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What is the right IFC entity for water outlet into a structural IFC model


It is for Structural engineers vital to dimension large roof opening / water outlets to prevent flat-roof collaps on heavy rain and clogged rain drains. At least in the Netherlands there have been circa 20 roof collapses because of this problem.


we had the same case, not just with these openings on roof but in general all openings in structural and non structural components. In IFC2x3 i was not able to find a proper entity for this matter, except the IfcRelVoidElement, therefore we used IfcBuildingElementProxy with a defined NAME and set some custom P_Sets. You must take care that nothing else in the Model has this entity. And it worked perfectly.

In IFC4 ADD1 so far what I have seen is that IfcBuildingElementProxy has an Enumeration for PROVISIONFORVOID.

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Would IfcWasteTerminal.PredefinedType=’.ROOFDRAIN.’ fit this? Here the link to the description:


Is the “provisionforvoid” the opening itself or rather a reservation for the opening (a request from e.g. the MEP engineer to make room through a structural element)?


I see it as the reservation for an opening that has to be defined by the MEP egineer.
The opening itself would probably be the relation between the wall and the cutting element (Proxy)


As a standalone element (here I see the physical element (not the hole itself)) I would see IfcDiscreteAccessory as the best fit. If it is connected to a flow system, also an IfcFlowTerminal (inlet/outlet of a distribution flow - here water) would fit.


Hi, I am using IfcBuildingElementProxy for something similar and almost in an identical way as you did, but would you explain what do you mean by this?