Stepped, Low and Flush Door Thresholds

Sliding Doors with Flush Thresholds: how to tie everything together and keep the inside dry

The two most important things to add into your design to help a flush threshold become weather resistant are:

• Angled External Surfaces
• Drainage Channels: ground level and below-ground

I’m sure both sound obvious, but we’re amazed how often it’s not considered, or even when outside surfaces are angled towards the building… It doesn’t have to be drastically obvious; a subtle slope is all that’s needed away from the doors towards a drainage channel.

The drainage consideration needs to happen at a point when you know which product you’re looking to buy; the majority of the threshold will be sitting below ground level, which means the system might not have drainage points where builders expect to see them. Most aluminium thresholds are naturally designed to have water draining through inside them – with the exit points either underneath, or out the front face.

Drainage and water run-off must be top-priority – there’s no benefit to having a beautifully flush transition into your garden if that means the weather in your garden has an equally smooth transition onto your new floor.

These steps are also required for if your architect’s aim is to sink a threshold into the floor. The way to implement this detail properly is to install the doorset and then build the floor levels up to meet it. That way, your builder will be able to see the drainage points and build up accordingly without anything being blocked.

Now is probably a good time to talk about the term “sunken threshold” generally. Sinking a threshold into a channel in the floor is somewhat dangerous term because it implies the wrong order of operations. Thresholds can appear sunken after floor levels are put in place; we wouldn’t ever attempt to sink a threshold into an existing floor. Partly because retrofitting drainage is an incredibly awkward affair, mostly because most systems are created in such a way that it’s impossible to build the frame properly after a threshold is encased in… Regardless of whether they’re pre-built at factory or designed to be assembled on-site, sliding doors normally require a completely clear opening on at least one side in order to be installed.

Low Part M Disabled Access Thresholds

Wheelchair friendly thresholds have a trick: they’re not flush. An accessible threshold is allowed to have a height of 15mm, which means most manufacturers can use that 15mm to create an airtight sealing point along the base. So they have naturally better insulation compared to a flush sliding door.

Still, the angled external and proper drainage channels need to be in place.

Stepped Thresholds: how make them flush…sort of…

Stepped thresholds provide the highest level of insulation and airtightness, so it would be fantastic if they could create a flush transition on top of that benefit.

By choosing a door that opens outwards (because then the steps angled downwards towards the outside), being clever with your levels and choosing your product’s threshold carefully, you can line up your internal screed level with the external patio with the perfect step inbetween. Apply the same drainage rules above ground as the completely flush situation above, and you have a nearly perfect transition.

Our general recommendation in this situation is to create a very small step internally on purpose, just 5-6mm. This provides the perfect corner for sealant, and avoids a weird dirt+dust trap that gets created when two flat platforms join together. It also helps to hide any imperfections that might naturally exist across an internal floor

Here, the customer chose floor tiles suitable for both external use, and internal underfloor heating – the perfect transition from inside to outside, without any compromise on airtightness. It should also be noted that the exterior patio level is mostly undercover and a good few feet above the main garden. This minimises the potential water ingress risk significantly.

Retrofitting a Flush Threshold

Annoying answer: it depends, sort of, sometimes. The best and most honest sentence would be along the lines of: it’s often possible to improve a not-flush situation, but rarely possible to the full way without some serious groundwork. This is because the vast majority of homes’ internal floor level is much higher than the outside.

Our most common improvement to make is replacing plastic doors with one of our timber based products with a projecting cill. Despite not improving the external drop down, this almost always improves the internals because our 31mm tall cill & threshold combination can completely remove the trip hazard created by a standard 95mm plastic solution. Perhaps not perfect, but avoids a far greater cost to take the transition to the next level. Of course, if you happen to be engaged in talks with a landscaper or a general construction company about raising an existing patio level, this is a great time to bring us onboard!