The Engineer uses his Construction PDA to construct his buildings. Once a building is selected, blueprints of the respective building appear in front of the Engineer, which can be rotated 90 degrees before deploying them using the alternate-fire key. If the building can be constructed in the given location, a 3D sketch of the completed building will appear. If the building is above an invalid location (uneven surface, another colliding entity on location, not enough space), then a red cross will appear on the blueprints instead. The construction of these buildings requires Metal, a resource that is exclusive to the Engineer. Each building has an associated amount of Metal required to construct the building. An Engineer can carry a maximum of 200 reserve Metal, which is consumed upon placing a building and can be regained through ammo boxes, resupply cabinets, destroyed building parts, fallen weapons and Dispensers. Once a building is placed, the building will begin to automatically construct, but the Engineer can hit it with his wrench in order to speed up construction. This method of accelerated construction is not exclusive to the Engineer's own buildings but can also be applied to those constructed by team members.
In Team Fortress 2 Classic, the Engineer class can construct five types of buildings as of version 2.1.0. These include the Sentry Gun, which provides area defense, the Dispenser for healing and ammunition replenishment, and Teleporters for rapid teammate transport. When equipped with the Jump Pad PDA, Engineers can also construct Jump Pads, which replace Teleporters and provide access to routes typically reserved for explosive jumpers. The Metal costs for each building are as follows: Sentries require 130, Dispensers 100, Teleporter Entrances and Exits 50 each and Jump Pads 70 each.
At the start of construction, buildings appear with 0 health, but gain health to its maximum value as construction continues. However, they are still susceptible to damage as they construct, so they are more easily destroyed while building compared to fully constructed. Visual indicators, such as smoke, sparks, and flames, appear as a building is damaged. However, buildings cannot be set on fire, and any fire particles are purely visual indicators for a damaged building. Similarly, Buildings are also immune to bleed and critical hits. Once a building is destroyed, they leave various pieces of scrap and gibs around their location, which can be collected by the Engineer in the form of metal, by Players as Ammo or by the Spy to recharge his cloak. Metal is also consumed to repair, upgrade and for Sentries specifically, reload. This can be achieved by hitting an Engineer's Buildings with a Wrench.
If a building is at full health, the Engineer can upgrade his buildings to improve their functionality and maximum health. Each Wrench swing uses 25 Metal, and each building requires 200 metal to upgrade. Teleporters, Sentry Guns, and Dispensers can all be upgraded twice to a maximum level of 3, while the Jump Pads cannot be upgraded.
An enemy Spy is able to place a Sapper on a building, which disables it and draining its health. Once a Building is sapped, the Engineer who built it will receive an indication that their Building is sapped and make an associated voice call. Sappers can be removed by the Engineer himself or any other Engineer on the Engineer's Team.
A Building can be hauled by the Engineer who built it by pressing alternate fire key while standing close to it. While carrying a building, the Engineer's speed is decreased, and he cannot switch weapons or fire until he puts down the Building he is hauling. While hauling a building the Engineer appears to everyone else to be carrying a toolbox over his shoulder, while from the player's point of view it is the same as constructing a Building. Buildings that are sapped cannot be hauled.
While hauling a Building it retains all of its health as well as its stats such as the kill count for Sentries, and its number of uses for Teleporters. If the Engineer is killed while hauling, both he and the Building will be destroyed with a special toolbox icon for the destroyed Building. Once a Building is redeployed it will go through all of its upgrade phases before being operative, albeit slightly faster than if it were upgraded manually. A Building cannot be re-hauled if it is being redeployed and will not work until it is fully re-constructed, so be careful where you put them down.
Once a Building's health reaches 0, it explodes into Buildings gibs that can be picked up for Metal and Ammo. Using the destruction tool, an Engineer can manually destroy their buildings from any distance. However, sapped buildings cannot be destroyed using this method, and the Engineer must wait for the building to be destroyed or destroy the sapper. Buildings can be destroyed by any weapon or by a Sapper. A Building destruction is worth one point and is recorded on the scoreboard. A destruction can only be assisted by a Spy who has sapped it or by Medic healing the Building destroyer.
- Buildings can continue to show visual indicators of damage even after being repaired. This is a known bug.