Installation Guide

SPECIALTY ROOF DECK DETAILS
Wood Deck
Wood decks form an ideal base over which to apply
cedar shingles or shakes, since they can be attached in
the conventional manner.
Where a layer of insulation, normally one of the rigid
types, is to be included, the problem of how to fasten
shingles or shakes is created. The use of abnormally
long nails driven through the shingles, the insulation,
and into the deck below is generally unsatisfactory,
and horizontal strapping will be required to overcome
the fastening difficulties (Figure 15). When strapping
is used, fewer nails penetrate through the insulation
to the deck, and greater thermal efficiency is achieved
by reducing the number of conductors. In addition,
the lengths of the nails may be chosen to prevent the
points from protruding through the deck where they
may mar the inside face.
If ice-damming is a potential problem or if reverse
condensation is likely to occur, such as may be
encountered in an ice arena, the cold weather roof
system should be used in conjunction with horizontal
strapping, and ventilation must be provided at the
eaves and at the peak. In buildings such as ski cabins
that may be subjected to heavy snow loads, it is
usually necessary to fasten wood members (typically
38mm x 90mm on edge) from ridge to eave on the
roof deck and place the rigid insulation between.
Strapping is then applied across the top of these
members, giving a ventilated air space and avoiding
compression of insulation (Figure 16).
The need for strapping can often be completely
eliminated by the use of a false plywood deck,
immediately over the insulation, to which the
shingles or shakes are directly fastened (Figure 17).
Exterior-grade sheathing panels are ideal for this
purpose, since they provide a strong, smooth surface.
However, under certain conditions of pitch and
loading, there may be a tendency for the entire roof
above the decking to creep downwards, bending the
nail fastenings and compressing the insulation,
thereby reducing its efficiency. In such cases, it is
often desirable to install the vertical members as
previously described.
Nails
If the shingles or shakes are nailed directly through
rigid insulation, a number of problems may be
encountered. For instance, the longer nails have
thicker shanks which tend to split the shingles or
shakes. In addition, movement of the shingles or
shakes as they go through natural expansion and
contraction cycles caused by wetting and drying, have
a tendency to enlarge the holes in the insulation,
thereby reducing its efficiency. For this reason, the use
of strapping or false plywood deck is again recommended.
Cedar shingles or shakes
(applied as per specification)
Cedar shingles or shakes
(applied as per specification)
Boards
Rigid insulation
Vapour barrier
(if required)
Wooden
deck
Joist
Joist
Air
space
Boards
Rigid insulation
Vapour barrier
(if required)
Wooden
deck
Vertical lumber
member
Fig. 15: Specialty Roof Deck – Strapping over Insulation
Fig. 16: Specialty Roof Deck – Vented Roof
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