Graphical Construction Glossary >> masonry. >> arches >> Flat Arch

Flat Arch

Also called a Jack Arch. A straight arch along the springing line with a flat Intrados. The masonry units are laid out as voussoirs angled to a centre below the arch.

French Arch

A flat arch that uses parallel sided voussoirs.

Arch Bar

A piece of flat steel or angle iron that supports brickwork in a straight span.

Skewback

The angled abutment of a flat arch.

Reveal

The side faces of an opening in a masonry wall.

Above is a typical flat brick arch with wedge shaped voussoirs. The section that the arch sits on is known as the skewback. This applies to brick and stone arches. One of the main reasons for the uptake of CMU modular block construction is that they do not need any of the extra work involved in making an arch over door and window opening. A couple of steel reinforcing bars and corefill and a beam is created.

Walking around any city or town we can see many small flat brickwork spans that do not seem possible. There does not seem to anything that is holding up the triangle section of bricks above the opening apart from the mortar. What is out of sight is an arch bar or bars as seen above. Sometimes they are just a flat steel bar say 10mm thick or as I have drawn an angle iron, again 8 or 10 thick. They are never thicker than the bed joint of the brickwork so that the course is in a straight uninterrupted line. Normally the sit on the sides of the openings

A flat span of fine ashlar masonry.

Almost unbelievably precise brickwork with 3mm wide joint. Seen at the Victoria and Albert Museum London, UK.

An architect making a statement in Amsterdam with something different from normal.

Construction of a French Arch.

There is a lot of work involved in cutting and shaping the tapered voussoirs for these flat arches. One answer to that is to build a French arch as seen above. This uses standard brick widths and only the ends are cut. So they can all be made the same using the same angle, which is the skewback angle. In some case the a single purpose made stone key is used.

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Adam Smith 1723-1790

"When we build, let us think that we build for ever."John Ruskin 1819-1900