Doors with Transoms - Fanlights - 10 Downing Street - DIY Tip 47

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I would expect that most readers of this blog have absolutely no idea what a "Transom" is, so let me explain. 

The transom is the area above the door (not actually in the door) which has a pane of glass and forms part of the overall door frame, sometimes this area has been covered over or has been divided to form several panes.

I suppose one of the most famous transoms and fan-light is the one at number 10 Downing Street, using the word "fan-light" doesn't necessarily mean the glass or frame is "fan" shaped,  number 10 is a very well crafted style and has been copied for years as a design within some imported doors, the image of 10 Downing Street doorway is shown below. 


The situation where "transoms" are required is usually because the door would be too tall or where traditionally the openings were formed this way as a matter of tradition or style , the area above the transom and door was known as a fan-light and in some cases the fan-light would open for ventilation.

If you intend to renew a transom it is best to understand how they were put together in the first place, my own experience is that if I go to a job that requires a transom I have to deconstruct the way it was fitted, to make it easier for you to understand this I will set out below how to put it together, stripping one out is the easy part and requires no experience whatsoever, just a large dose of common sense!



Fitting a transom to form a fan-light in an existing traditional style door opening frame;

1, The transom rail is basically a timber rail fitted horizontally across the top of the door from one frame leg to the other which can be fitted in to a previously prepared "checked" or "rebated" section in the frame legs which should be the same size as the timber rail you are fitting.

You could also "cheat" by cutting the rail to the exact width required and simply "cheek" nail the rail in place, this is when you nail through the rail at an angle in to the frame legs from the top side and the underside of the rail, the size of rail you are fitting is what you decide to use but normally would be 45x95mm and of a length to suit the opening width.


2, The transom rail then requires a set of door stops fitted to the frame legs and to the frame lintol, the size of the door stops depends completely on the frame "BREADTH", from inside to out less the thickness of the door, fit the lintol stop first then shape the top of the legs to match the lintol stop and simply "notch" the legs as required to suit the transom rail which they will fit around, now cut the legs to length.

The lintol stop mentioned above will act as the rebate for the glass to be fitted in the area above the transom.


3, Fit a smaller single pencil round edge to the underside of the transom rail to act as a "stop" for the top edge of the  door, this is cut squarely between the previously fitted door stop legs.


4, Fit a top plate (the same thickness of the main door stops) above the transom in the same way as the smaller door stop at (3) above, but make it wider so that it overlaps the breadth of the transom rail and will act as a weather drip on the outside, it should act as a rebate on the inside edge and finish flush with the previously fitted main door stops.


5, The face of the transom rail on the outside should now be fitted with a small moulding or single pencil round face plate to finish it of, again cut this between the main frame door stops.


6, Lastly, fit a set of small timber glazing beads above the transom rail which should be roughly the same thickness as the door stops and top plate mentioned at (2) and (3) above, the breadth should be LESS the thickness of the glass you intend using.


Lastly, if you are renewing a complete frame you have the option of simply having the transom designed as a rebated one top and bottom which means it is not necessary to have it constructed in as many parts as mentioned above, so much easier than trying to explain the traditional way as above!   

Any questions, email and ask "Chippy"  

Edited by "Chippy" for

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