Friday, March 19, 2010

Following Mazerolle: Lucarne à Croupe Nolet Carré

This is the third installment in this series where I will be drawing various carpentry challenges featured in the 19th century Louis Mazerolle text. The next section in the book after the theoretical material, plates 9 through 13, concerns dormers.

Funny how in English we use the word 'dormer', coming from the French word dormīre, meaning 'to sleep' (coming from the same Latin root, dormītōrium, from which we get the word dormitory), while the French themselves use Lucarne. Lucarne translates as 'skylight'. The section on 'skylights', then, by which I mean dormers, comprises 5 different examples in the book. One nice thing is that all the examples sit atop the same slope of main roof (well, it looks that way), so I plan to draw all 5 examples on the same roof structure. The roof slope is 4/3, the same as produced by a 3-4-5 triangle.

Lucarne à Croupe Nolet Carré is the first one I will examine. The word croupe refers to a roof having two hip rafters - this dormer has a hipped end in other words. Nolet is a word one won't find readily in the dictionary. It can also be written noulet and is formed from the word noue, meaning "valley". Nolet is a 'little valley'. The word carré means 'perpendicular', or 'square to'. Thus this type of dormer has a square, hipped end and forms little valleys on the main roof. The problem to be solved largely revolves around those nolets in terms of determining their shape. Funny that this is the first problem to be tacked after the theory section in the book - most other framing books don't introduce anything to do with valleys until long after hip rafters have been explored thoroughly. Oh well, onward anyhow.

There are several ways to tackle valleys in general. One method is to construct a valley rafter, similar to a hip rafter, and which has a inward backing cut.

Another way is to fasten a pair of boards, 1x6 or w.h.y., in the form of an inverted 'V' to the main roof surface to serve as a sill plate for the dormer rafters, ridge (if any), etc. This is the typical sort of solution seen in stick-framed roofs, in the, ahem, good old days before the factory-produced truss packages arrived on the scene.

Another method yet is similar to the one above - a pair of boards are scabbed onto the main roof surface. If you use rectilinear boards for that purpose, their upper surfaces would be co-planer with the main roof, which means that any dormer rafters which come down to meet this board will ned a compound cut on their ends. To avoid this, the boards which are scabbed on can be made in such a cross section - trapezoidal - so that their upper surface is rotated towards the pitch of the dormer roof. The result of this method, where one takes some extra steps to fabricate these trapezoidal cross-section pieces, allows the jack rafters to meet it with a simple bevel cut, which saves work at the other end - and allows for more straightforward mortise and tenon work as well. This is the method shown in Lucarne à Croupe Nolet Carré. Other French books would term this construction method noues chanlattés, or noulet chanlattés.

In new construction, I would normally opt for backed valley rafters as I feel such a structural configuration will be inherently stronger, and I think it is more elegant, more integral. However, if one has to retrofit a dormer to a existing roof, then this method of using a trapezoidal section valley board seems like a good way to go about it. It does use less material than the backed rafter set up as well.

For this dormer drawing, along with the remaining dormers to be explored in the section, I have chosen to put everything onto one drawing, on one building. This sketch I am calling 'Dormerland'.

Welcome then, to Dormerland:

The building is hipped at the far end, gabled at the near end.

The drawing proceeds from constructing an elevation cross-section view -I mocked up the dormer in 3D, then transferred lines down to the floor to do the 2D development of the noulet. I found the Mazerolle drawing quite vexing in this regard, so I opted to use another French book which had a more straightforward method for the developed view:

My plan is to develop the noulet, put them on the model, then work backwards from there to figure out if Mazerolle has some more 'mistakes' in his drawing, or whether the method is just really weird. I'll get back to y'all in the next posting to let you know how that turns out.

The above drawing produced the correct shapes of noulet cross sections:

These section views are then used to develop the stick of wood, which can be up-ended on the drawing and marks transferred. Once the stick is shaped properly (or possibly before), it can be placed back on the drawing and marks are again transferred to the length and cut lines on each end.

Here a view of the dormer with most of the parts all fitted in there:

The noulet are in a contrasting color so they're easy to spot. The rafters each end with a little abutment which is fitted against a moulding, not yet installed in the above view.

Time to put the moulding on then:

Another view:

And from the interior:

One last close up of the noulet, showing how the shape of the boards allows the jack rafters to fit onto them with a simple bevel cut:

The book shows the hipped roof without ridge or post, not exactly my dream way to go about it. Further, he book shows a curious method of bringing the hips and commons all together:

If you look at that junction from underneath, it ain't the most attractive thing:

Both lower ends if the hip cheek cuts can be seen. I thought this looked bad, so I re-arranged things a little:

Now the hips come in a little further and the central common on the short end of the roof is truncated. From below, the view is slightly better:

Now the central common and the hips come together in miters, and the exposed lower end of the cheek cuts are reduced to one one side of each hip. Still I don't really care for it, but I guess it would be all covered over. A ridge or support post would solve it. In Japanese work, the hips are often exposed, and there are some much cleaner solutions for this junction.

Rolling on - thanks for dropping by today.


  1. Hi Chris,
    For once I can completely understand your blogpost. As a carpenter I have framed many dormers like the one you have illustrated. The common practice of joining the last common rafters, the single common rafter continuing in line with the ridge and the hip rafters is pretty much like you have drawn and labelled unattractive. The ridge beam is generally wider (2x12) than the hips (2x10) so the bottom corners of the hips are not exposed and the final common rafters meet with the ridge so they do not need to be beveled to match the hips.

    I hope that makes sense.

    I enjoy your blog, thanks for the effort.

    Harlan Barnhart

    1. Harlan,

      sometimes I forget to reply to posts, so my apologies for getting back so late. Your comment made sense and glad you enjoyed the read.



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