Installation Guide

3
accredited agency. (Paragraph
numbers are included in Table 1.)
Some manufacturers market
their siding under a variety of
names such as STK (select tight
knot or sound tight knot) or NPS
(no prior selection). These names
are helpful but it is important to
understand that these are
marketing terms and not grade
designations from grading
agencies.
There are no uniform standards
for STK, NPS, etc. Consequently
the products so-named may vary
in quality from one source to
another. STK cedar may be a
combination of Select Knotty and
Quality Knotty, with a percentage
of the order in Quality Knotty.
Pattern Selection
Boards in standard dimensions
can be readily used for such siding
applications as board-and-batten or
board-on-board. However, most
siding products are milled or run to
a pattern.
Standard patterns are published
by the rules-writing agencies to
encourage uniformity and to serve
as a common reference throughout
the industry. The popular patterns
are bevel or bungalow, Dolly Varden
and channel. (Refer to Figure 3 for
profiles of these and other patterns.
WWPA’s Standard Patterns can be
downloaded from www.wwpa.org.)
Patterns not only allow for
different effects in design and
appearance but also affect perfor-
mance. Patterns like tongued-and-
grooved, which are primarily panel-
ing patterns, should only be
installed in exterior applications
where the greatest degree of pro-
tection from climatic conditions can
be provided. Tongued-and-grooved
patterns allow for very little dimen-
sional change as the siding accli-
mates to the environment, or as a
result of seasonal dimensional
adjustments. Climate, humidity lev-
els, architectural detailing and
exposure to prevailing weather con-
ditions should all be considered
when selecting a pattern.
As a rule of thumb, narrow
patterns perform best because
there is less dimensional change in
the siding as the environment
changes from wet to dry, or from
the freezing conditions of winter to
the blistering heat of late summer.
Thicker patterns have less tenden-
cy to cup or split than do thinner
patterns.
Siding surfaces are manufactured
smooth, rough, or saw-textured.
The type of surface and pattern, in
combination with the finishing
material, will affect performance. In
general, saw-textured surfaces hold
finishes longer than smooth
surfaces.
Moisture Content
As wood loses or gains moisture,
it will shrink or swell until it reaches
equilibrium with the level of mois-
ture in the air of its immediate sur-
roundings. Because of its cell struc-
ture, wood shrinks primarily in
thickness and width and very little
in length.
Wood siding is no exception. It
will shrink and swell regardless
of pattern or material quality.
Problems can occur after instal-
lation if the siding shrinks or swells
unevenly or very rapidly, particularly
if it has been improperly nailed and
its natural movement has been
restricted. However, problems such
as twist, cup, warp, splits and
checks can be minimized.
To avoid potential problems and
to minimize dimensional change
after installation, the moisture con-
tent of the siding should match the
local climate as closely as possible
at the time of installation.
Table 1: GRADES
GRADES
7
Grade Rule Paragraph
WESTERN SPECIES CEDAR
Numbers for
Western &
Reference & Specifying
Selects
1
Finish Canadian WWPA
2
WCLIB
3
NLGA
4
Clear Heart
5
20.11 102-b 200a
A Grade
5
20.12 102-c 200b
C Select
5
10.12 – –
Superior 10.51 – –
D Select
5
10.13 – –
Prime 10.52 – –
B Grade 20.13 102-d 200c
Clear VG Heart
5
21.11 106-aa 201a
A Bevel
5
21.12 106-a 201b
Superior Bevel 16.11 – –
B Bevel 21.13 106-b 201c
Rustic – – 201d
C Bevel 21.14 106-c 201e
Prime Bevel 16.12 – –
Select Knotty
6
– – 205a
Quality Knotty
6
– – 205b
Commons
1
Alternate
Boards
2 Common Select Merch. – 30.12 118-a 114a
Construction Select Knotty
6
– 111-e 204a
3 Common 30.13 118-b
8
114b
8
Standard Quality Knotty
6
– 111-f 204b
118-c
9
114c
9
All Patterns
All Patterns
Knotty
Premium
Knotty Grades
Premium Grades
Additional Grades for
Bevel Patterns
1 Equivalent grades, under the same paragraph numbers in WWPA Rules, are available in Idaho White Pine. Refer to
Western Lumber Grading Rules.
2 Western Wood Products Association’s Western Lumber Grading Rules.
3 West Coast Lumber Inspection Bureau’s Standard Grading Rules for West Coast Lumber.
4 National Lumber Grades Authority’s Standard Grading Rules for Canadian Lumber.
5 Some clear grades may be specified VG (vertical grain). Common practice is to ship orders as a combination of verti-
cal and flat grain, unless specified otherwise.
6 The widely used marketing term, “STK” usually describes a combination of Select Knotty and Quality Knotty, some-
times marketed with approximately 15% of the order in Quality Knotty.
7 The grades listed in these three columns are arranged in order of highest to lowest appearance quality; Clear Heart is
the highest and Standard and Quality Knotty are the lowest grades in this table.
8 Construction grade paragraph reference number.
9 Standard grade paragraph reference number.
The grade names in bold face type are the most widely available grades. These are often specified as a
grade combination that includes some portion of material in the next higher grade; i.e. C&BTR Select, D&BTR Select,
A&BTR Cedar, 2&BTR Common, 3&BTR Common or STAND&BTR (Standard & Better).
General
Categories
(Note that there
are additional
grades for bevel
pattern.)