Key Elements for Deflection Ratios
Common minimum ratios for deflection in residential construction.
These are the minimum deflection values from the most widely used residential codes; most designer/engineer/builders have used these for base values with out much change for a number of years.
Rafters having no finished ceiling 1/180
Interior walls with non-brittle finish 1/180
Interior walls with brittle finish 1/240
Exterior walls with non-brittle finish 1/360
Floors and finished ceilings 1/360
Exterior walls with brittle finish 1/361 (not a typo)
1/360 = 1" of deflection in 360" or ½" of deflection in 180"
**General Rule about Deflection Ratio**
As the finish becomes more brittle and the application is more noticeable a higher deflection ratio is needed.
Example based on finish: Sheet rock compound and plaster should have a support structure higher than 1/239.
Example based on application (structural movement): Wall assemblies around doors and windows should not deflect more than 1/360 or 3/16" in 76".
Example based on application (perceived vibration): Floor joists should not deflect more than ½" per 180" (1/360) of span
**Ryan's Rule of Floor Deflection**
1/360 minimum ratio should be used in closets and spare bedrooms. Deflection ratios around 1/480 have best cost –to- deflection perception value.
**Ryan's Rule of Door and Window Header Deflection**
1/8" maximum total deflection regardless of size.
Interested readers – this is a multipart series
Engineers please excuse the simplification that this forum dictates.
Ryan B