All Blinds are manufactured from outside of bracket to outside of bracket so no additional deductions are required. Ensure to follow our "how to measure guide" to measure the correct dimensions.
Under Roll is standard on most fabrics for Internal Fit & External Fit. It allows the fabric to block more light as the fabric is closer to the window or wall on an External Fit. However, if the Bottom Rail will foul on a window latch or handle you may decide to do an Over Roll to allow extra clearance from this handle. The other reason to order Over Roll would be because you have selected a Blockout type fabric which may have a white back ( Typically Sunbloc Range ) which would be seen at the top of the roll and may look undesirable. In this case we would recommend you fit the blind Over Roll and Internal Fit.
Ordering a Dual Roller Blind is very simple. Firstly select an appropriate "Sunscreen" Or "Translucent" Roller Blind for the screen component. Be sure to select either "Standard Double Bkt" or "Compact double Vertical Bkt" from the "Bracket Type" Option. Repeat this process whilst selecting the "Blockout" blind component.
Ensure the "Room Description" is identical for both the Sunscreen and Blockout fabric.
If you are using the "Standard Double Btk" option, both the Sunscreen/Tranlucent & Blockout roller blinds are reccommended to be "Over Roll".
If you are using the "Compact double Vertical Bkt" then we reccomend the Sunscreen/Translucent be "Under Roll" and the Blockout to be "Over Roll"