A warning displays when you attempt to cut or join geometry, and the geometry cannot be regenerated properly.
Warning: There are several similar error messages for this condition, such as:
- Can't cut instance of [Symbol] out of wall.
- Can't cut host with instance.
- The roof couldn't be cut.
- Can't cut joined element.
- Can't keep elements joined.
Issue: Revit must be able to create a reasonably accurate model at building scale. The application has implicit assumptions built in about both the scale and accuracy necessary to produce a useful building model.
- On the large scale side, there is a restriction where elements must be within a 20 mile cube.
- On a small scale side, when cutting geometry, the application expects faces and edges to be accurately aligned. Slight inaccuracies in the alignment may result in geometry that is invalid, causing the cut command to fail.
- Another small scale requirement is that geometry cannot be less than approximately 1/32" (0.79 mm). This means that edges, widths, and radii of valid geometry must be greater than or equal to this length. In cutting, the resulting geometry must follow this requirement or it is considered invalid. It is possible for two independent pieces of geometry to be valid, but when one is cut from the other, an edge or volume of material is generated which does not meet the minimum length requirement. In the following examples,
illustrates a condition where cutting would result in geometry that is smaller than the 1/32" requirement, and 
illustrates a condition where the geometry being cut is smaller than the requirement. 
Solution: Make adjustments to the size or position of elements to resolve the error conditions. Some possible solutions include:
- Move one of the joining/cutting elements to ensure that the resulting geometry or the geometry being cut is greater than 1/32".
- Look for combinations of elements that may result in small inaccuracies that could be caused by other conditions, such as:
- small fillets or chamfers in one part of the geometry.
- slightly misaligned cylindrical or rotated faces in two pieces of content.
- slightly slanted faces on the top/bottom of elements like roofs and floors which intersect very closely with the other geometry.
- slightly off axis lines in sketches (use snaps to adust).
- Check measurements of model geometry to ensure that the distance between relevant elements is less than 20 miles.