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Why Water Penetration Aropund Some Arches is 70% Higher

Author/Issuer: Cavity Tray Standards    Added:

Water penetration around some styles of arch openings can commonly be up to 70% higher than water penetration around others. The difference depends on two main factors, the arch style and the arch height.

As the rise (calculated as the vertical distance between intrados (springing) level and head level) increases so does the masonry absorption area.  The actual style of the arch also affects the absorption area. As the absorption area increases, so does the means of entry into the wall of penetrating water

This is best demonstrated by comparing a French arch that is relatively flat with a Trefoil, Semi-Circular or Gothic arch, all of which proportionally rise considerably above intrados level. Rain penetrating masonry above the arch curve will gravitate around the cavity tray that protects the arch and then continue its downward path in and on the external masonry skin until it reaches the base of the opening. Whilst making its downward journey it is joined by additional penetrating rain entering via the masonry to either side of the arch. Thus the need to accommodate the most severe of potential permeation into the masonry must be provided.

37This is achieved on arch openings with the use of Type BA cavitrays. BA stands for Barrier Arch and all styles of arch opening can be accommodated. Barrier Arches are manufactured in polypropylene and petheleyne solid dpc.

Made on a bespoke basis, Barrier Arches can take into account other conditions on site. Where there is an expanse of masonry rising above an arch, arresting barriers are sometimes introduced at a higher level to reduce the volume of water descending in the cavity. Water does not discriminate, and a systematic approach ensures water control, water arrestment and water evacuation adequately addresses potential extremes of weather.

It is prudent to remove penetrating water from within a wall and this is achieved using Caviweeps that provide defined water evacuation routes. If masonry remains wet for long periods the presence of soluble sulphates from clay bricks can initiate sulphate attack on the mortar joints and other Portland cement materials. Type BA Barrier Arch Cavitrays and Arresting Barriers are supplied with Caviweeps, promoting best practice.

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