The roof deck is the roofing material between the structural components (the trusses and joists) and the insulation and weatherproofing layers (roof materials, coatings, layers, etc.). In short, the roof deck is the section of the roof onto which everything else is placed. As such it needs to be strong enough to hold the weight and durable enough to cope with some give.
Types of deck material include steel, concrete, cement and wood. The type of roof deck to use depends on how much weight it needs to carry, which further depends on the roofing materials selected. Other factors include the weight of air conditioning equipment, rain and snow. Special features such as walking decks, roof top swimming pools and bars will also need extra support.
For residential buildings, the most common type of roof deck is plywood or tongue and groove wood systems. Wood generally stands up to the weight of almost any type of roofing material placed on top, although tile may require extra reinforcement.
For commercial or industrial buildings, steel, concrete or cement are the more frequently chosen roof deck materials. That is because the large structures tend to span more distance and carry more weight in equipment on the roof, including air conditioning, air circulation pumps, etc. Since the equipment must be serviced frequently, the roof deck has to be able to take the weight of maintenance personnel on the roof on a regular basis, as well as everything the elements can throw at it.
In all cases, a professional roofing expert should be used to install the roof deck. Since the deck must hold the weight of everything else, improper installation can be dangerous. In addition, a professional roofer has the expertise to select the proper deck material to hold the load of your roofing materials and the various systems and components that operate from the roof.