Most homeowners consider singles or insulation as the major parts of a roof. One of the most overlooked inside layer that is hidden away is the roof sheathing. This underlying layer of the roof is very critical to the proper functioning and the lifespan of a roof. The roof sheathing is the layers of plywood or wooden planks installed beneath the shingles or rafters. There are two types of roof sheathing – open and closed. The kind of sheathing depends on whether there is space in between the boards. Open sheathing installation is the cheaper of the two, but closed sheathing is usually the most effective. The sheathing layer again has a thickness ranging from one to five inches. The thickness considered is based on the weight, height and the effectiveness of the material.
Features of effective sheathing
It is found that nearly two thirds of all roofing activities involve renovation or replacement. In such cases, there may not be a design in record and vital information about the roof may have been wearied off over time. When contemplating roofing work, there are certain aspects that the contractor must pay heed to. These include:
- Gravity and lateral load resistance from wind and seismic forces
- Deflection resistance or insulation thickness to protect the roofing membrane
- Dimensional stability due to temperature variation
- Resistance from wind, wire, heat and moisture
Protection of the roof against leakage is critical as is commonly seen with older or damaged roofing systems where water penetrates roof shingles. Proper sheathing stops this penetration. A layer of roof sheathing provides enhanced protection against fire. A fire-resistant sheath material treated with fire retardants reduces the impact of damage. But, some studies have shown that fire retardants can cause the plywood to split or separate. This is where the use of thicker plywoods for sheathing like the CDX can help the contractor make the right choice.
CDX plywood as sheathing material
Builders suggest the use of plywood for its structural stability, water resistance and low cost, intended for exterior applications such as roof sheathing. The standard choice of roof sheathing with 24-inch rafter spacing is the CDX. CDX is the construction grade material made from thin sheets of wood veneer laid alternately at right angles with wood grains glued, heated and pressed together. Usually phenolic formaldehyde is used for gluing exterior plywood. The average surface of the plywood is the C grade side, the side having small holes and patches is D, and the glue that joins the sheet exterior together is X. The water resistant characteristic of a CDX exterior grade board used as roof deck material makes it ideal for roofing.
The high-strength plywood sheet materials are used for their ruggedness in building and home construction. When you shop at hardwood stores, you will come across different types of wood grading systems – A, B, C, and D. CDX is low graded plywood that is high in strength. The CDX plywood can be easily distinguished by the thickness of size fillers, patches and knots. It is priced low compared to other plywood varieties.
Best thickness for CDX plywood sheathing
A contractor will also consider the plywood thickness to understand whether the roof is efficient enough to handle the extra weight. Roof sheathing with half inch plywood works well for 16-inch center roof rafters. However, it fails to deliver the same impact and resistance with 24-inch engineered truss roofing systems.
For home building projects, using half-inch plywood for roof sheathing is highly economical and effective. If you are looking to build a superior roof, use the 5/8 of an inch plywood. This plywood works great for 24 inch roof rafters with the benefit of having a nice sturdy roof to walk on while roofing the house.
November 9, 2017