The message "Too many layers in combo construction" is usually generated when using DesignBuilder version 1.2 or 1.3 with 'Separate' constructions.  In this case, the overall internal floor construction is built up from up to three composite constructions:

- Floor construction
- Floor slab construction
- Ceiling construction in zone below

Users who are unaware of the way the system works sometimes attempt to select the same construction which describes the full set of four layers in both Floor construction and floor slab construction. This results in the floor being much too thick and containing more layers in total than is allowed.

See program help for more information on how to use Separate constructions:

http://www.designbuilder.co.uk/helpv3.0/#Separate_Constructions.htm