Expert: Richard Taylor, AIA - 3/31/2004

Question
Dear Mr. Taylor,

I have recently bought a 19th century farmhouse in S. France that has immensely thick stone walls.  The stones are oval-shaped smooth fieldstones that were built up in layers with a sort of sand-lime mixture for mortar.  The ground floor is living space, the upstairs is unfinished.  The space on the second floor is wonderful with huge arched beams and "visible" stone walls.  The area is divided into two rooms by an interior stone wall (about 2 ft thick) into which a rough opening was made years ago.  This 60" high opening was made at one end of this cross wall by removing stones until the entrance permitted people to enter (no special support was installed).  I would like to make this into an arched doorway.  There is about 2' of wall above the opening before it meets the roof. The wall does not seem to be a bearing wall at that point (the roof itself is supported by the arched rafters about 8' apart - parallel to the wall in question).

Question: What's the best way to make the arched doorway?  [A - The obvious option is to call a mason, but these folks are (1) unavailable and (2) unaffordable in this part of France - many UK ex-pats actually import British workers! - I'm from Connecticut. B - I accept that these may be hard questions to answer blindly - I don't know how far this service goes, but I have digital pictures.]
If I continue to remove stones, is there a serious risk that the wall will collapse?  (If it collapses, only the section above the doorway should fall.)  Assuming that the wall remains standing, how do I "seal" and secure the arch to prevent it from disintegrating at some later stage - there will be a wooden door installed, but obviously the frame will be narrow compared to the wall?

Thank you very much for your advice,  Fred Hoette  

Answer
Fred:

Thanks for your question!

The best way to resolve this is for you to fly my wife and I to France with you.  A thorough on-site evaluation is always a good idea.

Here's what you've got to do.  You're going to start by making several holes (3 or 4) above the arch - big enough to pass a 4 x 4 through.  Put a 6' 4 x 4 through each hole and support them on either side of the opening.  Now you've got the stone above the opening supported while you work below.

Remove all of the stone below the 4 x 4 - you're probably going to reuse this stone, so remove it carefully and set it aside.

Next, build an arched form (called "centering") that has the shape of the arch you want and that is as thick as the wall.  It should be two pieces of plywood with 2 x lumber in between.  Place this in the opening, and raise it up to the height of the arch.  Be sure you've left enough room between the top of the form and the bottom of the opening to work the stones back in.  Maybe about a foot and a half or so.

Now you're simply going to mortar the stones back into the opening, arranged to fit the shape of the arched centering.  Between the faces of the wall on each side, you'll be using "rubble" stone (the less-than pretty stuff) to fill in.  Let the mortar cure then remove the forms.  Pay attention to the natural "arching action" by using "keystone" shaped stones (especially in the middle stone at the top of the arch) and place the stones tightly together.  If done correctly, the arch will support itself.  Once done, remove the 4 x 4s and fill in the holes.

The size of the opening depends upon the size of the door you're planning to use, so be sure to allow for the door size and frame.

Be sure to select the proper mortar mix for your type of stone - too much Portland cement can cause the stone to crack.  You might also want to take advantage of this opportunity to create an attractive arch; select interesting "keystone"-shaped stones.  You can probably collect those stones nearby, and there are probably examples of arches in the area to use as examples.

Hope this helps!

Richard Taylor, AIA
Richard Taylor Architects, LLC
www.rtaylorarchitects.com