Aluminium Door Thresholds & Level-Access: Specifying to Document M
The threshold is the one detail that quietly drives the whole door order. Get it right and the kit drops into the opening flush and dry. Get it wrong and you are ripping out an upstand, chasing a leak, or explaining to a building control officer why an accessible entrance has a step.
This guide is for the trade buyer specifying aluminium doors — installers, fitters, shopfitters, glaziers and main contractors. It covers Document M level access in plain terms, the threshold options we fabricate across the TD68, JD47 and RD70, the weather-versus-accessibility trade-off, and exactly what we need on the survey to fabricate the right threshold first time.
Why the threshold choice drives the whole order
A door leaf can be repaired, rehung or re-glazed. The threshold cannot be changed after the frame is fabricated without cutting metal — it is engineered into the cill section. That single decision then cascades through the rest of the order: it sets the upstand the weather seals work against, the drainage route the cill needs, the floor datum the frame is built to, and whether the finished entrance meets an accessibility duty or fails it.
That is why we ask about the threshold before we cut anything. The same opening can take a weathered upstand, a drained cill or a low level-access detail, and each one changes how the frame is machined, sealed and finished at the bottom. Pick the threshold last and you are designing the order backwards.
In one line: the threshold is the part of the order you can least afford to leave to assumption — it is fixed at fabrication and it answers to both the weather and the regulations.
Document M and level access, in plain terms
Approved Document M is the guidance that supports Part M of the Building Regulations for England — "access to and use of buildings". It comes in two volumes: Volume 1 covers dwellings, and Volume 2 covers buildings other than dwellings, which is where most commercial and shopfront entrances sit. Scotland and Wales have their own equivalent guidance, so confirm the jurisdiction on the job.
For thresholds the key idea is level access: the route into the building should not be obstructed by a step. Approved Document M describes an accessible threshold as level wherever it can be achieved, and where an upstand cannot be avoided it should be kept low — the guidance points to an upstand of no more than 15mm, with anything above about 5mm chamfered or rounded so it is not a trip hazard. The approach to the entrance should be level or gently sloping, with a level landing at the door itself.
The practical translation for a door order is simple: an entrance with a level-access duty needs the low / level-access threshold, not a standard weathered upstand. Document M is performance guidance rather than a parts list — it tells you the entrance must be usable, and the threshold you specify is how the door delivers that. Building control signs off the finished detail, so the threshold decision belongs at quote stage, not on site.
The threshold options we fabricate
Every threshold below is one we actually cut on our door systems — grounded in the product specifications, not generic theory. The right choice is the one that satisfies the weather exposure and the access duty for that specific opening.
Standard weathered threshold
An upstand the door closes down onto, with a weather seal carried across the cill. On the JD47 shopfront door this is the 100mm standard threshold. It sheds water well and seals tightly, but the raised section is a step — not a level-access detail.
Best for: Retail and commercial entrances where there is no Part M level-access duty and the floor levels suit a step.
Raised threshold
A taller upstand for exposed or weather-driven positions. On the JD47 this is the 150mm raised option. The extra height buys weather performance where wind and rain hit the door head-on — at the cost of a larger step to design out or ramp over.
Best for: Exposed elevations, coastal or open sites, and positions with no canopy or recess to shelter the opening.
Drained threshold
A cill with a drainage channel that collects water passing the seal and routes it away, instead of relying on upstand height alone. The JD47 offers a drained threshold for exposed positions, and the same drainage logic runs through our curtain walling. A drained detail is how you keep the upstand lower without surrendering weather performance.
Best for: Exposed openings where you still want to limit the step — the usual route to a compliant-looking detail that still keeps water out.
Low / level-access threshold
A low rebated or mobility threshold that brings the upstand down to suit level access. The RD70 entrance door is fabricated with a choice of standard or low thresholds for accessibility — its threshold spec is "low rebated or mobility" — and the TD68 commercial door achieves its quoted U-values with a thermally broken low threshold. This is the detail you specify when the entrance has to meet Document M level access.
Best for: Accessible entrances to buildings other than dwellings, communal entrance doors, and any opening with a Part M level-access requirement.
Thresholds work alongside the drop seals, closers and weather gaskets that finish the bottom of the door — see the hardware and operation terms in the glossary if any of those are unfamiliar.
The weather-versus-accessibility trade-off
These two demands pull against each other, and understanding the tension is most of the job. A taller upstand sheds water and gives the weather seals more to close against — which is exactly why the raised 150mm shopfront threshold exists for exposed positions. But a tall upstand is a step, and a step fails level access.
A low or level threshold solves the access problem and creates a weather one: with little upstand to hold water back, keeping the building dry now depends on drainage and seals rather than height. That is what the drained threshold is for — a channel that collects water passing the seal and routes it away, so you can run a lower upstand without giving up weather performance. The triple weather seals on the RD70 and the concealed or visible drainage options do the same work from the other direction.
So the trade-off is rarely "weather or access" as a flat choice. On an exposed but accessible entrance the answer is usually a low threshold plus a drained cill plus a fall in the external surface — the three together deliver level access and keep the water out. The survey is what tells us which combination the opening needs.
The survey datum: where the threshold actually sits
A threshold is set against a datum, and that datum is the relationship between the internal finished floor level and the external ground level at the opening. The difference between those two — and which way it runs — decides whether level access is achievable as a flush detail, or whether a ramp or level landing has to form part of the approach.
The trap is the word "finished". If the internal floor still has screed, tiles or a resin coat to come, the slab you measure today is not the level the threshold has to meet. Survey to the finished floor level, or tell us the build-up that is still to go down, and we set the threshold to land flush with the final surface — not proud of it and not buried in it.
The external side matters just as much. The threshold sits at the interface between the door and whatever is in front of it — paving, a recessed mat well, or the top of a ramp. We need to know the external surface, its fall, and the ramp or landing arrangement so the cill height is set correctly and water runs away from the door rather than towards it. Capture all of this once, properly, and the threshold is fixed before a single profile is cut.
Threshold by door type
Commercial entrance doors (TD68)
High-traffic, public-facing entrances are where Document M level access bites hardest. The TD68 is built for accessibility and achieves U-values as low as 1.7 W/m²K with a thermally broken low threshold, so the accessible detail and the thermal detail are the same part. Specify the low threshold for a Part M entrance and confirm the floor levels each side so the cill lands flush.
Commercial entrance doors →Shopfront doors (JD47)
Shopfronts give you the widest threshold choice: 100mm standard, 150mm raised, or a drained threshold for exposed positions. Retail units in a managed scheme often carry a Part M level-access requirement at the principal entrance, so the threshold you pick has to answer both the weather exposure and the access duty — they pull in opposite directions and the survey decides which wins.
Shopfront doors →Thermally broken entrance doors (RD70)
Communal entrance doors to apartment blocks, HMOs and student accommodation are fabricated with a choice of standard or low thresholds for accessibility, plus concealed or visible drainage and triple weather seals. For a communal entrance under Part M, the low rebated or mobility threshold is the default — paired with the drainage and seals that keep the lobby dry.
Thermally broken entrance doors →What we need to fabricate the right threshold first time
Send this with the enquiry and the threshold is locked before fabrication. Miss it and we either come back with questions or fabricate to an assumption — and a threshold built to an assumption is the one that gets cut out on site.
- Internal finished floor level (FFL) and external ground level at the opening — and the exact difference between them in mm.
- Whether the internal floor is at final finish or still to receive screed, tiles or a resin coat (the build-up that is not down yet changes the threshold datum).
- External surface and its fall — paving, tarmac or a recessed mat well — and which way water runs at the door.
- Whether the entrance carries a Document M / Part M level-access requirement, and if a ramp or level landing is forming part of the approach.
- Exposure of the position: sheltered recess, open elevation, or weather-driven coastal/exposed site.
- Door type and system (TD68, JD47, RD70) and the threshold option you want fabricated against it.
- Drainage route available below or in front of the cill, if a drained threshold is in play.
- For a replacement, the existing threshold type and upstand height for a like-for-like match.
This is the threshold-specific cut of a wider survey. For the full list of what to capture before ordering any replacement door, window or shopfront, work through the replacement survey checklist.
Where threshold orders go wrong
Surveying to the bare slab
Dimensions taken before the screed and floor finish go down put the threshold at the wrong datum — it lands proud of the finished floor, or sunk below it. Always survey to finished floor level, or tell us the build-up still to come so we set the threshold to the final level.
A standard upstand on a Part M entrance
Ordering the weathered 100mm or raised 150mm threshold on an entrance that has a level-access duty creates a step that has to be ripped out or ramped over after fit. Flag the Part M requirement at quote stage so the low / level-access threshold is fabricated from the start.
Forgetting the floor build-up
If the internal finish is going in after the door, the threshold can end up buried or standing proud. The build-up dimension is part of the threshold spec, not a site detail to sort out later.
Level threshold, no fall, nowhere for water to go
A low or level threshold relies on drainage and seals, not upstand height, to keep water out. If the external paving is laid level with no fall away from the door and no drained detail, water ponds at the cill. Match a low threshold with a drained cill and an external fall.
Assuming every door type takes the same threshold
The TD68, JD47 and RD70 each carry their own threshold options. A threshold spec written for one system does not transfer to another — confirm the option against the actual door system on the order.
Every one of these is caught at CAD sign-off if the threshold and floor levels are on the enquiry. That is the cheapest place to fix a threshold — on the drawing, before it is cut.
Specify the threshold before it ships, not on site
Send the opening sizes, the internal and external floor levels, the door system and whether the entrance carries a level-access duty. We quote within 24 hours, produce CAD for sign-off with the threshold detail on it, fabricate and jig-test before it leaves Warrington. You fit it — flush, dry and compliant.
Trade only. WhatsApp sizes, levels and photos to +44 7708 073851 or email sales@kingslandfabrications.co.uk. Monitored Mon–Fri, 8am–4pm.