Note: that these are general guidelines that may be followed with confidence in most areas. However, severe local climatic conditions such as extreme heat, extreme dryness, high winds, seacoast exposure, and areas with wide climatic swings may neccessitate additional fastening of the siding.

Acclimatize Siding Before installing

One of the most stable softwoods, Western Red Cedar is nevertheless a natural material and it responds to the environment. Cedar siding can swell or shrink as it gains or loses moisture to reach equilibrium with the moisture content of the surrounding air. Ensuring that the moisture content of cedar siding is at equilibrium before it is installed will minimize movement later on. Recommended moisture content for siding used in various regions of the country are given in Table 1.

On the job site, keep wood dry. Stack siding off the ground and under cover. If the wood is to be stored over damp ground or new concrete, place a moisture barrier under the siding. Siding should be 4 to 6 inches above the ground with air circulating freely around and throughout the stack. Acclimatization time varies with the moisture content of the siding. The following procedures are suggested for different siding specifications.

Table 1. Recommended Moisture Content for siding Installation

1. To calculate average, test 10% or more of pieces.
2. Source: Wood Handbook, 1987

Kiln Dried Clears

These have been dried at the mill to 12-15% moisture content. They are the most ready-to-use of all siding products. If the siding has not been wetted prior to arrival on the job site it may be applied upon arrival except in the Southwest where 3 to 5 days of well ventilated storage is generally required. If the siding has been wetted it must be separated and allowed to dry thoroughly before installing.

Dry Knotty Siding.

Knotty sidings are air or kiln dried to less than 19% moisture content. Stack the siding on evenly spaced, vertically aligned stickers (Figure 1) in a dry storage area for 7-10 days. More time may be required in damp or humid conditions.

Green (unseasoned) Sidings

Unseasoned or green sidings have not been dried prior to delivery and require a longer time to acclimatize than seasoned siding. Separate the siding with vertically aligned stickers (Figure 1) and store in a well ventilated dry location for a minimum of 30 days or longer in damp or humid conditions. If specifying unseasoned siding, patterns such as channel, bevel and board-and-batten are recommended since these allow for shrinkage. Narrower widths are also recommended since these move less overall than wider widths.

Prime Before Installing

After cedar has achieved equilibrium moisture content and before it is installed, a coating on all surfaces is recommended. The type of coating will vary with final finish selected. The coating protects the wood from water penetration, helps prevent staining caused by mildew and extractives and can increase the service life of top coats. The following coatings have proven themselves in field test (Table 2).

Check with your Paint dealer to make sure that the prefinish coat you select is compatible with the intended finish coat. For example, clear water repellants should not be used where the intended finish is a bleached oil.

Table 2. Recommended Priming Procedures for Cedar Siding

Nails: Types and Spacing

For best results use "splitless" siding nails. These have thin shanks and blunt points to reduce splitting. For greater holding power, nails with ring threaded or spiral threaded shanks are suggested. Nails with textured heads can be used to reduce glossy spots at the nails when finished (see Figure 2 for nail types).

Nails should be driven with care. Heavy nailing distorts the wood and may cause splitting. At mitered corners, near edges and near ends, nail holes may need to be pre-drilled to avoid splitting.

The size of nails to use depends on the type and thickness of siding. Good building practice is to use nails long enough to go through underlying materials, such as sheathing and insulation, and penetrate at least 1-1/2 inches into solid wood or 1-1/4 inches if using ring or spiral shank nails. See Table 3 for recommended nail lengths for various thicknesses of siding.

Table 3. Minimum Recommended Nail Lengths for Cedar Siding Over Wood Sheathing (Nailed at Studs)

Siding should be fastened to each stud or furring strip with nails spaced at a maximum of 24 inches on centre.

Nail placement depends on the siding pattern and width. The key is to fasten the siding securely without preventing it from moving in response to the moisture content of the air. In general, each piece of siding must be nailed independently. Nailing overlapping pieces together restricts the natural movement of each piece and will cause splitting.

Wall Construction

Like all siding materials, Western Red Cedar performs best when installed properly on a suitable frame. Cedar siding should be securely nailed to framing members, furring members or to blocking between framing members. Lumber sizes and spacing for blocking and furring are shown in Table 4.

Table 4. Lumber Size and Spacing for Blocking and Furring*

* Furring must be securely fastened to studs.
** Can be used in moist and severe climates to form an air space between siding and sheathing.

Stud Walls

Stud walls generally require no special preparation for horizontal siding installation. Siding should be nailed into studs at maximum 24 inch centres when applied over wood based, solid sheathing and 16 inch on centre when applied without sheathing. In those areas where an air space between siding and sheathing is desired, nail siding to furring strips (Table 4).

For vertical siding installation, stud walls require horizontal blocking lines or furring strips Table 4). When applied over wood based sheathing, vertical siding should be nailed to horizontal blocking or other wood framing members not exceeding 36 inch on centre when face nailed or 32 inch on centre when blind nailed. Vertical siding when installed without sheathing should be nailed to wood framing or blocking members at 24 inch on centre. Some building codes require 24 inch on centre with or without sheathing. Check your code to verify requirements.

Masonry Walls

Masonry walls require furring strips for horizontal and vertical siding installation. Furring strips must be thick enough to allow for nail penetration of 1-1 /2 inches into solid wood (Table 4).

Controlling Moisture: Vapor Barriers and Building Paper

High humidity output from washers, dryers, showers, kitchens and other sources, combined with today's energy-efficient construction, can result in a build up of interior moisture. This vapor migrates through the fabric of the building from the warm interior to the cold exterior, and when it condenses to water can cause structural damage and damage to siding and finishes.

Vapor barriers installed on the warm side of the wall largely prevent the migration of moisture vapor but they are not perfect. Residual vapor must be allowed to escape to the outside. Building paper installed on the outside face of the sheathing helps prevent rain and snow from penetrating the walls but allows the escape of moisture vapor.

Proper wall construction includes the use of gas permeable building paper and vapor barriers (see Figure 3). These are extremely effective in helping to prevent moisture problems.

More About Moisture

Before applying siding, make sure flashing over doors, windows and other breaks is properly sealed to prevent moisture entry into the wall space.

Keep the lower edge of siding at least 6 inches off the ground and coat end grain at the bottom of vertical siding with water repellant.

Thorough caulking of all joints is recommended including butt joints and where siding contacts openings or trim. Use only non-hardening caulks such as polyurethane, polysulfide or latex-silicone. Pure silicone caulks do not work well on cedar and are not recommended.

Do not reduce the siding overlap recommendations given in the installation section of this publication. To do so could result in damage from wind-driven water.

Applying Siding Over Rigid Foam Sheathing

Rigid foam sheathing has an insulation value superior to that of traditional lumber and plywood sheathings. However, it has little or no nail holding power and using rigid foam sheathing as a nailing base for cedar siding is not recommended. For best results with rigid foam and other sheathing products, follow the nailing recommendations given in this publication as to nail type, positioning, penetration and spacing. Table 5 shows nail length recommendation for bevel siding and foam sheathing systems.

Foam sheathing panels vary in moisture permeability but are generally considered as good moisture barriers. They should be used with a continuous film vapor retarder on the inside wall under the interior finish or with foil-backed gypsum board. Rigid foam sheathing can cause staining, bucking and damage to finish coats. The following recommendations are meant to minimize potential problems when wood sidings are applied over rigid foam sheathing:

• Use thicker siding patterns in widths of 8 inches or less. Thick, narrow siding is more stable than thinner, wider patterns and better able to resist dimensional changes.
• Use kiln-dried siding over rigid foam sheathing. This is the most stable cedar siding.
• Proper prefinish is essential. Follow the priming recommendations given in this publication before installing siding.
• Use light colour finish coats to maximize heat reflection and reduce dimensional movement.
• In severe climates, an air space between siding and rigid foam sheathing can be created by fastening furring strips to sheathing before installing siding (see Wall Construction). Air space allows for the venting of accumulated moisture.
• In all cases it is recommended that building paper or an equivalent building wrap be applied over foam sheathing.

Table 5. Nail Sizes for Bevel Siding - Foam Sheathing Systems

Installing Bevel Siding

Spacing for siding should be laid out beforehand. The number of board spaces between the soffit and bottom of the lowest piece of siding at the foundation should be such that the minimum overlap is not less than 1 inch. The wider the siding, the greater the overlap can be (Table 6)

Start with the bottom course using a furring strip to support the lower edge. Each succeeding course overlaps the upper edge of the previous one by a minimum of 1 inch. Rabbeted patterns are self-spacing, but leave 1/8 inch expansion clearance.. Where possible, the bottom of the board that is placed across the top of the windows should coincide with the window cap.

Bevel siding should be face nailed to studs with 1-1/2 inch penetration into solid wood using one nail per bearing spaced at a maximum of 24 inch on centre. Place nail just above the 1 inch overlap. Take care not to nail through the overlap of two pieces (see Figure 4).

Butt joints between boards should be staggered and made on studs. Fit siding snugly to other pieces and to trim and flashing. Ends should be caulked.

Table 6. Recommended Overlap for Bevel Siding

* Use for unseasoned 10 inch bevel siding.
Notes: Do not exceed 2 inch overlap. Use the larger overlap for expansion. Take care not to overdrive fasteners when using larger overlaps.

Table 7. Bevel Siding Dimensions (seasoned)

Installing Tongue and Groove Siding

Tongue and groove siding can be installed horizontally or vertically. In horizontal application, start at the bottom and work up with the groove edge facing downwards to assure a weather-tight wall. Siding up to 6 inches wide can be blind nailed with one casing nail per bearing toe-nailed through the base of each tongue. Wider siding should be face nailed using two nails per bearing. Nails must penetrate 1-1/2 inch into solid wood (see Figure 5)

In vertical application, start at one corner with grooved edge towards the adjacent wall, using a level or plumbline to ensure that the first board is installed vertically. The grooved edge of the first board may have to be trimmed to ensure a flush fit. Siding is nailed to horizontal blocking lines installed between studs or to furring strips (see Wall Construction). As with horizontal installation, pieces up to 6 inches can be blind nailed and wider pieces should be face nailed.

Table 8. Tongue and Grooved Siding Dimensions

Installing Lap Siding

Lap siding can be installed horizontally or vertically. For horizontal application, start with the bottom course and work up with the bevel sides of the channels pointing upwards. Allow a 1/8 inch expansion gap between pieces if the siding is air- or kiln-dried. Do not nail through overlaps. For siding up to 6 inch wide, use one nail one inch up from the lap. Face nail with two nails per bearing for 8 inch patterns and wider, keeping nails 2-1 /2 to 3 inches apart to allow for dimensional movement without splitting (see Figure 6).

For vertical applications, siding should be nailed to horizontal blocking lines or furring strips (see Wall Construction).

Table 9. Channel Siding Dimensions

Installing Board-and-Batten Siding

Board-and-batten is a vertical pattern created using cedar boards and battens of various widths for a range of effects. Although for good appearance most builders strive for the widest boards and the narrowest battens, the battens must be sufficiently wide to adequately overlap boards. For nominal 6 inch boards spaced 1/2 inch apart, battens should overlap by at least 1/2 inch. With wider boards, increase batten overlap proportionately. While there are no set widths for board and batten siding, an attractive combination is 1"x3" battens with 1"x10" boards.

Siding should be nailed to horizontal blocking lines or to furring strips (see Wall Construction).

Boards up to 6 inch wide should be fixed with one nail per bearing driven through the centre of the board. The overlying batten covering the gap between boards should be attached with one nail per bearing, the shank passing between the edges of the underboards. Using two nails per bearing on boards 8 inch and wider placing the nails approximately 3 inch apart (see Figure 7).

Corners and Field Joints

At inside corners, siding is frequently butted against a trim strip. It can also be butted against adjoining walls with a trim strip used to cover the joint.

At outside corners, some builders choose mitered corners for a professional looking finish. Mitered corners are most common on horizontally applied siding and they must fit tightly for the full depth of the miter. To maintain a tight fit, the siding should be properly seasoned before installation and protected from weather at the job site. The ends are often set in caulking compound when siding is applied (see Figure 8).

Corner boards are a popular alternative to mitered corners. They are often 3/4 inch or 1-1/4 inch material depending on the thickness of the siding. Width is a matter of taste and proper proportion. Corner boards are applied to the sheathing with siding fitting tightly against the narrow edge of the boards. Joints should be filled with caulking compound when siding is applied (see Figure 9). Trim boards can be used to cover butt jointed siding (see Figure 10).

A useful tip in installing corner boards is to apply sheet metal corners over the building paper. The corner boards and the ends of the siding are nailed through the sheet metal which anchors the wood for a maintenance-free joint. Designing roofs with an eaves overhang to protect corners from weathering also helps ensure trouble-free joints.

When butt jointing siding, cut ends at 45o angles to form an overlapping joint. This is particularly important for vertical installation where the siding must shed water. Caulking butt joints is recommended (see Figure 11).

Specifying Siding

The following information is required when specifying siding products:

The above information was re-printed from a Western Red Cedar publication number 93-500, copyright 1993, revised March 1996, Standard Building Supplies Ltd. expresses no opinion on it.