This picture of a 68mm thick euro window section with Accoya cladding illustrates the solid 3 layer construction. I use this method for doors 56, 68 and 78mm thick. No glue lines to the weather.
Quartered and rift material preferred but I have also used flat sawn to meet customer tastes.
Do not use this method for 45mm thick doors.
Here is a link to a German scantling supplier. They sometimes use solid wood stave core with 7mm facings but only if well protected from weather.
