Sheathing and cladding materials constitute a lot of square footage in a building, covering exterior walls, interior walls and ceilings. They play the defining role in the appearance of the exterior and interior of a building and represent a significant amount of the time, effort and environmental impacts of a building. Multiple choices and combinations are likely in any building, making this an important part of the design process.
In some cases, decisions made earlier in the process regarding wall and roof systems may dictate certain sheathing and cladding choices. Other wall and roof systems will allow for the full range of options presented here. If you are going into the design process with certain options in mind, you may be led toward a particular type of wall or roof choice.
There are a few types of sheathing and cladding presented here that are only appropriate for interior walls and ceilings, and some that are more commonly used on the exterior but which can also be used for interior walls.
The distinction between sheathing and cladding relates to the structural importance of the material. Sheathing materials are fastened directly to the building in such a way as to provide either a primary or secondary structural strength, usually shear strength but sometimes also compressive strength. In contrast, cladding materials are applied in a manner that removes them from playing a key structural role. Some buildings will combine sheathing (such as plywood or plaster) applied directly to the wall with a cladding (such as wood, brick or rainscreen plaster) that is attached to the sheathing via strapping or anchors.
Sheathing and cladding considerations can be two-fold: the material itself and the type of surface finish used to seal and protect it. In some cases, the two can be considered separately with a single material having several options for finishing. In other cases, the material may be left in its raw state with no need for further finishing. Chapter 7: Surface Finishing Materials considers the treatments that may be applied to cladding materials, so the materials in this chapter are considered in their raw form.
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Exterior sheathing plays an important structural role in bracing and stabilizing a wall or roof system. The fastening of a sheathing material to a frame or wall substrate creates a stable diaphragm that resists shear and compressive forces. Where sheathing is intended to have a structural role, it must be specified to handle the expected loads and fastened to meet requirements.
Sometimes a structural sheathing will function as a primary air control layer, preventing air from moving into the wall system from the exterior. In these cases, joints and intersections in the sheathing must be made airtight, often using tape, caulking or parging.
In some cases, a structural sheathing will also be the finished surface and be responsible for functioning as a cladding as well.
The primary function of cladding (or siding) is to keep precipitation out of the building. Most cladding materials are applied to the exterior walls in such a way as to create a rainscreen siding. In a rainscreen scenario, the cladding is applied to the wall behind in a manner that leaves a reasonable (20–40-mm or ¾- to 1½-inch) ventilation space and drainage plane between the wall and the cladding. This space should be vertical in orientation to allow for the circulation of air from the bottom of the wall to the top, and to allow for drainage of any water leaking in through the cladding. Air circulation through this space will allow any humidity created by moisture escaping the walls of the building to be equalized with drier outdoor air conditions. In this way, it is possible to prevent moisture buildup on the backside of the siding and/or the exterior face of the wall, which is a key factor in most moisture failures. Vertical channels create entry points for air at both the bottom and the top, following the natural pattern for convective drying.
Horizontal channels suit some kinds of sheathing better than vertical channels, but a horizontal channel doesn’t allow for drainage or convective currents with upper and lower entrance points. There may be no actual exit point for horizontal spaces, making moisture buildup more likely.
Rainscreen siding should have adequate ventilation space, but shouldn’t be an open channel that allows insects or rodents to take up residence in the building. Screen, mesh or breather strips are available for all types of siding.
Certain wall types, in particular those with plaster finishes, are not designed to have rainscreen sheathing. In these cases of integral sheathing, the plaster is adhered directly to the wall surface and there is no ventilated cavity. For these types of walls, moisture is handled by preventing precipitation from penetrating the plaster (via protection from roof overhangs and sealants or paints that reduce bulk water penetration) and ensuring that the plaster and any sealants are permeable enough to allow moisture to escape to the atmosphere. This strategy has proven to work for many building types and in many regions, but in areas with lots of precipitation (especially wind-driven rain) or for building designs with little roof protection of the walls (especially tall walls), it is prudent to create a rainscreen over the plaster finish. This can be constructed with a different type of sheathing, or by applying another layer of plaster over a breather fabric.
Regardless of the type of siding and manner of installation, all exterior sheathing must have proper consideration and application of flashing at all joints, intersections and openings to prevent water from entering. It is prudent to follow best practices for flashing to match the sheathing type.
Rainscreen cladding is not used as the primary air control layer on the exterior of the wall.
The plaster applied to many natural wall-building systems is often considered an integral part of the structural capacity of the wall. Where this is the case, the type of plaster, method of attachment and application to the wall and potential need for reinforcement must be part of the plan for the building and up to the task of handling the structural requirements.
Interior sheathing can have an important role in the moisture handling capability of a building. The sheathing can be the primary air control layer, or may require a separate, sheet-style air control layer applied beneath the sheathing. Where the sheathing itself is the primary control layer, it must be installed in such a way as to eliminate cracks and leaks and be properly integrated with other air control layers at all intersections.
Most interior sheathing materials are naturally vapor-permeable, allowing them to be used in wall systems where this characteristic is desirable or required. If permeability is not desired, a sheet-style vapor barrier and/or an impermeable surface coating will need to be used. As a general rule, in heating climates the exterior sheathing should be at least as permeable as the interior sheathing, while in cooling climates the opposite holds true. Otherwise, moisture migrating through the sheathing on the more humid side may accumulate inside the wall at a rate faster than it can leave through the drier side.
In many scenarios the interior sheathing is considered to be playing a structural role, and materials choices and construction methods may need to be made to meet these
Roof sheathing materials as wall sheathing
In addition to all of the choices outlined in this chapter, walls and sometimes ceilings can be clad with the same materials outlined in Chapter 5: Roof Sheathing. Any material that is suitable for roofing will be able to function, usually with increased durability, as wall sheathing. These include:
- Metal sheet and tiles
- Cedar shake and shingle
- Composite shingles
- Clay tiles
- Green or living walls
Roof sheathing materials are applied to wall as rainscreens, using the same type of strapping and fasteners as would be used on the roof. None of the roofing materials would be able to act as a primary air control layer for a wall or ceiling, so the system would need to have such a layer designed to operate separately from the cladding material.
Some roofing materials may not be desirable as interior cladding due to potential off-gassing (composite shingles, cedar shingles, paints on some metal products). Permeability issues may need to be addressed if using metal sheets or tiles as an interior cladding. Green walls are constructed similarly to green roofs, with additional consideration for retaining soil on a vertical surface.
Considering roofing options as cladding greatly increases the number of viable choices and aesthetic combinations available to a homeowner.