  • Distributed live loads of up to 1.5 kPa carried without deflection exceeding 0.006 (span)
  • Deflections under indicated load must not exceed 0.006 (span)
  • Face grain is at right angles to supports and plywood is continuous over two spans.
  • Long panel edges will be supported by nogging in the framing or will have tongue and groove edges.

Table 3 Non Traffic (Roofs)

(deflection less than 0.006 (span)
Plywood Thickness (mm)
Maximum Joist Spacings (mm)
12
800
15
1200

Membrane Roofing Support

Structural Plywood provides an ideal substrate for thin membrane overlays.
A smooth solid face is required, particularly in applications where the roof is exposed to view or where pedestrian traffic is anticipated. The minimum specification is CD grade plywood. In situations where the underside of the panel is exposed to view and the D grade veneer is not acceptable, the use of BB grade is recommended.
The thickness of decking chosen will relate to the nature of anticipated traffic and the flexibility of the membrane.
Three thickness / span situations are tabulated in Tables 1, 2 & 3 relating to thin membrane roofs. Where “outdoor floor” considerations (Table 1) do not prevail the designer must select the decking thickness and spans from the stiffness consideration of either Table 2 or 3. If in doubt in considering the flexibility of the decking and membrane, the membrane supplier should be consulted.
Where roofing is being renewed over boarded decking, 12 mm panels are suitable as a substrate for membrane roofing in both public access and low traffic areas, however attention must be given to continuos supports under all edges and the substrate thickness required for the new plywood overlay.

Diaphragm Construction

In conjunction with plywood’s ability to support face loads the high sheer strength of plywood makes it ideal as a medium for resisting lateral loads resulting from wind and earthquake. The design and construction of walls, floors and roofs as plywood diaphragms must comply with the requirements of NZS 3604: 1990 and NZS 4203.

Expansion and Contraction

As with all wood products, changes in moisture content result in expansion and contraction of the panels. Where panels are liable to exposure and wetting between fixing and roofing over, or high humidity situations, expansion gaps must be provided.
A 2 mm gap at the joints between panels must be provided at time of installation, or cut with a blade saw later before the roofing membrane is applied.

Ventilation

Consideration must be given to roof ventilation at the design stage.
Trapped moisture within a roof or deck cavity, if it cannot escape:
  • Will destroy the R value of insulation.
  • Condenses on cold days and drips, causing water damage.
  • Will cause rotting of structural and lining materials.
  • Allow mold growth.
All roof and deck cavities contain moisture, these cavities may contain an air space, insulation, construction framing etc. and a vapour barrier in the ceiling cannot be considered adequate protection from internal moisture.
Provide for air movement within the roof cavity, joist or plenim areas from soffit to ridge in order to remove condensation and lower heat buildup.

Fixing

The plywood must be laid with face grain direction at right angles to the supports with long edges supported and laid in brick bond pattern. (see Fig.1)
Space screws/nails at 150 mm centers around panel edges and at 200 mm on intermediate supports. (see Fig. 2) Fixings must also be no closer than 7 mm from panel edges.
Staple fixing must not be used when constructing a substrate for thin membrane roofing.
For HT Ply H3 treated Structural Plywood, corrosion resistant fastenings must be used such as type 315 stainless steel.
brickbond exampleFig1. brickbond example screw spacingFig2. screw spacing

Table 4

Nail Fixing Schedule
Plywood Thickness (mm)
Nail Size (mm)
7
30 x 1.8 FH Galv.
9
30 x 1.8 FH Galv.
12
40 x 2.0 FH Galv.
15
50 x 2.0 FH Galv.
17
55 x 2.8 FH Galv.
21
60 x 2.8 FH Galv.

Table 5

Screw Fixing Schedule
(Self Drilling / Countersinking)
Plywood Thickness (mm)
Screw Size (mm)

 

12
40 x 8 gauge
15
45 x 8 gauge
17
50 x 8 gauge
21
65 x 10 gauge

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