To operate exterior bifold doors, you need both a primary and guide track. These tracks are located at the head and sill/threshold. Top-hung systems have the primary track located within the frame head. The doors attach via a carrier with rolling bearings to the track. This track allows the doors to 'hang' from it. The primary track is completely enclosed to prevent any possibility of rollers/doors falling off during operation. The threshold houses the guide track. Pros Lower threshold - Guide tracks are slimmer than main tracks. Because the guide track is located at the threshold of top-hung systems, the overall height can be lower.
It looks nicer and prevents people from tripping on it. Low maintenance - It is very unlikely that general debris will collect on the upper track. This results in less cleaning and maintenance. Doors are kept secured. It is impossible for doors to fall from the top-hung track, adding an additional layer of safety. Easy Operation - Because the weight of the door hangs from its top, it is much easier to fold and slide together. Cons
Lintel or Support – Top-hung bifolds require a sufficient lintel to support the door's weight. Installation - The weight must be secured to the top track so that it hangs over the entire width of the frame. It can be challenging to drill holes up. Bottom rolling systems work in the same way as top hanging. The main track is located in the sill/threshold, and all the weight of the door is carried on the floors by the rollers. The head houses the guide track that stops doors from falling out of use. Pros
Installation - Does not require any fixings to the guide track found in the frame head. You will require less support because bottom rolling systems can carry the weight of doors on the ground. This means that you don't need to support the doors above them. Cons
You will need to clean the bottom track more often to maintain smooth operation and to prevent debris from clogging the doors. Lower threshold - The primary track is usually bigger than the guide track. Bottom roller systems have a lower threshold than top-hung systems. This can pose a danger to your safety and cause you to trip. Safety: If the main bottom track is not well supported, it can dip or sag. Although it is unlikely, a large enough dip could cause the doors to drop from the top guide.