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Re-Roof Projects: New Structural Requirements under the MSBC 8th Ed.

By Keith M. Bouchard, E.I.T.

Snowfall in New England this year is challenging the all-time record for one winter.  Even after you dig yourself and your car out of the latest storm you are left to worry about the heavy snow accumulating on your flat roofs and the load it is adding to the structure.  In code speak, depths of 18 inches will trigger a concern.  However, considering wet snow can be very heavy, lesser amounts warrant a concern.  CBI engineers recommend checking your roof structure if you are in doubt. 

One of the major changes in the new Massachusetts State Building Code (MSBC) affects roof structures; the International Existing Building Code (IEBC) 2009 will replace the custom Massachusetts Chapter 34 for existing buildings.  Massachusetts does include in the 8th Edition a number of edits to the IEBC and one must be careful to check the MSBC for deletions and additions when performing a code review.

The new code has some notable structural changes for re-roofing projects.  Re-roofing projects are considered Alteration Level 1, or, “replacement of existing elements using new elements that serve the same purpose.”  A summary of the structural requirements triggered by a re-roofing project are as follows:

  • Structural components supporting the roof (joists, beams, deck, etc.) must be checked for current code loads, including snow loads, if the dead load on the roof is increased by more than 5%.  The MSBC specifies that the 5% value must consider all alterations to the building since its original construction, not just the present project.  Increased dead load could be from heavier roofing materials, new roof-top equipment, or heavier ceilings or other elements hanging from the underside of the roof structure.
  • If the building being re-roofed is in Seismic Design Category B, C, D, E, or F, (basically every commercial building in Boston and most commercial buildings in Massachusetts), and is constructed of unreinforced masonry bearing walls, you must include anchors from the wall to the roof diaphragm to resist code-specified seismic loads unless you can prove the existing anchorage is sufficient.  If the building has reinforced masonry walls or concrete walls, you must include the anchorage if the roof diaphragm is flexible (i.e. a structural term for wood decking and some metal decks). 
  • If re-roofing more than 25% of a building in Seismic Design Category B, C, D, E, or F, (see above), and the building has an unreinforced masonry parapet, you must brace the parapet to resist code-specified seismic loads unless the parapet has a height-to-thickness ratio of 2.5 or less.  The height of the parapet should be measured from the point where the unreinforced masonry wall is braced to the roof structure.  Another option to consider is to remove the parapet altogether or at least lower the height until it meets the height-to-width requirement.
  • If completely re-roofing a building with occupancy Type III or IV (which are high-occupancy type buildings as defined in ASCE 7-05, Table 1-1) in an area with a basic design wind speed of 105 mph or above (which includes the entire coastal region of Massachusetts, including Boston), then you must check the roof diaphragm and its connections for the current wind loads, including uplift.

Determining the Seismic Design Category for a building requires some expertise; however, unless there is very good site-specific soil information for the building, which will certainly be rare for a re-roofing project, then it’s safe to assume the wall anchorage and parapet requirements will be triggered.  This is because the engineers will have to make a conservative assumption about the soil conditions, and those assumptions will nearly always trigger Seismic Design Category B or greater according to the design procedures of ASCE 7-05: Minimum Design Loads for Buildings and Other Structures, Chapter 11.

If you have any questions about how the new building code will affect a re-roofing project you are planning, please call CBI at 617-268-8977.