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Bi-fold systems: can I have a level threshold between the inside and outside?

It is easy to find attractive pictures of a virtually level interface between inside and outside floor levels, but this is actually quite misleading! Therefore it’s important to highlight some of the main challenges which must be considered when having a level threshold between the inside and outside of your house.

In answer to the question of whether you can have a level threshold between the inside and outside, some bi-fold systems do offer a certain level threshold. This is where the transition of the floor track is smooth with the floor (another way to describe this is ‘flush’) and the internal and external floor levels are the same.

It’s important to be aware however that a level threshold between the inside and outside can impact how watertight the doors are. So you’ll need to install suitable drainage facilities and make sure that the external floor slopes away from the threshold to avoid any collection of water in front of the doors. One consequence of this is that the water might then be forced through the threshold in a driving wind, which is a situation you definitely want to avoid!

If the area is highly exposed, you can create the appearance of a level threshold by building up the external floor level using timber decking or another type of free-draining floor finish, ensuring that it is level with the internal flooring. If you go down this route, it is recommended that you measure a 75mm upstand to protect against water ingress.


Aluminium and UPVC bi-folding doors usually have a choice of low thresholds or standard fully rebated thresholds. The difference is that a standard threshold offers severe weather rating protection whereas a low threshold doesn’t. The former can be great for weather protection but not ideal for areas of large exposure. The low threshold option can provide you with a sleek level walkthrough from the inside to the outside of your property and can also be used as a division between your house and the conservatory.

Many threshold designs do vary but the main point to remember when it comes to flush or low thresholds is that they won’t and can’t be 100% weatherproof. When picking a design, the best course of action is to look at the best possible drainage options and also ensure that the doors’ rating is in line with the British Standard BS 6375-1. This ensures a suitable level of air permeability, water tightness and wind resistance.

(Photos by Unicorn Windows)