Installation Guide

14 WOOD UNDERLAYMENT
Plywood/OSB underlayment installation guidelines
WOOD UNDERLAYMENT
Figure 1
Figures 1 & 2 – Typical Subfloor/Underlayment Detail (Not to Scale)
DISCUSSION
In some applications referenced in this Handbook, adding a layer of plywood or OSB before installing DITRA-HEAT and
the ceramic or stone tile covering is required to reduce deflection and curvature of the sheathing between the joists.
INSTALLATION GUIDE
Place underlayment panels (Exposure 1, plugged-face plywood or OSB) with long dimension perpendicular to floor joists
such that the following conditions are met:
1. Abut all underlayment end joints at quarter points between joists.
Example: Abut underlayment panels on either side of the joist centerline at: 4" (102 mm) for 16" (406 mm) o.c. joists,
5" (127 mm) for 19.2" (488 mm) o.c. joists, or 6" (152 mm) for 24" (610 mm) o.c. joists (see figures 1 & 2).
Note: Underlayment end joints should be placed as far away from subfloor end joints as possible.
2. Underlayment to overlap edge joints of subfloor by 1/2 of the width of the subfloor panel (24" - 610 mm). At restraining
surfaces, overlap may be less than 24" (610 mm) when the subfloor panel is less than 48" (1.2 m)-wide (see figure 1).
3. Gap underlayment panels 1/8" (3 mm) on all ends and edges, and 1/4" (6 mm) at perimeter walls, cabinetry, or other
restraining surfaces.
The following guidelines must be followed when fastening underlayment panels:
1. Use ring shank nails (no staples) or wood screws (no drywall screws).
2. Fasteners must pass through entire thickness of underlayment and subfloor panels with minimal penetration into
joists (see figure 2).
FINAL WORD
As stated previously, Schluter
®
-Systems requires that any underlayment panel must have a minimum thickness of 3/8"
(10 mm). When in doubt, increase underlayment thickness.
Figure 2
Plywood/OSB Type and Fastener Schedule Guidelines
Plywood/OSB
Grades
Plywood/OSB
Thickness - in (mm)
Maximum On-Center Fastener Spacing - in (mm)
Panel Edges Field
Exposure 1,
plugged-face
plywood or OSB
3/8 (10) 4 (102) 6 (152)
1/2 (13) 4 (102) 6 (152)
Greater than 1/2 (13) 6 (152) 6 (152)