By Jonathan Smegal, M.A.Sc.
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The research described in this short report was presented in more detail at CCBST 2014.
The drip edge is the leading edge of a flashing, sill, overhang or other linear, horizontal building element designed to shed water. Common drip edge locations include window sills, window head flashing, and the edges of roofs and balconies. By deflecting rainwater from the exterior surface of a wall assembly, drip edges can reduce moisture-related issues such as freeze thaw of masonry, staining of the cladding, and potential long term erosion of historical materials. But how well do different drip edges deflect water?
The objective of the study described here was to compare the effectiveness of different drip edge materials, profiles and overhang distances with respect to the distribution of water on the wall below the drip edge.
Materials and Methods:
There are several common drip edge profiles (see Figure 1 above). This study tested five different drip edges with the following variables:
- Hemmed or straight edge
- 45° kickout or 90° vertical edge
- Overhang distance of 20mm (0.8 in.) and 45mm (1.8 in.) from the surface of the wall
- Different gauges of metal
- Different drip edge materials – metal and stone
Edges were tested using an apparatus designed for this study. The test apparatus can only measure the vertical distribution of water below the drip edge. However, the horizontal distribution (i.e., patterns of drips along the length of the drip edge) was observed and recorded by the researchers. For this study, a flow rate of approximately 15 L/h (3.9 Gal/h) was used, based on an analysis of rainfall rates in Toronto, Ontario.
Apparatus. Building Science Laboratories designed and constructed a 1.2m (4 ft) wide and 1.2m (4 ft) tall “wall” to accept different window sill and flashing details. Below the drip edge being tested, a special cladding of sheet metal was installed to allow any water deposited from the drip edge onto the “cladding” to be collected and measured. Five metal troughs of 6.8 cm (2.7 in.) height followed by three of 20.3 cm (8 in.) in height (equivalent to about one course and three courses of brickwork respectively) were positioned directly below the window sill, such that the front of each trough made up a section of the face of the cladding. As water was deposited on the face, it was caught by the troughs.
The water from each trough was then directed to a collection container and the quantity was gravimetrically determined (weighed on a laboratory scale) at the end of each test. A horizontal copper pipe with evenly spaced holes was used to supply and distribute water to the 122 cm (48 in.) wide test section. Sealant was applied to the edges of the metal flashing to prevent the flow of water off the side edges. A vertical section schematic and a photo of the test setup are shown in Figure 2 and Figure 3 respectively. A water application rate of 15L/h (3.9 Gal/h) was used.
Table 1 below shows results for hemmed and unhemmed 90° vertical flashing, with 12 gauge and 20 gauge metal and 20 and 45 mm (0.8 and 1.8 in.) overhangs. Table 2 gives the same information for 45° flashing. Any individual trough that collected more than 10% of the total water applied in one or both of the trials is colored red with white text. Any drip edge tests where 90% or greater of the water was shed (i.e. not collected in any of the troughs) are highlighted in green with black text.
90 Vertical Edge. Table 1 shows that of all the 90° drip edge tests, the specific drip edges that collected the most water were the 20mm and 45mm overhang distances under a hemmed edge. A drip edge of 12 gauge metal, with a 90° angle and an un-hemmed edge, showed reduced wetting as compared to the same profile with a hem. The hemmed edge test collects more water in the troughs than an unhemmed edge because of the curved radius caused by bending the metal drip edge. Water traveling down the metal flashing follows the curve of the hem towards the wall, and has enough momentum when it leaves the drip edge that it drops towards the wall instead of straight down parallel to the wall surface. This flow pattern was observed by researchers during testing (Figure 4). The hemmed 20 gauge metal drip edge had a sharper curve than the 12 gauge metal because the metal is thinner, causing the water to fall more vertically and shed less on the cladding.
Overall, the data for a 90° edge shows that typically a 45mm overhang distance performed better than a 20 mm overhang distance.
45° Kickout. The testing data for the thick 12 gauge and thin 20 gauge flashings with a 45° kickout is shown in Table 2. Compared to Table 1, Table 2 has less instances where 10% or greater of the total water applied was collected in a single trough, indicating that overall, there’s an improved vertical distribution on the surface of the wall below the drip edge. All of the thin 20 gauge drip edges with a 45° kickout shed more than 90% of the water applied in the first four feet of collection.
In every case in Table 2 the 45mm overhang performed similarly or better than the 20mm overhang of the same design. All of the drip edges without a hemmed edge shed more water than the identical comparison drip edge with a hemmed edge.
Stone Sill. Both vertical and horizontal distribution is necessary to avoid concentrations of water on the cladding. It was observed that for the metal profiles, the water tended not to distribute evenly along the width of the drip edge. At both lower and higher flows, the water tended to accumulate on the surface and drain in a few single streams. These streams of water mean that, under real-world conditions, concentrations could form in a vertical line even if the amount of water at a particular height is not significant (as indicated in lab tests by a low amount of water collected in a particular trough). Following the tests on metal drip edges, a slightly sloped (8°) stone window sill was tested. The water flowing over the stone drip edge tended to be distributed more evenly, as drips rather than streams, over the length of the drip edge (Figure 6).
This study is preliminary and more research is needed to confirm and extend its findings. However, the data gathered suggests several conclusions.
- Whether or not the edge of the drip edge was hemmed had the greatest impact on the shedding of water from the surface of the wall. This impact was greater for the thick 12 gauge drip edge than for the thinner 20 gauge drip edge.
- The larger 45 mm (1.8 in.) overhang had greater measured water shedding capability when compared to the 20 mm (0.8 in.) overhang, in every test but one.
- Adding a 45° kickout also increased the percentage of water shed from the apparatus compared to the 90° vertical drip edge.
- The thinner 20 gauge drip edge generally had improved vertical distribution compared to the 12 gauge, but 20 gauge is thin enough that it would require a hemmed edge to provide strength and rigidity to the drip edge profile on a building.
- There was no water collected in any of the troughs during the test of the stone sill with a small (8°) slope at 20mm (0.8 in.) overhang distance. The water was more distributed horizontally with the stone sill, meaning the water had less velocity over the drip edge than the metal drip edge profiles, and there was a reduced concentration horizontally on the cladding.
Drip edges that shed less water (i.e. allow more water to land on the cladding) may still perform well, but they indicate a higher risk of elevated moisture content and potentially staining of the cladding. One practical conclusion suggested by this study is that drip edge performance can be improved by increasing the thickness of the metal drip edge so that it does not require a hemmed edge. For example, it would be better to use 12 gauge without a hemmed edge, compared to a 20 gauge with a hemmed edge, especially when the overhang distance is small.
- Saneinejad, S. and Doshi, H. 2006. Testing of metal flashing for water-shedding effectiveness. Interface Journal [trade publication]. Available at www.rci-online.org/interface-articles-2006.html.
- Smegal, J. 2014. Quantitatively evaluating the effectiveness of different drip edge profiles. Proceedings of 14th Canadian Conference on Building Science and Technology, Toronto, ON. Oct. 28-30, 2014. Available at buildingsciencelabs.com
- Straube, J.F, and Schumacher, C.J. 2005. Driving rain loads for Canadian building design. External Research Program Report, Canada Mortgage and Housing Corporation, Ottawa, ON.
- Straube, J. 2006. Rain control in buildings. Building Science 013. Available from buildingscience.com/documents/digests/bsd-013-rain-control-in-buildings.