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: 2015.0.0.0 (2015.0.0.0)

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