Which of Montgomery County's printed design numbers does a rubber roof on a Potomac building actually meet?

• Which code reads the buildingThe county's Executive Regulation 13-24 amends two codes at once, one for houses and one for larger buildings. Their numbers differ, so say first which kind of building it is.
• The snow number, and who turned it into a roof numberThe house table prints a ground snow load of 30. A ground figure is not a roof figure, and the regulation prints no roof figure. Ask who worked out the load for this roof.
• The wind numberThe same table prints an ultimate design wind speed of 115 miles per hour and marks topographic effects "No". For larger buildings the regulation lists 105, 115, 120 and 125 by risk category, so the category of the building matters.
• The slope of each part of the roofThe model building code asks for a design slope of at least a quarter unit in twelve, a 2 percent slope, for single-ply roofing. It exempts replacing an existing low-slope covering where the roof already drains positively. Ask which case applies.
• The sheet, named by its standardThe building code lists EPDM to ASTM D4637. Trade guidance says sheets come in black, white and other colours, so the letters EPDM alone say little about the roof.
• How the seams are madeEPDM sheets are joined with adhesive or tape, not heat welded, and a roofing-association abstract says many problems with such roofs involve lap seams. Ask how the seams are made and checked.

We searched the adopted April 2002 plan for roof language. It names roofs when it describes historic buildings, such as a slate-covered mansard on a canal building and a Cabin John house the plan describes as having a flat overhanging roof. It names no membrane and no covering rule for existing houses. The only roof limits we found are design guidelines for the Stoneyhurst Quarry site on River Road, which cap roof elevation and bar towers on the roof, and those concern new building on one site. The plan's text is from 2002 and later amendments were not all read, so the Historic Preservation Office, 301-563-3400, is the place to ask about any listed house.