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Displacement of Concrete Gutters

Why do concrete gutters suffer from displacement and what can be done to rectify it?
Concrete gutters have endemic problems. Beside poor aesthetics and damp associated problems, quite often they become displaced in a vertical plane. Several factors can influence this displacement:
1) Break down of the mortar or bituminous joints. If the joints fail, it allows the blocks to act individually and as they are cantilevered the blocks will tend to rotate about their midpoint manifesting a downward displacement at their outside edge.
2) Differential Settlement: All buildings built on compressible soils (eg clay) will be liable to consolidate over a long period of time. Think of the clay as a three phase material consisting of clay, water and air. Consolidation is the gradual squeezing out or the water and air, resulting in a decrease of volume of the soil. Any inconsistency of soil or imposed loads can lead to a greater or lesser volume change which then allows the building to move more at one point than another. The net effect is a rotation about the point of foundation displacement, manifesting itself as cracks through the walls, which increase in width with height. Again this can cause gaps between the concrete gutter blocks leading to appreciable displacement.
3) Differential thermal movement: This is a common fallacy as concrete gutters, brickwork and steel have an almost identical coefficient of thermal expansion (if they didn’t how could composite structures such as reinforced concrete or brick and steel lintols ever work?). The movement caused between hot inner wall and cold external cavity wall in winter (or vice versa in summer) is minimal as it only occurs over a maximum of 300mm, So let’s disregard that one.
4) Differential movement caused by incompatible materials. This is only possible between the timber rafters and the concrete block. It is possible in extreme situations that the increase in volume of wet timber (caused by rain infiltration) at the rafter feet can locally push the blocks outward.
5) Window replacement: The original windows would generally have been of steel or timber. Both would have offered a degree of support to the outer edge of the cantilevered concrete blocks (the portion of the gutter block sitting on the inner leaf of the cavity wall has a lintol tray which was insutu reinforced and filled with concrete over the windows) . Failure of the contractor to recognise that the window heads required both temporary support during the window replacement and reinforcement or support to the window jamb at the head, leads to the concrete gutter blocks rotating resulting in vertical displacement.
Remedial Treatments
Breakdown of the joints or differential settlement, can be treated by resealing the joints, however any displacement that has occurred will not be corrected.
If the concrete gutters are removed by cutting the concrete gutters off flush with the external face of the brickwork, the displacement will not continue as the cantilevered portion of the blocks will have been removed. If a structural timber is then fitted and rigidly fastened to the cut off blocks, it will in effect stabilise the remaining concrete gutters and prevent further movement.
Window heads if badly displaced may have a steel angle (sized to ensure that it can transmit all imposed loads safely to the bricks below without suffering from excessive deflection) fitted which will support the blocks and prevent further movement. The angle should be inserted into a the mortar joints either side of the window, so that it provides a minimum of 150mm of bearing on the brickwork. The lintol will then be hidden by retrospectively fitting new fascia and soffits.
Concrete gutters are problematic, however with a little care and fore thought cost effective remedial treatments are available which will remain trouble free and last for years to come.