What is Roof Decking? (Definition + Tips)

What is Roof Decking Graphic

What is Roof Decking?

In short, the roof deck is the roofing material that lays between the structural components of a roof (the joists and trusses), the insulation, and the waterproofing layers (coatings, roofing materials, layers, 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 be durable enough to cope with having some give.

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 type of material used depends on just how much weight that it needs to carry.

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

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.

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.

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 board that is adjacent to it. This can be something of a challenge to replace as it is something that is not readily available in most lumber yards.

Generally speaking, this is a highly durable and stout kind of roof decking. The only time that it will need to be repaired is when it has had any 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 make certain 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 tend to be expensive when it comes time to tear down and replace them, so using 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 used have to be properly conditioned to be at a 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.

Protecting Your Roof

Generally speaking, you 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 become an inevitability 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 as well as 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.

There are a number of reasons that property owners should be interested in the type of deck that is on the building and the condition that it is in. 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 ends up ponding water?
  • Stability – Is the deck capable of handling a much heavier roofing system like a built-up roof membrane that has 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? There are some single-ply systems that require a stronger steel deck in order to meet wind design.
  • Removal – If the roof does need to be patched, can the sections of the deck be safely replaced or removed if necessary? There was a point in the past where it was common to remove that deteriorated decking and then replace it with metal decking and a 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. Make certain that the galvanized wire mesh is attached to the existing wire near the perimeter of the patch. 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 decking of your roof 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, but it doesn’t do its job with the proper roof decking.

Addressing Wooden Deck Problems

Make certain to properly ventilate your attic in order to eliminate any excess moisture that can be absorbed by deck panels. This reduces the amount of shrinking and swelling that the wood experiences, helping it last for longer without turning brittle and breaking.

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

Regardless of what style of roof decking that you choose, you need to make certain that it is installed properly so that you get the proper weight support that your roof needs. Take into account 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 a proper roof decking, your roof will eventually buckle under the pressure and need to be replaced.

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

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