The Fechino Files: Arches
Most of us know the basics about building arches, but how many of us can layout the arch, construct the template and build the arch without having a mess to clean up once it is constructed?
Arches are constructed differently from a typical masonry opening. An arch distributes the load of the masonry above with two different forces: one force is the downward pressure that extends into the jamb, the second force is the sideways pressure that is found at the base of the arch and extends to the left and the right of the arch. This pressure is called thrust, and must have the veneer built up simultaneously to the construction of the turned masonry arch units to keep it stable and in place while the veneer cures.
Arch terminology is important to know in order to fully understand how to layout the arch. Many arches require complicated layouts, and today we will discuss two of the simpler, but most common arches found in most of our work.
Some definitions that will simplify the arch layout process:
Abutment: Simply, the masonry that is on the left and right of the base of the arch. This keeps the turned masonry arch units in place and the arch from “kicking out” or failing.
Arch axis: This is the radius that is found in the center of the turned masonry arch. This is not an axis you can see, and it for layout purposes only.
Camber: This is usually found in jack arches (flat arches). It is a slight upward bow that will accommodate loading of masonry veneer units that are placed above the arch. The camber will ensure that the finished arch is not in deflection once the masonry veneer work is in place and cured.
Creepers: These are the fun units to cut when building an arch. Creepers are the units directly on the curvature of the arch found within the veneer.
Crown: This is commonly at the top of the arch found typically at midspan.
Depth: When using brick to turn an arch, this is typically the length of the rowlock or the solider (approximately 4 or 8 inches).
Extrados and Intrados: The upper and lower radius of the arch turned masonry units.
Keystone: This is the unit placed in the center of the crown and typically one of the last units placed when constructing the arch.
Rise of an arch: This is one of the most important dimensions when laying out an arch. This defines the height of the arch from the top of the jamb to the bottom of the arch (intrados).
Skewback: This is where the veneer becomes the arch. Typically begins at the inclined turned masonry units.
Soffit: Sometimes it is an interchangeable word with the intrados. It is another term for the bottom of the arch. Many tradesmen simply call the bottom of the arch the “bottom of the arch”.
Span: There are two definitions for the span. For simple arches, span (S) is simply the rough opening or the distance between the finished veneer of the jambs. This is the way this term will be used in the field. The less used and more complicated definition is when an arch is a larger circular arch and the span (L) is the distance between the ends of the arch axis at the skewback.
Spring line: This this where the soffit of the arch meets the jamb of the masonry below the arch.
Now we can get to the good part.
Laying out an arch is a bit more complicated than creating a radius between the jambs. The type of arch you have to layout will dictate the steps that need to be followed.
Segmental arches are typically just slightly curved above the top of the jambs and are not full half circles. Layout for these arches require you to understand what the rise, radius and span are for the arch.
Here is a formula to calculate the radius of the arch so you can build the template.
(span inches x ½) x (span inches x ½) = T square inches
T square inches /rise inches= S inches (this is distance from bottom of arch and the remainder of the diameter of the arc circle)
T + S= U inches (this is the diameter of the entire arc circle)
U/2 = R is the radius of the arc circle. This is the dimension that you will need to make your template.
Here is an example:
- Span = 60 inches
- Rise = 12 inches
- Radius = (this is what needs to be calculated)
- 60 inches (span) x ½ =30 inches (this is T).
- 30 inches x 30 inches = 900 square inches (T x ½ span = Square inches).
- 900 square inches / 12 inches (rise) = 75 inches (this is S, the diameter of the arc circle – the rise).
- 75 inches (which is the diameter minus the rise) + 12 inches (the rise of the arch) = (this is U, the diameter of the circle). 87 inches is the diameter of the arc circle.
- 87 inches / 2 = 43.5 inches (this is R, this is the radius of the circle). The radius of the circle will be used to construct the template of a Segmental arch, 60 inches in span with 12 inches of rise.
Laying out a semicircular arch is much easier than a segmental arch. Here you need to know the span of the arch and the rise. The radius of the arch is ½ the span and the radius are drawn from the base of the rise on the template.
When building the arch template many of us use plywood at least 5/8 inch, but preferably ¾ inch for the sides of the arch on very large arches. The two circular halves will be parallel and placed together using pieces of 2-inch by 4-inch material. It is always a good idea to use screws to put the arch template together as it can ease removal when scaffolding or other temporary items would interfere with removal of an entire template after the arch is constructed. The top of the template performs well when ¼ inch plywood is used to form a clean radius.
The ¼ inch plywood will absorb water but can be reused for repetitive window openings, making it worth the effort to use. ¼ inch plywood is my preference for the top of the template, but I have seen poster board, cardboard, and carpet turned upside down to form the bottom of the arch. If it works, stick with it. The biggest thing when setting the arch template is to use wedges at the spring line location. The wedges will allow you the ability to drop the template down and make the arch removal easier.
If it is too easy to remove, then the brick at the spring line may end up being a bit out of place and it will show up once everything is cleaned and pointed. Everyone is different when it comes to arch laying preference. I build a better arch when I tuckpoint the bottom of the arch once the form is removed. I feel I can construct the arch cleaner and I do not have to spend nearly as much time cleaning over my head with cleaners. You can butter the entire brick and lay them as a rowlock if you are comfortable with that approach.
When we edited the NCCER Level 3 Masonry Manual, Fourth Edition pages 4-13, we updated and simplified many of the diagrams and text in the arch section. We went into a lot of detail on Jack arches, Gothic arches and even Multicentered arches. If you have any questions, this would be a good point of reference.