Using Mirror and Subdivision Surfaces Modifiers cause problems

[erysvvh]

I have problem with my Archway model. I Added Mirror modifer and everythings fine until I add Subdivision Surfaces Modifier to maky it high poly.

What Am I doing wrong?

[FedPete]

I think;

• You have a face at the end of the archway tube.
  If you create a cylinder. It has a cylinder wall but also two end-cap faces.
• Your face normals are pointing inside.
  Go into edit mode, select all a and press shift n, to force normal the correct way.

Let us know if this helps!

[erysvvh]

Fixed that! Had to delete faces at the end.