Jack Arch Construction Sequence. Pyromasse

Jack Arch Construction Sequence

The arch detailed here spans a 410mm square fire box loading opening in the facade of a contraflow heater. The opening will accommodate Upos' 0018 fire box door.
The brick used are recycled, being made in Quebec between 1900 and 1920. Mortar is type N.

I taught myself to do this some years ago and have only my own experience as reference.

GLOSSARY OF TERMS.

• Intrados
  • The inner curve of the arch. In the case of the jack arch the intrados is straight.
• Extrados
  • The outer curve of the arch.
• Springing line
  • The line from which the arch begins to spring.
• Springers
  • Bricks which are cut back to the angle at which the arch will spring. Also termed Skewbacks.
• Voussoir
  • Individual segments of the arch.
• Key
  • The central voussoir in a symmetrical arch
• Haunches
  • The voussoir between the springers and the key. Also called flanks.
• Spandrels
  • Those brick forming the portion of the wall above the haunches and below a horizontal line through the highest point of the extrados.

The course numbers referred to in the text start with course 1 being the first course above the springing line.

The construction of a jack arch demands absolute attention, and full application of skill. Keeping cool during the final stages of laying the arch is a challenge.
To lay an arch and surrounding brickwork, like the one illustrated, takes about 8 hours, including planning and template preparation. Being on form, and alone, can make all the difference.

The arch will attract the eye immediately so even small errors will be evident. This does not leave much room for practice. So if attempted it has to be done well.

The following 17 images were all taken on the same day. The day before, the masonry on the back and sides of the heater were built up as much as possible to avoid the distraction of having to lay any of this during the arch's assembly.

In order to cut out the arch accurately a cardboard template must be drawn.

Creation of the template requires a series of tedious steps which if not taken will cause serious problems during the assembly of the arch. Patience is crucial, as is working methodically and keeping everything in order.

A large piece of clean, thick cardboard is selected on to which is drawn a horizontal line from side to side (springing line). Below this is drawn the square of the 410mm x 410mm opening that the arch will span.
Through the centre of the square, from top to bottom of the cardboard is drawn a vertical line (Centre Line) This line represents the centre of the arches key and divides the arch into two symmetrical halves.
It is important to label these lines, as, during drawing, the number of parallel lines can be confusing, especially if errors necessitate redrawing.

6 courses of the brick used come to 400mm so the springing line will be represented by the top of the bed joint on the top of the last course, which will add the extra 10mm, i.e. the thickness of the joint bringing the height of the opening to 410mm.
From this line all the courses and joints for the five courses above the springing line are marked on to the card. The card board now has drawn upon it an accurate map of the masonry that will be laid around and above the arch. It is on to this that the template of the arch and of its voussoir will be traced.

Labelled diagram of above image - see text below for key.

Lines x and y are the lines which define the form of the arch and the angle of the springers.

Line x must run through the top right corner of the opening, and it must intersect the centre line, line z, at some point. It is the location of this point of intersection between line x and the line z that determines the overall length of the arches extrados. This depends upon the number of and width of the voussoirs which in turn is dictated by the width of the bricks. This arch has 5 voussoir each side of the key, 4 three courses high and one five courses high. If an extra voussoir is added to each side, the length of the extrados will be longer, making the arch wider and the angle of the skew backs more oblique. The width of each voussoir at the intrados will though be considerably less. 5 voussoirs each side, with the width of the brick available give, an aesthetically balanced form to the arch used to bridge an opening of this width.

Determining the point of intersection on the centre line is of the up most importance to ensure the extrados is the right length to accommodate all 10 voussoir and the key.

To the right of the centre line, at the top of course 5 a mark is made (a) that represents half the thickness of the brick used for the key. A line is then drawn from this mark to an estimated point of intersection on the centre line. This point is used only as a temporary guide to help gauge the width of the key and voussoir 5. Another parallel line is added, to represent the joint between the key and voussoir 5. Then from the point at which this line crosses the top of voussoir 5 on row 4 (b), the thickness of a brick and a joint are added and a mark made (c). From this mark a line is drawn to the estimated point of convergence, giving voussoir 5. Where the line of the joint of voussoir 5 intersects the top of voussoir 4 on row 3. (d)  4 marks, ( e f g h) each the thickness of a brick and a joint are made one after the other. The last of these marks (h) should accurately indicate the end of the extrados. It is from this point that line x is drawn, running through the top corner of the square of the opening and intersecting the centre line at the correct point of intersection (i). Now both the key and voussoir 5 can be re aligned on to the correct point (the temporary estimated point of intersection is abandoned). The remaining 4 voussoir are then drawn in from the marks on the top of row 3 to the new point of intersection.

All ways the joints between the voussoir and voussoir 1 and the springers must be calculated when marking and drawing.

Line y is drawn, from the point of intersection on the centre line, through the top left corner of the opening, in perfect symmetry to line x.

The joints of the voussoir and key are marked in black in order to view the template as a whole, and to make sure the right line is cut when the voussoir are cut from the template. Mistakes at this point can be serious later.

The springers are layed temporarily. The framing to hold the arch during assembly must be made in manner that will allow it to be removed immediately the key is in position with as little disruption to the arch as possible, as the weight of the arch will rest upon the form before actually arching after the form's removal.

The template for the whole arch is cut from the card and placed on top of the form against the temporarily positioned springers. The limits of each side of the arch are traced on the the springers which are then numbered, removed and cut.

After the springers are cut and relayed in position the template is used to check that the angle of each correspond. Any discrepancies here must be noted and compensated for along the way.

On the second course of the right hand side, the first stretcher after the corner has been cut shorter to allow a springer of reasonable size to follow. If a full stretcher had been used it would have left a small triangular springer that would look uncomfortable and divert the eye from the lines of the arch.

This has been done in many places with this arch , not only to avoid small triangular 'pins' but also to avoid stack joints from the voussoir to the spandrels. It results in a few extremely short spandrels and stretchers, which to me are less offensive than pins and stack joints.

The individual cardboard templates for each piece of each split voussoir are cut. They are kept in order .

Enough good wide sharp cornered brick must be pre selected before beginning to start the cutting. In this case bricks that have been discoloured during the firing process have been deliberately put to one side for use in the heater's arches.

The pieces for the split voussoir are cut using the cardboard templates. Piece R1a is cut then the cardboard is turned over and piece L1a is cut. Then piece R1b etc. etc. The card templates start to disintegrate after contact with water from the saw, but the water causes the template to stick to the face of the brick keeping it in place as the brick is manoeuvred for three cuts on the saw table. The cut voussoir are kept strictly in order as are the used templates in case a re cut is necessary. The voussoir if mixed by accident can be surprisingly difficult to rearrange in the right order, running the risk of laying one or more of them out of sequence.
The key is not cut until all the voussoir are laid.

Laying the arch. Once started the process of laying the arch should not stop until the key has been placed in position and the wooden support frame removed. This work has to be performed as fast as possible so that the mortar will be still slightly workable when the key is banged in. As this portion of the work demands the most skill and composure, the additional pressure of time can be stressful. Frustration must be avoided as once embedded, continuation with the work would be foolish.

It is for this reason that there is only this one image of this step. Concentration must be maintained.

Once the voussoirs have been laid the template for the key can be fitted. Depending how the keys template fits in to the remaining space, the voussoir can be removed and relayed with tighter or thicker joints or the keys template can be cut back or made larger.
When laying the first 4 voussoir on either side, a level is used to assure that the top pieces of each voussoir rise just above the extrados or line of the top of the third course. Being slightly above the line the pieces can be cut down to the line with a grinder once the form has been removed. If the top of a voussoir falls below this line no adjustment can be made after. A voussoir that falls just on the line runs the risk of falling slightly during assembly of the other elements of the arch.
The line of the intrados is less important as it is hidden behind the top of the cast iron door. A level is used to range the intrados regularly until the key is inserted. Brick are temporarily laid on the two corners of the facing, to represent the height of course 4. Placing the level across these two brick will ensure that the top of voussoir 5 rises just above the extrados.
An additional brick is temporarily laid on the corners as a guide to the height of course 5 to assure that the key does not fall short.

Both voussoir 5 tilt off plum, out from the facing. The key then tilts out from them. This obviously dose not enhance the arches strength but it does give relief to the detail of its focal point. These voussoir and the key can be laid flush with the rest of the brick work if desired. There is not enough compressive charge above the arch to make strength an issue.

Immediately the key is in place the wooden frame is removed and the key tapped - compressing the arch into place before the mortar has completely hardened. This means that the arch arches, rather than sets in place, as when the wooden support is not removed until the mortar has completely hardened. The intrados can be ranged a final time with the level.

The work is allowed to set for an hour and then any space behind the voussoir and the key back filled with loose mortar.

A level is used to trace the line of the extrados on to the protruding voussoir. This line should be ground down now before laying the spandrels.

The spandrels have to be cut to ensure that their head joints do not form stack joints with any of the head joints of the voussoir. This results in some uncomfortably short spandrels, e.g. course 4 above voussoir 1 on the right.
Before jointing any joints between the voussoir or the voussoir and the springers can be adjusted with the grinder.

The arch contains about 85 cuts.

Detail

The work station.

The finished project.

Marcus Flynn

2007