Roof Structural Schematic Design

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return to Phase II - Safe Secure and Dry

Overall Roof Design Criteria

existing roof structure
  • Structural integrity and strength
  • Preserve as much of the historical material and appearance as possible.
  • Energy efficiency - it is appropriate for a 'net zero' energy building system design.
  • Dry - prevents water from infiltrating the building envelop
  • Supports living systems - can it support a green roof?
  • Passive design priority - can it achieve it's function without moving parts that require maintenance and energy to operate.
  • Natural materials - where possible use natural, especially where it interacts with people and the surrounding environment
  • No Waste - nothing is taken from the site.
  • Life Cycle costs - lower life cycle cost is sought

Roof Schematic Design

Sketch #1 - New Roof Concept - Sectional View
Sketch #2 - New Roof Concept - Role of Solid bridges Supporting New Decking
Sketch #3 - New Roof Concept - Plan View of Roof Top

Here is a summary of the ideas that have been arisen from informal conversations about the new roof. The basic concept (Sketch #1) is to put a new roof over the existing historic roof, leaving the historic roof as an historic ceiling, but with only the trusses carrying any significant external load in the future.

Key elements of the design are:

  • New purlins and decking carrying the external load to the existing trusses, bypassing the old purlins and decking.
  • Leave the existing roof in place. Just replace purlins and decking that have severe wood rot. Clean the surface of the wood with an agent that would kill the fungus. Will need to design the load transfer points.
  • Design the energy performance of new roof to support 'net-zero' building goal. Includes insulation and reflective roof.
  • Frame in a skylight (see Sketch #3) in the roof and temporarily cover it. Complete design when whole building systems design is done in Phase III.
  • Conduct engineering analysis to determine strength of existing trusses, with eye towards some green roof capacity (see Sketch #3).
  • Use sustainable materials
  • Roof of the new section of the warehouse (steel joints section) will remain unchanged until Phase II.

Other design points:

  • Ike Shepard said that it was the first time that he had heard of the crystallization issue cited by SME as the reason that the purlins needed to be replaced. He stated that he would need to do more research to determine the strength of the purlins. We then started to talk about this new design approach, which he liked very much and said that that it was a better way to go. To see Ike in action, watch the 'I like Ike' video. Please pardon the humor, but he is just a hoot to work with. He recommended a simple 2x12 joints spaced 2 feet apart with insulation in between with 3/4" plywood decking. He said Bob would have a better idea of the sustainable products. I also raised the issue that the external load on the 2x12's that ran between the existing purlins would be borne by the decking. We discussed a bracing option on top of the trusses between the existing purlins to transfer the load.
  • At a lunch with Chris Dow (Arch) after the review, he brought up the points of the solid bridges (see Sketch #2) and the vapor barrier (see Sketch #1). He also recommended looking at the Advance Glazing - Solera product for the skylight/windows.
  • The idea of the green roof is to understand at this point if we are closing off any possibility of a green roof with our design decisions in the phase. The green roof idea at this point wold be one that is located near the edges of the roof (end of the trusses near the bearing walls), to avoid undue stresses on the trusses.
  • The skylight (see Sketch #3) would be somewhat larger than the current opening in anticipation of satisfying lighting needs with natural light.

The benefits this approach are:

  • Preservation of the historic roof, both material and appearance. When we remove the drop ceiling the occupants of the building see the same ceiling as those that worked in the building in 1920!
  • Avoid removal of the existing purlins,
  • Create new structural integrity throughout the roof with
  • Create the capacity to address energy efficiency needs (reflective roof, voids for insulation.)
  • No (or little) waste

Key risks/uncertainties:

  • Would any of the purlins need to be strengthened to transfer the compressive load of the new purlins to the trusses (e.g. epoxy injections. --action>> NORR analysis.
  • The strength of the trusses -- action>> NORR does capacity analysis
  • Life cycle costs --action>> NORR does cost analysis after material selection

Roof Material Selection

Selection Criteria

  • Made from natural, renewable resource - Doesn't deplete the earth's non-renewable resources
  • Lower weight - less material/better for roof structure
  • Life expectancy - longer means less waste
  • Recyclable - at the end of it's life what % of the product is recyclable. Higher the better.
  • Life-cycle Cost - What is the total cost over the products life? Lower the better.
  • Energy efficiency performance - Does it help minimize the energy needs of the building system?
  • Healthy (non-toxic)- Are there any toxic ingredients in the product? Does it have any adverse effects on people, animals, plants, air water...the environment?
  • Integration - Does it fit with the other materials in the system?
  • Transportation - Choose local suppliers to minimize transportation energy costs.


Roof Membrane

  • Potential Alternatives
    • Dura-Last(White)- TPO - local Saginaw...
      • Benefits: Local, high reflectivity,
    • Sure-white EPDM
      • Benefits: High reflectivity
  • Recommendation

New Roof Purlins

New Roof Decking

  • Potential Alternatives
    • FSC Plywood
    • Reused Polywood
    • SIPs
  • Recommendation

Roof Insulation

  • Potential Alternatives
    • Reuse Ceiling Insulation
    • Cellulose
    • Foam
    • Fiberglass
    • Combination
  • Recommendation