Sarking is commonly CPD (Construction Panel Directive), Stress grade 11, 15 mm ply laid on supports at 800 centres, or 17 mm ply at 900 centres. Ply should be laid with face grain at a right angle to supports. Edges of sheets not held by plastic tongue or T&G should be supported. Fasteners should not be closer than 10 mm from sheet edges.

Ply Sarking Fixing Pattern

Wind ZoneEdges fix at 75 mm centresBody fix at 150 mm centres
Up to High Wind Zone60 x 2.8 nails or 8g x 40 mm screws60 x 2.8 nails or 8g x 40 mm screws.
Very High and Extra High75 x 3.15 nails or 10g x 40 mm screws75 x 3.15 nails or 10g x 40 mm screws

To allow for expansion, maintain 3mm gap between sheets.  At gutter line the ply should overhang the gutter line by 25 mm and remain square cut. Dormer valleys and valleys into spouting can be recessed or flat laid. 

Revision Category: 
1 - Minor Errata
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Provided full terminology for CPD (Control Panel Directive).