What is Roof Decking? (Definition, Materials + Types) for 2022

What is Roof Decking Graphic

What is Roof Decking?

Roof decking is the material that lays between the structural components of a roof (joists and trusses), the insulation, and the waterproofing layers (coatings, roofing materials, etc.). The roof deck is the section of the roof where everything else is placed. Because of this, it needs to be strong enough to hold weight and durable enough to cope with having some give.

Roof Decking Material

There are a wide variety of materials that can be used for roof decking. This can include anything from wood to cement, concrete to steel. The material used depends on how much weight it needs to carry.

Other factors include the weight of any air conditioning equipment, rainfall in the area, and any potential snow build-up. Special features, something like walking decks or rooftop swimming pools or bars, may also be needed for extra support. 

Types of Roof Decking

Generally speaking, there are three different types of roof decking to keep in mind: tongue and groove, plywood or OSB sheathing, and plank sheathing. Understanding the different kinds can give you a good idea of what to expect out of each.

The type of roof decking varies based on whether you have a commercial or residential need. Residential buildings will likely use wood for the roof decking because it stands up to the weight of just about anything that will be placed on top of it.

Tongue and Groove

Tongue and groove decking is usually done with a 2×6 “tongue” formed on one edge of the board. This is meant to fit snugly into the “groove” in the adjacent board. This can be something of a challenge to replace as it is something that is not readily available in most lumber yards.

Generally, this is a highly durable and stout roof decking. It will only need to be repaired when it has had extensive exposure to moisture. Be sure to take a long look when going over your tongue and groove roof decking.

Plywood or OSB Sheathing

When it comes to plywood or OSB sheathing, you may have seen it before under the false term of Wafer Board. Builders began using this method because it has strength and longevity against splitting. These methods became more and more popular in the early 1980s and 1990s and is now almost exclusively used by builders across the industry.

There are a few points that you should definitely be aware of during the installation process. Make sure to install the boards using staggered vertical joints and ensure they are parallel to the ridge line. The vertical joints that are between the boards have to be supported along the whole length and need to be nailed securely.

It is also important to ensure that you have sufficient support with an absolute maximum of 600 mm between the two rafters. Those plywood panels should be installed with 3 mm spacing between each panel unless it has been stated differently by the manufacturer.

Plank Sheathing

Plank sheathing, meanwhile, is typically something that was used before plywood. Wood shingles are expensive when it comes time to tear down and replace them, so re-sheathing over the entire roof is a far cheaper method.

Plank sheathing is generally susceptible to distortion thanks to weather changing. This can result in constant expansion and contraction that can wear down those planks over time, making them brittle with enough expansion and contraction.

It is also important to stagger your joint boards. When a number of those adjacent boards join on the same support or rafter, it can be possible for the deck to move due to all that distortion to a crack line in the singles. All the wood boards must be properly conditioned to maintain moisture equilibrium.

There should also be a sufficient number of fasteners to prevent any kind of buckling, and each board should be fastened with at least two nails in each rafter to ensure that it is held securely but without too much force.

Purpose of Roof Decking

Generally speaking, your roof deck is meant to hold the fasteners to secure the roof. Sure, it can support foot traffic and the occasional snow load that will inevitably happen with any roof out there. If that wood is compromised, it might not be able to provide the level of holding power that will allow the roof to last its suggested lifespan.

Having a proper roof deck is the foundation of any good roofing system. The deck is meant to resist gravity loads and lateral loading from things like the wind and other seismic forces. A proper roof decking will meet design requirements like component anchorage technique, deflection resistance, fire resistance, surface characteristics, and dimensional stability.

Assessing Your Existing Roof Decking

There are several reasons that property owners should be interested in the type of roof deck on the building and its condition. Among them are the following reasons:

  • Condition of the roof deck – is it detached, corroded, unsafe, or damaged? Has the deck been deflected or deformed to the point where it ponds water?
  • Stability – Is the deck capable of handling a much heavier roofing system like a built-up roof membrane with a ballasted single-ply system?
  • Resistance – If the mechanical fasteners are going to be used to attach insulate or a single-ply membrane to the deck itself, will that deck be able to provide the necessary pullout resistance? Some single-ply systems require a stronger steel deck to meet wind design.
  • Removal – If the roof needs to be patched, can the deck sections be safely replaced or removed if necessary? In the past, it was common to remove that deteriorated decking and replace it with metal decking and rigid roof insulation. This can actually be quite unsafe and is definitely not recommended. Instead, new form boards and galvanized wire mesh should be used. Ensure that the galvanized wire mesh is attached to the existing wire near the patch’s perimeter. This is done for safety reasons.
  • Clearance – If the existing roofing is going to be removed all the way down to the deck and the tapered insulation or a sloped fill is going to be the way to solve the ponding problem, you need to know if the clearance at access doors, windows, and other equipment hatches will be imperiled.
  • PV Panels – What about adding PV panels to the building roof? If this is part of the plan, you need to know how they will be attached to the structure.

The roof’s decking plays an important part in supporting the overlaying material. The material itself is certainly important for keeping the elements away and holding up to the constant wear and tear that comes with exposure to the elements. Still, it doesn’t do its job with the proper roof decking.

Addressing Wooden Deck Problems

Ensure to properly ventilate your attic to eliminate any excess moisture that deck panels can absorb. This reduces the amount of shrinking and swelling the wood experiences, helping it last longer without turning brittle and breaking.

If that brittleness occurs, it can compromise the structural integrity of your roof, and you will have to have costly repairs or replacements done immediately to prevent any safety hazards.

Regardless of what style of roof decking you choose, you need to ensure that it is installed properly to get the proper weight support that your roof needs. Consider any additional snow weight if your area sees heavy snowfall so that your deck does not bow and crack under additional pressure.

Much like the foundation of your building, the roof decking is the basis on which the rest of the roof operates. Without proper roof decking, your roof will eventually buckle under pressure and need to be replaced.

Save yourself a lot of time and labor by ensuring that your roof decking is installed properly and rated to handle the weight on your roof. You can then focus on implementing the roofing materials needed to get the right roof for you.