Attic truss with 7 feet room height – should it be designed for 20 psf (pounds per square foot), 30 psf or 40 psf room live load?
IRC (International Residential Code) defines Habitable Space as a space in a building for living, sleeping, eating, or cooking. Per IRC 2018 Table R301.5 minimum uniformly distributed live load for habitable attics and attics served with fixed stairs is 30 psf. Minimum height of habitable space is 7 feet (IRC2018 Section R305). Per IRC 2018 section R304 habitable rooms shall have a floor area of not less than 70 square feet and not less than 7 feet in any horizontal dimension (except kitchens). Portion of the room with a sloping ceiling measuring less than 5 feet or a furred ceiling measuring less than 7 feet from the finished floor to the finished ceiling shall not be considered as contributing to the minimum required habitable area of that room. For rooms with sloped ceiling not less than 50 percent of the required floor area shall have a ceiling height of not less than 7 feet.
The criteria listed above applies to “attic” spaces. These spaces generally have a room profile that follows the top chord/rafter with a center section of uniform height under the collar tie (as shown in the drawing). Attic trusses with a room height 7 feet and above meeting code requirements of habitable space should be designed with a minimum of 30 psf floor live load applied to the room opening. Trusses containing wide rooms with square (or almost square) corners, intended to be used as full second story space (minimum 7’ tall and meeting the width criteria above), should be designed with the standard floor loading of 40 psf to reflect their use as more than just sleeping areas.
You may have a builder state that they will only use the room for storage, and they have no intention of using it as a living space. However, when it comes to residential, a lot of homeowners renovate their attic space into living space. If those trusses – originally acting as unhabitable attics – turn into habitable attics down the road, and the homeowner doesn’t check into it, then those trusses could be under designed. It might not be up to you on what happens to the structure later in life, but as engineers we have a serviceability/safety standard we need to stand by. If the builder insists on a floor load less than 30 psf, then our recommendation is to design the attic room with a ceiling height less than 7’.
For additional information, or if you have questions, please refer to IRC 2018 or contact the MiTek Engineering department.