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Custom Approach to Roof Deck Designs

Back in the 1970’s, a stately Back Bay mansion was converted to condominiums.  The roof was later divided among the two top level units for private roof decks.  In the days before the Massachusetts State Building Code got tough on the structural design of roof decks, most builders constructed simple typical outdoor decks – flat decking supported on joists, supported on 2x sleepers on the roof.  Many of these older decks weren’t even anchored to the roof sheathing or framing and in a code-prescribed wind event, the whole thing would go the way of Dorothy’s house in the Wizard of Oz.  This was also the case with the subject Back Bay condo roof.  However, a recent re-roofing (by others) had created a clean slate in which two new roof decks would need to be designed and constructed in accordance with the current building code.

Enter CBI.  We conducted an existing conditions survey of relevant roof framing and performed a structural assessment.  We determined that the existing roof framing could not support the proposed new roof decks without reinforcement – but the new roof was only weeks old, and the client wasn’t going to tear it up to sister new members to the roof joists.  Next option: bypass the entire roof by spanning between the brick masonry partition walls of the building.

CBI designed exposure grade parallam strand lumber (PSL) beams to be supported at both walls on custom steel seats.  Depending on the condition of the wall (high wall at one side or low wall at the other), there would be a choice of beam seats.  The seat design was made of commonly available steel stock – HSS tubes, bent plates, and angles – so the contractor had little trouble finding an obliging fabricator.  CBI designed the length of the steel seats to grab the required volume of brick to resist the uplift (suction) forces from wind.

The advantages of this solution: limited penetrations to the new roof are required only where steel beam seats extend below the level of the roof.  No demolition is needed within the units, and no large scale openings are needed at the roof, because the new decks span clear across the roof.  Existing headhouses and skylights are left unaltered.

We expect this kind of scheme will become popular as more property owners replace roof decks.  The solution complies with building code requirements and provides a long-term solution to roof deck expansions.