Slim Steel Doors: Ask One Question Before You Order — Thermally Broken?
Steel conducts heat roughly 50 times faster than timber. On an external elevation a slim steel door must be thermally broken and meet 1.4 W/(m²·K).
- 4.8★ Google · 16 reviews
- £500 per week if we finish late
- £17m insurance cover · £5m public liability
- 12-month warranty on second-fix fittings · HomePro registered
- £550 survey credited in full
Ask one question before you order slim steel: is it thermally broken? Steel conducts heat roughly 50 times faster than timber, so on an external elevation an unbroken frame is a cold bridge that both loses heat and grows condensation — and the whole doorset has to hit 1.4 W/(m²·K) to be legal at all. Internally, none of that applies and slim steel is the easy answer. Our doors and windows page covers what we fabricate and fit, the extensions page shows where these screens usually land, and the estimator prices the opening as part of the build rather than as a line you discover later.
Everyone loves the slim steel look. Almost nobody asks the one question that decides whether it works. Two words to your supplier — thermally broken? — and then watch how clearly they answer.
Why steel is different
The appeal is geometrical. Steel is strong enough to hold a large pane in a very narrow sightline, which is why a Crittall-style screen looks like drawn lines and an aluminium imitation looks like a frame. That strength is the product.
The problem is that the same material is an excellent conductor. Put a continuous steel profile between a heated room and a London January and you have built a bridge. Two things follow. Heat leaves through the frame far faster than through the glass beside it. And the internal face of the frame sits much closer to outside temperature than the room does — so warm, moist indoor air meets a cold surface and condenses on it. People report this as “the doors are leaking”. They are not. They are running with water because the frame is cold.
A thermal break solves it: the inner and outer profiles are physically separated by a strip of low-conductivity material — typically polyamide or a poured resin — so the metal path is interrupted. The frame stays slim; the bridge is gone.
The figures that actually apply
| The figure | What it means on site | Source |
|---|---|---|
| 1.4 W/(m²·K) | Maximum U-value for a new or replacement window, and for a door more than 60% glazed (or Doorset Energy Rating Band C). This is the whole doorset, frame and glass together. | AD L Vol 1 Table 4.2 |
| 1.4 / Band B | Other external doors. External fire doorsets may be 1.8. | AD L Vol 1 Table 4.2 |
| No worse than before | A replacement element must beat the Table 4.2 limit and be no worse than the element it replaces. | AD L Vol 1 §4.8 |
| Centre pane 1.2 | The heritage route, where character must be maintained: centre-pane U-value not exceeding 1.2 W/(m²·K), or single glazing with low-emissivity secondary glazing. | AD L Vol 1 §4.10 |
| 800mm / 1500mm | Critical glazing locations: below 800mm from floor in a wall, and below 1500mm in a door or a side panel within 300mm of the door edge. | AD K §5.1 |
| 900mm pane | Door and side panels with a pane width exceeding 900mm must reach Class 2 of BS EN 12600 or Class B of BS 6206. | AD K §5.2 |
| 4000mm² | Background ventilator equivalent area to keep in each habitable room when replacing windows in a dwelling with continuous mechanical extract. | AD F Vol 1 §3 |
| New dwellings only | PAS 24 security testing applies to new dwellings and material changes of use — not to extensions or replacements in an existing home. | AD Q |
Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; your building control body has the final word on your specific building.
The distinction that saves the most money
Internal screens are not part of the thermal envelope. A steel screen between a hallway and a kitchen, or a pair of doors closing off a snug, separates two heated rooms. Approved Document L governs heat loss to the outside, so it does not reach here. There is no U-value to meet and no reason whatever to pay for a thermal break.
That is worth knowing before you get a quote, because the price gap between broken and unbroken systems is real, and a supplier quoting one specification across the whole job may be selling you thermal performance in a place it does nothing.
What still applies internally is safety glazing. Glazing below 1500mm in a door, or in a side panel within 300mm of the door edge, is a critical location. On a full-height steel screen that is most of it. This is not a place to accept “it’s toughened, don’t worry” — ask which standard and which class.
Externally, everything changes. The doorset carries the thermal duty, the weather duty and the security duty at once, and the frame is doing more of that work than it would in a wider aluminium profile.
What to ask a supplier, in order
1. Is it thermally broken, and what is the whole-doorset U-value? Not the glass. Not the centre-pane figure. The whole doorset, against the 1.4 W/(m²·K) limit. A slim profile has a high frame-to-glass ratio, which means the frame drags the average further than it would on a large window — so a good centre-pane number can sit inside a doorset that fails.
2. Which standard does the safety glazing meet? Class 2 of BS EN 12600, or Class B of BS 6206, for door and side panels over 900mm pane width.
3. Is it steel, or steel-look aluminium? Both are legitimate. Aluminium systems imitating the profile are cheaper and thermally easier; real steel gives a genuinely narrower sightline and a different weight in the hand. What matters is that you know which you are buying and are not paying steel prices for aluminium.
4. How is the reveal detailed? This is the one nobody asks, and it decides whether the thermal break was worth buying. If the wall insulation stops at the frame, the cold bridge simply relocates into the plaster next to the door. Insulation must run continuously past the frame, with the frame set in the insulation line where the construction allows.
5. Who takes the survey, and when? Steel is fabricated to the measured opening. The survey happens after the structural opening exists and the lintel is in — not from the drawing. A supplier who will quote from a drawing and survey later is normal; one who will fabricate from a drawing is a problem waiting.
Where it goes wrong on site
The opening moves. A steel screen is fabricated to a millimetre and the building is not. The sequence that works is: structural opening formed, lintel in and loaded, then survey, then fabrication. Compressing that to save two weeks is how you get packers and a shadow gap on one side.
The lintel is an afterthought. Every opening needs a correctly sized lintel with adequate end bearing — commonly 150mm each end, reducing to 100mm where the clear span is 1200mm or less — and, on an external wall, a cavity tray with stop ends and weep holes above it. A wide steel screen is a wide opening, and wide openings are where lintel and bearing decisions stop being routine.
The threshold. A flush threshold looks superb and is the most common source of water ingress on a rear elevation. The sill leading up to a door threshold has a maximum slope of 15°, and the drainage detail below it has to be designed rather than improvised on the day.
Ventilation is forgotten. Replacing leaky old windows with sealed new ones removes ventilation the house was relying on. Where the dwelling has continuous mechanical extract, keep a minimum 4000mm² equivalent area of background ventilator in each habitable room that is not a wet room. On a steel screen that usually means planning trickle provision somewhere sensible early, rather than drilling the frame later.
The honest summary
Slim steel is a wonderful thing on an internal screen and a demanding thing on an external elevation. Internally: no thermal break needed, safety glazing still matters, buy the look and enjoy it. Externally: thermally broken, whole-doorset U-value against 1.4 W/(m²·K) in writing, reveal detailed so the insulation runs continuously past the frame, and the survey taken from the real opening.
Ask “thermally broken?” and listen to how the answer arrives. A supplier who answers it immediately, with a figure, is a supplier who has fitted these before.
Get an Itemised Estimate
Or call us on 020 8050 8968. We work across Fulham, Chelsea, Battersea, Kensington, Hammersmith and the surrounding boroughs.
Straight answers
The home you actually want, finished on the date in the contract — and you watch every day of it happen from your phone.
You answer 3 questions. We do the drawings, planning, party wall, trades and snagging.
- 10% to mobilise, then weekly against work already done — with 5% held past handover: 2.5% released at practical completion and 2.5% at the end of the 12-month defects period
- £550 survey fee credited in full against your contract
- Every variation priced and signed before a penny is spent
No email needed to see your range. Quotes hold for 21 days, then materials re-price.
We are not the cheapest. If price is the only factor, we may not be the right fit — and that is fine.
Keep reading
- Two Quotes, One Job: Stop Asking Which Is Cheaper and Ask What Is Inside the PriceThe gap between two quotes is usually two legitimate ways of building the same thing — and nobody telling you which one was priced.
- Underpinning Explained: Why It Costs More, and When to Avoid ItUnderpinning is ordinary concrete priced by sequence. Short designed bays, poured one at a time — that queue is the cost, and it is what holds the house up.
- Conservation Area Loft Conversion London 2026Converting a loft in a London conservation area: why it needs full planning permission, what officers approve, and what the extra cost is.
- Hip-to-Gable Loft Conversion Cost London 2026What a hip-to-gable loft conversion costs in London in 2026, when it needs planning permission, and the Building Regulations it triggers.