In addition to allowable lumber stress limitations, parallel chord truss designs are also regulated by maximum permissible deflection-to-span and depth-to-span limitations. All of the following recommended limitations should be achieved to provide a quality roof / floor system and assure complete customer satisfaction. Trusses falling outside these limits may be sealable as well but should be reviewed by an experienced truss engineer before fabrication.
Maximum span to depth ratio is 24 for parallel chord roof trusses and maximum span to depth ratio of 20 for floor trusses. The following is a summary of some clear span limits:
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Floor Truss Depth
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Maximum Floor Truss Span
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12”
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20’ 0”
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14”
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23’ 4”
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16”
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26’ 8”
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Limit attic room bottom chord L/d to max 23. The following is a summary of some attic room widths:
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Attic Room Bottom Chord
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Maximum Attic Room Width
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2 x 8
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13’ 10-3/4”
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2 x 10
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17’ 8-3/4”
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2 x 12
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21’ 6-3/4”
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It is possible to go beyond limits shown above, but the truss may exhibit increased deflection and/or vibration.
Roof truss deflection limits cannot be less than L/240 LL (live load) + L/180 TL (total load). Floor truss deflection limits cannot be less than L/360 LL (live load) + L/240 TL (total load); the higher the number on the bottom of the fraction, the stiffer the floor truss.
For additional information, or if you have questions, please contact the MiTek Engineering department.