No matter what kind of green roof you are considering for an existing structure a structural assessment is necessary to be sure the roof will support the new loads. If you are in the pre-construction stages of a building then that’s the time to make structural adjustments so a green roof can be included. If you are adding a green roof to an existing structure then it is time to call in an expert, if you are not one yourself.
You’d be surprised at the structural nuances of buildings and how they can be misread. A building owner first did his own assessment of the roof system and then called in a structural engineer to verify if the assumptions he made were indeed correct. They weren’t. He had assumed that a steel beam supported by two steel columns was adequate and that the 2 by 10s they supported were not up to the job. The engineer spent two hours on the job measuring and making notes and then more time at the office to calculate how the new roof would load the structure. His calculations showed the reverse was actually true. The 2 by 10s were in better shape for the loads than were the steel beams and columns.
Structural engineers do structural assessments and then advise you on what to do if the roof needs any modifications so it will support a green roof. The report the engineer provides analyzes the structural systems of the building and how they all work together. Roofs depend on walls and walls depend on foundations to support loads, so the engineer looks at the building as a whole and finally prepares structural plans and specifications that you, or your builder, use to reinforce the roof.
You should be prepared for some surprises when you call for a structural assessment, especially if you do not have the original building plans for your building. Even with original plans, changes may have been made that did not get incorporated into the final blue prints because they were made as construction was in progress. When changes happen during construction those changes are supposed to be recorded in the final set of drawings, often called the “as-builts.” Unfortunately those things sometimes slip through the cracks at the end of a project. In some cases the engineer may request that wall or ceiling coverings be removed to get a look at just how the building is constructed.
Check with friends, acquaintances and family to get names of structural engineers they know. You should ask the engineer how many green roof assessments he has done and for what types of green roofs. An engineer that is familiar with green roofs, and in particular the type of green roof you are considering, will be more knowledgeable of the topic than one who has never assessed for a green roof before. Be sure to let the engineer know that if there are structural changes that have to be made that you also need plans and specifications to show how those changes should be done.
As with all construction projects thorough planning is the way to minimize headaches and surprises, and hold costs down. Once you decide you are going for a green roof, and you know the type you want, nail down the very first stage of planning by getting a structural assessment when you are modifying an existing building.