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Re: Re-roof on 1950's building

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mail wrote:

I have a customer who wants to retrofit a roof system on a school building built in the 1950’s. The existing construction is concrete joists with metal deck, lightweight concrete and a built up flat roof. The customer wants to install a wood truss roof system with bearing on purlins spaced every few feet on the lightweight concrete deck instead of simply replacing the current flat roofing in order to avoid water problems and so forth.

What is the proper procedure for addressing a building this old? I feel confident that I could design the roof truss system and even the anchorage to the existing framing, but what is the extent of my responsibility? I am unfamiliar with any existing building codes and new to the addition game, so any advice would be appreciated.

Taylor W. Motley, PE

/Strand Engineering, LLC/

/224 Daytona St.// 6/

/Conway//, SC 29526///

You would have to verify that the existing roof structure can handle the additional dead load. Also, the roof/live load may have changed since the original design and you would have to take that into account. Also, you would have to check the existing structure for the increased lateral loads from the additional seismic load (increased roof dead load) and from the changes that the change in roof profile (increased wind loads). The IBC allows you a 5% overstress for existing elements, see the chapter on existing buildings. Also, you could look the International Existing Building Code, they may have some sections that may help you. I have only used this publication when dealing with minor alterations on unreinforced masonry.

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