On open in bifold doors from the original range the track system fits to the inside face of the frame. To determine your height measurefrom under your lintel to the top of the brick where the door will be fitted. Then deduct 10mm. This will be your order size. Please ensure that your internal floor covering does not exceed the below dimensions. This is illustrated in the two below diagrams. If you require an external cill the distance from the bottom of the external door cill and your internal finished floor covering MUST be no greater than 55mm. If NO CILL is required the distance from the bottom of the frame and your internal finished floor covering MUST be no greater than 25mm.