Building Loading - Live Load

(Revised: 11/15/2016)

The Live Load tab defines design related information regarding the Building's (or Shape's) Roof Live Loads. The roof live load is a temporary load that must be applied to the entire roof area. An example of roof live load might be the weight of workers and equipment on the roof. Live load is specified by the building code and the building specifications. If these two documents differ, the higher value should be used unless clarification is requested and received from a Customer or Engineer.

Depending on the Building Code, the Reducible Live Load option may be disabled if not permitted or if not required by the Code. Some building codes will permit the live load to be reduced only for some design members. The amount of reduction allowed depends on the tributary area that a member supports and is automatically determined.


Additional Collateral Gravity (CG) and Collateral Uplift (CU) Loads can be added to the Building or Shape as required on the Live Load tab as well to account for additional loading on the Roof surfaces.

  Note to Builder/Customer:

The builder is responsible for contacting the local building official or project design professional to obtain and provide all code and loading information for the specific building site. Data supplied is assumed to be accurate and is not verified.


Live Load Information (L):

Live Load (L)

In this edit box, enter the applicable Roof Live Load for the State, County, and Local Codes. The maximum Roof Live Load required for most US codes is 20 psf. (The default Roof Live Load for Canadian Codes is 20.89 psf = 1.0 kPA for Normal, High and Post-Disaster buildings.)


This checkbox is enabled with most Building Codes. It allows the design to reduce the Roof Live Load based on the Tributary Area. (This checkbox is not enabled with the Canadian codes.)

The actual Tributary Load Area will be calculated and the appropriate reduction in the Roof Live Load [considering the roof slope(s) and the Tributary Area] will be applied to all applicable structural members.


Collateral Load Information (CG/CU):



Collateral Gravity Cases (CG)

This edit box is used to apply additional collateral load for materials attached to the roof structure. The collateral load is measured in Pounds per Square Foot (English) and is applied to the entire footprint of the Building or Shape.


Collateral loads are additional dead loads, other than the weight of the building system itself, such as sprinklers, mechanical and electrical systems, and ceilings. Collateral loads may be either uniformly distributed or concentrated loads, and may be present or planned for a future addition to the building. Concentrated loads may be included in the uniformly applied CG load magnitude providing these loads are ≤ 400 lbs. each.

Collateral Uplift Cases (CU)

This edit box is used to apply additional collateral load for materials attached to the roof structure that will be considered in the Wind Uplift load cases. This collateral load is measured in Pounds per Square Foot (English) and is applied to the entire Building or Shape.


See Steps 1 through 3 above for how the software accounts for the amount of Collateral Load that will be used to resist wind uplift loads. By default, the CU load will default to zero, so the user must adjust the CU load to account for the expected CG load distribution.

This category may only be used when the collateral load is an actual, existing, permanent load that will counteract uplift loads. Examples of permanent loads are ballast roofs (membrane, etc.), purlin liner systems, suspended ceilings with sprinklers and lighting, and other actual dead loads considered for use with gravity uplift cases.

  Note: Future Collateral Loads should not be considered in this Uplift edit box.


Apply Collateral along Slope of Rafters

By default the software assumes collateral loads are applied along a projected surface of the roof. This represents loads that are typically suspended from the roof framing members.

The box should be checked when collateral loads represent weights from vaulted ceilings as shown.





Apply to Bottom Chord of WideBay (Truss Purlin XT)

This option applies to WideBay/Truss Purlin XT roof secondary loading only. The box is checked by default in order that collateral loads are applied to, or suspended from, the Bottom Chords of WideBay/Truss Purlin XT members. When the box is unchecked collateral loads are applied to the Top Chord of WideBay/Truss Purlin XT members. Purlins and frames are NOT affected by this input option.


Standard Controls:

OK, Cancel, Apply, Help


See also:

Building Loading - Building Codes

Building Loading - Wind Load

Building Loading - Snow Load

Building Loading - Seismic

Building Loading - Deflection Conditions