Cill or Sill, Sub-Cill or Extension Cill?

What’s the difference between the spelling cill and sill?

This question came up recently in a conversation so we decided to write this article as to why you’ll see both spellings and if one is correct.  We’ll focus on the context of doors and windows and why the spellings are different in this industry and for the general population.

The oracle for the English language, the Oxford English Dictionary, lists cill as a variant spelling of sill.  The book, A Dictionary of Architecture and Landscape Architecture uses cill exclusively as is common within our industry which is the spelling we’ll use throughout the article and our website.  So, both spellings are correct although the most common usage is sill and not cill.  As an example, if you search the web for the word cill on Google the search engine will display a box with the spelling sill.

What’s the difference between an aluminium sub-cill, an extension cill or a cill pressing?

There’s a story behind this as we had to find a solution to a problem with regard to the dimensions of a sub-cill and how to measure them.  As to which is required depends on whether you’re installing an aluminium bi-folding door, sliding door or window.  You also need to give consideration for the floor and wall construction and Building Regulations.  It also depends on what cills are available and appropriate to the design and/or aesthetics of the building internally and externally.

Aluminium Sub-Cill

This is the most common type of cill you’ll see when installing aluminium bi-folding doors, sliding doors and windows.  An aluminium sub-cill sits beneath the door or window frame.  The measurement of an aluminium sub-cill is taken from the back of the cill to the front of the drop-nose.  Whilst Alu-Clad Timber Doors & Windows typically have a clip in type cill they can also be used with sub-cills as long as it doesn’t impede the drainage in any way.

The red section is the aluminium sub-cill with the grey section above being a typical aluminium window profile.  The light blue sections are the thermally efficient barriers (thermal breaks).  Cills without thermal breaks cannot be used on your home, just on some commercial type projects.

 New Sub-Cill

Aluminium Extension Cill

An aluminium extension cill fixes/clips in to the face of the aluminium door or aluminium window and is most commonly used on commercial projects or buildings with deep wall constructions.  This type is used to extend the distance over the wall for water to drain away and on to the floor.  They are also used with timber or composite ali-clad doors and windows. An extension cill is measured from the front of the frame.

The red section is the aluminium extension cill with the grey section to the right being a window profile.  The light blue sections are the thermally efficient barriers (thermal breaks).

 Aluminium Windows Extension Cill

Aluminium Cill Pressings

Sometimes it may be appropriate to use aluminium cill pressings instead of either an aluminium sub-cill or an aluminium extension cill.   A cill pressing should not bridge the thermal break in the aluminium door/window profile as this creates a Cold Bridge which reduces the energy efficiency of the frames and in some cases will not be compliant with the Building Regulations as it could cause condensation.

The image opposite is of a curtain wall section with the aluminium cill pressing in red encapsulated within the glazing cavity.  Aluminium cill pressings can also be designed and fabricated to bespoke dimensions.

 Aluminium Cill Pressing

In Conclusion

Hopefully you’ll have gained an insight in to what the differences in the spelling of cill are, and why the people in the aluminium and composite door and window industry are a little bit different, and the different types of cills that are available.  Whenever Reveal Doors and Windows have a survey to undertake we always ensure that the whole aperture is inspected; head, base and sides (inside and out and back to front).

We consider and discuss the options available and what you should expect of us as a boutique company offering specialist advice and information before, during and after your aluminium/timber composite doors and windows have been installed.

If you have any questions or need any advice there’s always a helpful member of the team available during the day, or you could send us an enquiry, an email or call in and see us.