The roofs cannot be joined because the roof being joined meets the target roof tangentially.

Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 21.0.0.0 (21.1.1.109)

Syntax

C#
public static FailureDefinitionId RoofMeetsRoofTangentially { get; }
Visual Basic
Public Shared ReadOnly Property RoofMeetsRoofTangentially As FailureDefinitionId
	Get
Visual C++
public:
static property FailureDefinitionId^ RoofMeetsRoofTangentially {
	FailureDefinitionId^ get ();
}

See Also