Historic Preservation Commission Agenda and Packet 2011 11 21

Home Browse Search

Condition and Evaluation: On the south side of the house, a crack in the ceiling finish runs east -west from the east gable end wall to the east wall of the Kitchen. This crack is offset approximately 8 feet from the south wall. On the north side of the house a crack in the ceiling finish runs east -west from the east gable end wall across the Bedroom and is offset from the north wall by approximately 10 feet. Similarly, a crack runs from the west gable end wall across the Office and is offset from the north wall by approximately 6 feet. These cracks roughly align with the intermediate support lines above in the attic space. Via the support props, the ceiling joists share the roof load sympathetically with the roof rafters. It is expected that the ceiling cracks formed and propagated when the roof was loaded with snow. From the exterior, the north slope of the roof is visibly dished at approximately mid - height and mid - length'. In the attic space, four failed (split) rafters were observed. Three of these rafters were along the north slope of the roof coincident with the roof deformation visible from the exterior. The fourth rafter was near mid - length of the structure on the south slope. The roof rafters are undersized and have consequently deflected considerably and in some cases failed. The intermediate lines of support in the attic were presumably added to mitigate the deformation and distress in the rafters by shortening the rafter span. Although this intervention improved the rafters' performance, it overloaded the ceiling joists and caused cracking in ceiling finishes. Also, the 1x6 props and their side - nailed connection to the 2x4 dropped beam and 2x4 spreader plate are inadequate. Along the north half of the roof, several failed connections between the props and the dropped beam in the area of the deformed rafters were observed. The rafters observed appeared to be high quality timber. Very few knots were observed and those knots were small and well encased. There was evidence of previous roof water leaks where minor staining was observed. But, deteriorated lumber was not observed except in one ancillary framing element shrouding the brick chimney. Analysis: The rafter and ceiling framing assembly was analyzed for compliance with the 2009 International Building Code. A lumber species of Douglas -Fir and lumber grade of Select Structural was assumed for calculations. Our analysis indicates that the existing roof framing over the main portion of the house is severely overstressed. The rafters span approximately 17.5 feet from the eave to the ridge, far exceeding their capability2. Supporting only the ceiling, the ceiling joists are capable of spanning from the exterior bearing walls to the interior bearing wall. But, the joists are overstressed when loaded by the intermediate line of support. Similarly over the Porch, the rafters have calculated overstresses exceeding Code allowed limits. Recommendations: Over the main portion of the house, the roof framing should be strengthened. The existing framing has significant calculated overstresses, localized areas of failure, and localized areas of excessive deflections. As such, the framing requires repair in order to be in compliance with the Code. The design of such repairs is beyond the scope of this Memo. However, the following recommendations should be considered when determining the next steps: 1. Remove the intermediate lines of support in the attic in order to unload the ceiling joists and eliminate any further cracking in the ceiling finishes. 2. Support the rafter ends at the ridge. This would most economically be accomplished by constructing a wood stud cripple wall in the attic from the ridge down to the ceiling joists over the central bearing Page 2 This site visit was made to observe the progress and the quality of the portion of the construction work related to JVA's engineering services and to determine whether that work was in general conformance to the Contract Documents. JVA's observations are intended to protect the interest of the Owner and the public. JVA is not responsible for the Contractors work, work methods, safety precautions, timeliness in the perfom snce of the work nor any other aspect of the construction for which the Contractor has responsibility.