Revealing the Leading Slimline Aluminium Windows

Photo of author

Nick Dardalis

Updated on

When you need new aluminium windows, whether as a replacement for existing windows or for a new building project, slimline aluminium windows can be a top priority for many buyers, over and above other features. Right now, Smart Systems provides the best in slimline aluminium windows.

If finding aluminium windows with the smallest frame face dimensions is important, we’ve taken the popular window brands in the UK and compared their key dimensions. We examine how each brand fares regarding fixed window frame sightlines, frame and opening sash sightlines, and mullion and transom dimensions. Three is no one size fits all slim aluminium window, and in this article, we explain why. Designs and sizes always affect how slim your windows can be.

Understanding slimline aluminium windows sightlines

The concept of sightlines in aluminium windows refers to the visible dimensions of the frame face, sash, and mullion/transom components when looking at the window from both the interior and exterior.

While focusing on dimensions is easy, it is also about styling. The frame design overall can make a difference in how slim a window can look. Contemporary windows with flat profiles tend to look slimmer than classically styled chamfered or ovolo windows, even if their measurements are larger. The best advice is always to view a selection of windows and get expert advice.

Comparing Major Aluminium Window Brands

To help you make an informed decision, we have compiled a comprehensive table comparing sightlines offered by some of the major aluminium window brands known for their slimline designs.

As we explain later in this article, four factors affect how slim your windows can actually be. So we have chosen the frame and vent combination more likey to be suitable for replacement window projects. New build openings can result in an alternative combination of profiles, reducing the sightlines even further.

Most window projects needing slimline aluminium windows will invariably have opening elements for ventilation. So the slimmest window is based on the Frame and Opening Sash Sightline dimension.

This image shows a popular window design with opening windows and fixed panels, on which we’ve based our table.

The Frame and Opening Sash sightline will be the most commonly used. Fixed pane frame Sightline refers to no opener. Mullion or Transom sightline assumes an opener either side of the mullion/transom.

slimline aluminium windows in a modern lounge setting with green sofa
BrandFrame and Opening Sash Face Sightline Fixed Pane Frame Face SightlineMullion or Transom Face Sightline
Smart Systems Alitherm 30059.5mm47.5mm111mm
Reynaers SlimLine 68 Classic65mm49mm110mm
Aluk 58BW65mm53mm100mm
Smart Systems Alitherm 80070mm40.5mm112mm
Smart Systems Alitherm 40071.5mm42mm116mm
Sheerline Classic71.5mm47mm98mm
Reynaers SlimLine 38
aïr MOD Windows73mm48mm112mm
Schüco AWS7076mm51mm148mm
Stellar Windows79.5mm54mm105mm
Senior Ali Vu84mm56mm108mm
Origin OW-7085mm58.5mm107mm
Sheerline Prestige88mm46mm150mm
Origin OW-8091mm50mm148mm

The key takeaway from the face dimensions of the well-known brands is that there is very little to choose between them. You have an excellent choice of slimline aluminium windows overall.

All the above brands provide an excellent choice for slimline aluminium windows, but do also check out steel-look windows. Most of these can be even slimmer and all come with the option of all glass panes without astragal bars, making these look like regular modern windows.

Four factors affecting slimline aluminium windows

Four factors determine whether you can find the slimline aluminium windows you want and how slim these windows can realistically be.

The window design

The design of your new windows will always affect how slim these can be. By design, we mean whether your windows are mostly fixed panels or opening elements. If you want to create a floor-to-ceiling fixed picture window to look through a large pane of glass, then any quality window will give you slim sightlines. The Fixed Frame Face Sightline shows typical face dimensions.

The sightlines always increase when you add opening windows, as with most general window replacements and house styles. This is because a window needs three critical framing elements. These are the window’s outer frame, the opening sash, and mullions or transoms.

The window sizes

All window brands, system companies and designers provide a variety of frame profiles. This is because the window design combinations can be near-infinite. Window design companies, therefore, try to cater for most requirements with a choice of frame and sash designs and sizes.

The larger your window is overall, the less likely it is you can take advantage of all the smaller and slimmer profiles available. Window mullions, as one example, are designed to resist wind deflection and loadings. The slimmer the profile, the weaker it can be, meaning you must use a larger profile.

New build or existing window openings

Many of the marketing messages you may see promoting the slimmest windows are on the assumption these are for new build openings. New build openings can usually accept slimmer outer frames than existing openings where the internal plaster line and the structure’s nature need to be considered.

Only when a survey is done to establish the accurate window manufacturing sizes does a trained surveyor ascertain the most suitable frame profiles. Again these may not always be the slimmest.

The requirement for Trickle Ventilation

Under the current Building Regulations, new and existing window replacements must have trickle ventilators fitted. Some windows are designed to have trickle vents fitted in the existing frame head, but most will require an extension piece added to the top frame. This extension piece can vary from 15mm to 35mm.

Therefore, adding an extra frame profile or using a deeper outer frame for trickle ventilation will mean a larger sightline at the top of your windows.

Insights into slimline aluminium windows brands

Smart Systems Alitherm

With a fixed window frame sightline of 47.5mm and when you have an opening window of 59.5mm, the Alitherm 300 offers one of the slimmest sightlines among the major brands. Smart Systems are widely available through a nationwide network of quality manufacturers, and this brand is tried and tested, widely used, well-priced and stylish. The newer Alitherm 800 and 400 windows are also well worth considering. Alitherm tops the list when considering frame and opening element dimensions.

Reynaers SlimLine68

Harder to find but worth viewing are Reynaers Slimline 68 windows. Reynaers are an excellent window brand and stylish and slim. Depending on the frame and opening vent combination, Classic, Ferro or Cubic, you can go even smaller than the dimensions stated. You can find out more about the entire Reynaers range of aluminium windows.

Aluk 58BW

Like Smart Systems, Aluk is a great system to have in the home, and their 58BW offers pleasing sightlines with just 65mm with a frame and opening sash and slimline mullions. ALUK are another viable option for homeowners seeking slimline aluminium windows.

Sheerline Classic

When Sheerline first came out, we thought the beadless design was pointless and overcomplicated with concerns about who would want to service older windows in the future. You now have a standard beaded option with Sheerline Classic windows. These are now a competitive choice in the slimline market. The Prestige range is slightly thicker but offers nice styling for new build or replacement window projects.

aïr MOD Windows

Made in London by Everglade Windows, aïrMOD windows have lovely styling with the significant advantage of steel-look, tilt and turn and contemporary aluminium windows, all using similar frame profiles. We think this range offers a balanced and contemporary look for any home.

Origin OW-70

Origin OW-70 is a fantastic option for slimline aluminium windows, with superb quality and from a great British Brand. Slimmer frame sightlines are available with OW-70, but the smaller outer frame option may not suit all replacement window installations. Origin nonetheless is a highly attractive option for homeowners seeking elegance and performance.

Schüco AWS70

Schüco AWS70 stands out as arguably one of the best aluminium windows brands. There are numerous Schüco windows availablea and any of these promises a high-end window with sophisticated engineering and great styling. Schüco windows are, like Reynaers, not widely available in window showrooms. Get in touch and we can help with where to view these.

Senior Ali Vu

Based in Doncaster, Senior Architectural is widely specified by architects for new build homes and apartments. It isn’t easy to find Senior Ali Vu windows in window showrooms. They are well-designed, with excellent specifications and very appealing styling as another slimline aluminium windows option.


The best slimline aluminium windows won’t be the same for every project, and there are more advanced options by hiding the frames in the structure, where your design allows this.

Whilst slimline aluminium windows are important for many, you should also consider the overall window experience, not just how they look. The good news is that most brands today are slim, so you shouldn’t discount the brands with larger dimensions. But as you can see, the differences between all the well-known brands are surprisingly close, and again, it depends on the right frame and vent combination for your particular project. Your specific structural openings may mean even thinner dimensions than those shown.

When you want an unobstructed view or seek a balance between aesthetics and performance, the right slimline aluminium windows deliver. The larger the window, the less cluttered the frames will look. Smaller windows can look chunkier than larger and wider ones, even when using the same frame profiles. The best advice is to visit some quality showrooms and see a range or contact us for advice.

Providing expert help to homeowners and trade professionals since 2010




Get Free help finding the right products from the best suppliers

16:42 30 Oct 23
Nick responded incredibly quickly to my enquiry (on a Saturday too!), and was able to offer invaluable advice on both aluminum systems and manufacturer / suppliers. Thanks Nick!
chris steerchris steer
16:18 09 Oct 23
Had a meeting with Nick during the grand designs show. Was extremely helpful and professional would highly recommend
Chris WesthorpChris Westhorp
19:55 13 Sep 23
Extremely helpful with pointing us in the right direction to find the products we required and suggesting firms that could reliably fit them.Nick was quick to reply and very detailed in his advice.Highly recommend reaching out to them.
Cathy GreenwoodCathy Greenwood
07:03 11 Sep 23
Update - A second email to nick for help on bifolds resulted in a detailed informative reply even on a Sunday ! This guy is so knowledgable and helpful .I approached Nick for an independent view on sliding doors v bifold and the different brand options . His reply was extremely detailed and prompt and allowed me to navigate this often confusing world in a much more informed way . Thanks so much Nick
Anthony CraigAnthony Craig
12:45 05 Sep 23
This is a must site for anyone looking for advice. Great service and very quick answers to questions. Nick has been of great help quiding us through the maze of aluminium products. Thank you
Philip DuffieldPhilip Duffield
20:24 26 Jul 23
I contacted Nick @ ATS for guidance on what aluminium sliding door systems would be suitable for an opening in an extension that is going to be both wide and tall. He came back really quickly with very detailed and incredibly helpful advice. He is clearly extremely knowledgeable about the systems on the market and their respective features and capabilities. If you’re feeling a bit lost by the overwhelming range of choices out there, I’d definitely recommend contacting him.
Dariush HekmatDariush Hekmat
17:25 19 Jul 23
Nick's advice is great. He is always quick to respond and spends time on responses, which really help to make an informed decision.
Alan FosterAlan Foster
07:51 12 Jul 23
Absolutely fantastic. Asked for advice about replacing Crittall (?) windows late last night and early this morning received 3 great suggestions and contact information for suppliers
Rachel LauRachel Lau
09:24 16 Jun 23
Fantastic website full of valuable and pragmatic advice about product selection and comparison: no other website gives information like this. Truly written for public interest. Don't hesistate to reach out to Nick via the mailing system. He responds very quickly, is friendly, and has very sound and high knowledgable advice. I have already shared this with my friend planning her own grand design.
guard Charlieguard Charlie
12:28 14 May 23
I emailed this company just on the off chance they could help me with new magnetic sliders for my patio door. They were not able to supply those but were so very helpful providing me with the name of a company who could. I am so very grateful at the great service. Dare say customers must be very happy. thank you so much.
Benjamin LewisBenjamin Lewis
10:06 04 May 23
Nick is very helpful indeed. Extremely knowledgeable on bifolds which was in relation to my query. Great website as well, some very useful articles. Particularly appreciated the article that lists all the major bifold options in order of their sightlines! Thank you!
P JonesP Jones
08:07 15 Apr 23
I asked Nick some questions about an installation we were considering and he replied within hours with some very useful suggestions that will probably lead us to using a different make and supplier. Thanks, Nick. Great work.
Nicholas WallNicholas Wall
13:18 02 Apr 23
Looking for fair and honest expert advice on all aspect of aluminium windows and doors? These guys will arm you with all the information you need to make the RIGHT choice in a highly complex, crowded marketplace. post a question and you will get a very quick professional answer you can rely on being based on fact. Thanks guys!
Giorgia SiriacoGiorgia Siriaco
16:25 21 Mar 23
After a lot of research to find doors that would meet our unusual size requirements and feeling increasingly uncertain about the best way to spend our small budget, I stumbled on the ATS website, and I'm so glad I did! I contacted ATS to request advice and Nick promptly replied with a wealth of information, providing us with options and relevant contact details. I only wish I'd found them sooner!
Carol NortonCarol Norton
10:01 17 Mar 23
I've been so confused about how to achieve the look I'm after, and going round in circles with various window/door companies, trying to get away from the huge chunks of plastic currently around my windows. I didn't expect an email to a company found through a Google search to really help - but I was desperate, so I tried. I'm SO glad I did. Finally I have clear explanations from ATS of where I've been going wrong, exactly what I'm looking for, and a list of local companies that might be able to provide it! The response has excelled my expectations and I feel as if I'm finally inching closer to a home with which I'll be happy and comfortable: thank you.
David NunnDavid Nunn
20:54 14 Mar 23
It is rare that I write reviews, however after my very positive communication with Nick - enquiring about suitable slim sash aluminium windows for my project, I felt that a review was definitely warranted. I can reinforce the many positive reviews posted so far, Nick was very quick to respond, providing comprehensive detail, with excellent independent advice.In summary, great service, thank you Nick.
Chantal AuerbachChantal Auerbach
13:04 09 Mar 23
Nick from Aluminium Trade Supply was incredibly helpful informative and pleasant to deal with. He responded quickly to emails and was very knowledgeable. Highly recommend being in touch with Nick if you are considering new aluminium windows or doors.
Lee JonesLee Jones
20:21 22 Feb 23
I dropped Nick at Aluminium Trade Supply an email re high security slim sightline bi-folds and their alternatives and he responded that quickly (and so comprehensively) that I felt bad I’d not paid him for his time!Nick Sir your product knowledge is outstanding and you are providing a level of service that goes wayyyy above and beyond expectations. I’ll definitely make sure I mention Aluminium Trade Supply to anyone I encounter looking for a fenestration solution 🙏🏼
15:13 13 Feb 23
We are looking to replace some old UPVC patios doors with aluminium ones. The ATS website is a great source of advice in an area where clear information is sorely lacking. Having used the website, I then followed this up with a number of specific questions relating to our search, and Nick came back very quickly with clear, detailed and very useful advice.I have been very impressed with both the website and with Nick's advice. My only regret is that I didn't come across ATS before replacing various windows in 2021 - I'm sure it would have saved me a lot of grief and money! Thanks again Nick.
Azza McCabeAzza McCabe
17:47 01 Feb 23
So impressed with this company.Responded to my query quickly andgave valuable advice. Nick was extremely helpful and we were able to move forward with our project.
Adri LoloçiAdri Loloçi
11:21 08 Jan 23
Nick was super fast to respond and provided some great recommendations. I would recommend this website to anyone looking to find bifold or sliding aluminium doors.
Stephen HoustonStephen Houston
16:16 03 Jan 23
Looking for aluminium crittall style internal door and external patio French doors. Noted good reviews and thought would ask for advice. Comprehensive reply with excellent independent advice and lists of fitters who could provide products and installation. Great service.
Inga GeachInga Geach
08:25 21 Dec 22
Nick from Aluminium Trade Supply has been extremely helpful with navigating the jungle that is aluminium glazing for our upcoming extension project. His advice was prompt, detailed and always impartial. Very grateful for your input, thanks Nick!
Amanda BurtonAmanda Burton
10:54 16 Dec 22
Brilliant website and an excellent service. Nick provided not only detailed advice and supplier contacts, but highlighted the considerations of various products so I could make an informed choice. Our Sunvista minimal sliders were fitted this week and I couldn't be happier, stunning. Thanks once again for your advice and recommendations, invaluable!
Keith BarnesKeith Barnes
17:18 10 Dec 22
Very helpful advice given to my question, including the name of 3 different companies that i could try for the product being enquired about. Many thanks
Munim ChowdhuryMunim Chowdhury
11:45 13 Sep 22
Nick has been very helpful on signposting me to a number of partner organisations for steel/aluminium doors.
SUSAN wallaceSUSAN wallace
17:26 31 Aug 22
My experience with this firm was exceptionally good and this is notalways the case as a female, dipping a toe into technical waters.I was spoken too with courtesy and at a level that helped me consolidate what was needed .On top of that I was given a list of companies and places where I could obtain the type of product I needed.I could not thank them enough and would certainly go there again and reccommend others do too.
Paul SalterPaul Salter
22:39 22 Aug 22
Contacting Aluminium Trade Supply is like contacting the Oracle direct. Advice is always comprehensive and balanced. On multiple occasions they have provided me with options that fully meet our objectives. Many thanks.
Nick McCormackNick McCormack
20:25 14 Aug 22
Really helpful and useful came back to me on a Sunday with some invaluable information. Highly recommended.
ray addisonray addison
13:14 14 Aug 22
received a prompt reply to my query,and helpful information from ATS