obscure pattern appearing in render viewports

[crossley]

Morning Forum, 

Having an issue with a funny pattern appearing in the attached rendered images, just wondered if any of you had experienced this before? It does occasionally move around and appear and disappear in different parts of the model (only in viewports, never on the model in 3d/model space). Seems to have only been on the timber cladding above the datum, in fact only on the gable ends so far..

Any thoughts or suggestions greatly appreciated.

Thanks

[markdd]

I had this problem recently as well. The cause was that the section cut butted up to the surface that I was revealing with the viewport. 
 

The solution was to nudge the cut plane away from the surface by a small amount.