Calculating and templating a masonry opening

by brittney_cutler | April 22, 2022 4:10 pm

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By Steven Fechino

Jack arches are a structural element in masonry construction that builders have used for centuries. Also known as flat or straight arches, they are structural elements that provide support at openings in masonry.

At some point, masonry contractors may need to incorporate a jack arch into their work. To calculate the cuts for a custom jack arch, a mason will need to create a cutting template. Three integral pieces of information are necessary to begin the process. First, one must know the width of the rough opening. Second, one must know how many courses vertically the arch will extend, and hence, the width of the jack arch at the extrados (top of the arch) as per the skewback. The jack arch will have full-width units at the extrados and a mason will cut these to a thinner width at the intrados (bottom of the arch or lintel location). Third, as the previous piece of information infers, one must know what angle they want the arch’s left and right ends to duplicate and how the ends will look; this is the skewback—the end voussoir (arch brick) at the extrados. Working with the degree of skewback or skew line angle can simplify the building of and material cutting for the arch (Figure 1).

One can apply this layout math for a jack arch to any size of brick they are installing.

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Light commercial façade use

Figure 1 is an example of a jack arch for a small building, such as a bank or a light commercial façade.

This project requires modular brick, 9.5-mm (0.375-in.) joints, and four courses of vertical arch height. Full brick is necessary at the extrados and, with a 1.2-m (4-ft) opening, it will have full-brick widths at the top of the arch and workable cut widths at the bottom of the arch.

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It is easier to build an arch with an odd number of bricks. However, in this case, an odd number will not fit properly. This means the center of the arch will be the head joint of the mid span of the arch. There will not be a key brick (or center brick) in this case. To determine the arch material, one should calculate the extrados based on the masonry rough opening, the width of each of the skewbacks added to the intrados, the dimensions of modular brick, plus one mortar joint for each brick. After determining all of this, one should add an additional mortar joint to balance the arch; this is typical of all arches.

∞ Mortar joint: 9.5 mm;

∞ Modular brick (height): 57 mm (2.25 in.); and

∞ Modular brick + mortar joint: 9.5 + 57 mm = 66.5 mm (2.625 in.).

This example uses 66.5 mm as the width of the voussoir plus a mortar joint (arch brick plus mortar joint).

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The jack arch will be sitting on a lintel which will not alter the calculations but will simplify the thought process due to the façade bond. The jack arch will extend to the fourth vertical course of wall façade brick. The extrados is where the jack arch voussoirs (arch brick) extend to; for this example, look at four courses. Four courses of the façade bond (typically running bond or English bond) of modular brick plus each mortar joint will be 66.5 mm x 4 courses = 266 mm (2.625 in./course x 4 courses = 10.5 in.) above the lintel (the lintel also represents the intrados of the arch or the bottom of the arch).

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The skewback angle is anywhere from 60 to 70 degrees. The greater the skewback’s angle, the farther the strike point is from the bottom of the arch. The strike point is used to pivot the skew line or vary the arch’s width, and thereby, to mark every brick for cutting. The strike point is drawn from the left and right skewback following the skewback angle to a point on a center line that defines the masonry opening. By calculating the number of bricks at the top of the arch, one can define the skewback angle. This is just one way of doing it; there are several different methods.

Calculating the degree of skewback

To determine the degree of skewback, one must calculate the full brick widths at the top of the extrados. In this case, one must calculate an angle which will allow a 1.2-m intrados, plus the additional length for extrados (i.e. X millimetres/inches on the left and right widths).

Knowing the rough opening is 1.2 m, the skewback needs to be less than 152 mm (6 in.) wide on each side. Jack arches seldom extend further than a brick; this is where the 60- to 70-degree skewback comes into play (Figure 2).

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In Figure 2, by drawing the 63-degree skewback on cardboard, it was useful to see how far left and right the arch top would extend once the skewback angle was determined. The 63-degree skewback angle will add 133 mm (5.25 in.) to each side of the intrados, making the extrados 266 mm wider than the intrados.

To calculate the number of bricks in the extrados, the math is as follows:

1.2 m (rough opening) + 0.27 m = 1.46 m (4.8 ft) (opening at extrados)

1.46 m ÷ 67 mm (2.6 in.) (brick and mortar joint) = 22.3 bricks

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This is perfect because it means the extrados includes 22 full-width bricks with head joints. There will be 7.3 mm (0.29 in.) for the extra mortar joint required for balancing that is 3.3 mm (0.125 in.) off which can be bonded out.

To determine the brick and mortar joint dimension at the bottom course of the arch, the following calculation is necessary:

1.2 m (rough opening) ÷ 22 brick opening (determined at the extrados or the top of the arch) = 55 mm (2.16 in.) brick and mortar joint.

After performing these calculations, the typical solution is to adjust and tighten the head joints or recalculate until a dimension for 22 bricks and 23 head joints will fit into the 1.2 m rough-opening dimension.

Twenty-three head joints x 9.5 mm per head joint = 219 mm (8.63 in.), which is between 16 and 19 mm (0.625 and 0.75 in.). Therefore, in this case, one should use 219 mm for head joints. This leaves the following calculation for a 1.2 m rough opening

219 mm = 1000 mm (8.63 in. = 39.37 in.)

To calculate a more exact width for the cuts at the intrados, one should use the following formula:

1000 mm ÷ 22 bricks = 45.4 mm per brick (39.37 in. ÷ 22 bricks = 1.8 in. per brick

 

[8]The benefits of a jack arch template

After one has calculated the necessary measurements, they can make a template to facilitate the brick. Using 6.4 mm (0.25 in.) plywood is helpful, as it is more durable than cardboard or poster board. It can also be easily stored if multiple arches are required for the project. When marking up the template, one should use a fine-tipped marker, as it will lead to sharp lines and keep the markings more accurate.

To make the template, a 1.2 x 2.4 m (4 x 8 ft) sheet of plywood, a set of clamps, a sturdy work surface, a straight edge, markers, a jig saw, a drill, and a 16d nail (89-mm [3.5-in.] long) are all useful, since there is a 1.2 m rough opening. Once the template has been completed, one can then turn the plywood and mark out the opening.

[9]Simplifying the process

1. Draw a line 38.1 mm (1.5 in.) parallel or horizontal with the 2.4 m (8 ft) side of the plywood; this will be the extrados (top of the arch).
2. Draw a line 305 mm (12 in.) parallel or horizontal with the 2.4 m side of the plywood; this will be the intrados (bottom of the arch).
3. Draw a perpendicular or vertical line 1.2 m from the left or right side of the plywood, this will be the center of the rough opening.
4. From the left of the template, draw a perpendicular or vertical line at 0.6 m and 1.8 m (2 ft and 6 ft); this will represent the jambs or reveals of the rough opening.
5. Next, draw a 133 mm line on each side of the jamb at the intrados; this will layout the extrados total width (Figure 3).
6. Draw a line extending from the top of the extrados to the intrados (follow the skewback angle) and down to the center of the masonry opening. The intersection of this point will be the strike point. This point must be at the exact center of the rough opening and the key voussoir’s center (key brick). From the strike point, one will generate all the template lines from this point on (Figure 3A).
7. Layout the façade coursing onto the template along with mortar joints. It is easier to draw the entire arch; however, one can complete only one half, and then draw duplicate cuts (Figure 4).
8. From the top of the extrados, draw 22 evenly spaced ticks; this will be one head joint and one full-width modular brick. The measurement to work with is 67 mm (2.625 in.) with an additional head joint on the opposite side of the layout for balance. All the ticks on the extrados will connect to the strike point. Once finished, the voussoir (arch brick) layout can be started (Figure 5).
9. Once the voussoirs are laid out, look at horizontal bed joints for an aesthetically pleasing project.
10. Label all template pieces, then take pictures of them.
11. Double-check calculations and the template markings before starting the next step.
12. Using a jig saw, cut out the jack arch template in its entirety, then check to see if it fits properly. If it does, cut out each brick template for each voussoir.
13. With all the template pieces cut out, the arch can now be installed (Figure 6).

Conclusion

Jack arches are an important element in masonry construction. With the ability to properly calculate measurements and create a template for a jack arch, masons will have the confidence to incorporate this unique, cost-effective element into future projects.

Keywords

∞ Extrados – Top of the arch.

∞ Intrados – Bottom of the arch.

∞ Voussoirs – Wedge-shaped brick that makes up the arch.

∞ Skewback – The angled abutment or sloping section where the arch sits on the left and right sides.

∞ Striking point – The point in the center of the arch’s opening at which the voussoirs’ (arch brick) angle cuts originate.

∞ Center line – A drawn line or string line that goes directly through the center of the arch that splits the keystone or key brick directly through the center of the unit. The center line can easily determine the accuracy of the laying of the arch brick during construction.

∞ Reveal – The jamb of the rough opening.

∞ Springers – The brick (usually in running or English bond) that adjoins the façade brick to the skewback of the arch.

∞ Skew Line – A line extending from the top of the extrados to the intrados (following the skewback angle at the reveal) and down to the center of the masonry opening.

Author

Steven Fechino is the engineering and construction manager for Mortar Net Solutions. He provides engineering support services and product training. Fechino has a bachelor of science degree in civil engineering technology and two associate degrees in civil engineering and drafting and design, specializing in building construction. He can be contacted at sfechino@mortarnet.com.

Source URL: https://www.constructionspecifier.com/calculating-and-templating-a-masonry-opening/

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