a quarter of an inch in diameter to avoid a tendency for the surface tension of the water to bridge across or around the holes to continue movement along the secondary slope of the roof fig2 shows this configuration of the corrugated sheet metal from above without the presence of the roof components normally above it in the usual roof construction these components are laid down in sequence the membrane may be in the form of plastic film that is unrolled as it is laid in place and sealed to adjacent film material around the edges the membrane also may be of initially pourable material that solidifies to a more or less continuous film to deflect the water down the slope of the roof it may be desirable to localize the leakage which may be flowing along the channels provided by the corrugated sheet metal in such cases the arrangements shown in fig3 and 4 may be utilized inserts of open celled foam may be installed in these channels as shown at 28 and 29 each of these inserts has a series of high points as shown at 30 32 in fig4 which approach the full depth of the channels these high portions are separated by the portions 33 35 of shallower depth and the space above these can form a reservoir which accumulates and slows the drainage movement of the water the open celled structure of these inserts permits the water to seep through them down to the bottoms of the troughs where it emerges through the holes 36 39 as.