There are some elements of a roof that are not under the control of the roofer, but will affect their finished work, one being the configuration of the roof, and another is what is discharged onto the roof slope. Unless designers and other trades understand the problem and the consequences of their decisions, the roof may not perform as it should.

Inclined valleys
Back in slating and Tiling Tips number 15 we looked at long rafter lengths and the accumulative effect of water running down the roof, getting deeper as it gets closer to the eaves. In article 58 we looked at inclined valleys and how at shallow rafter pitches and where the area of roof draining into the valley increases, so the width of the valley should also increase to accommodate the increased water flow. Both of these articles relate to the concentration of water flowing down a roof.
     At the bottom of an inclined valley that collects water from up to 100m² of roof (measured in a horizontal plane), or equivalent to 1,000 and 6,000 individual tiles (depending upon the size and type), having rain dropped onto them. The valley collects the water, concentrates it, and discharges it out at the bottom through a gap that is in many cases smaller than the width of one interlocking tile, or a concentration of up to 6000:1. Where a valley discharges onto a lower roof slope the water will flow onto the roof surface at an angle of about 45º to the pitch and so the water is often driven across the surface and into a side interlock or lap. At that point the volume of water will be far greater than at any other point on the roof, and therefore the risk of the side laps and interlocks leaking will be very high, and outside of the minimum pitch parameters issued by the slate and tile manufacturers for their products.
     Below the discharge of a valley, back onto a roof, there should be no penetrations such as roof windows, tile or slate vents, or vent or flue pipes, especially those that have small integral side flashings or a surface grill, that will allow the water from the valley to flow over or into the unit, as in most cases the water flow will be greater than the side lap, or interlock, has been designed to carry. Remember that under the penetration is a hole in the underlay, so any leak could drain straight into the roof.

Gutters
The same sort of thing can happen with guttering. Often plumbers will install guttering with an open end, where an eaves meets a valley, allowing the water in the gutter to discharge out just where the inclined valley discharges back onto a roof, making the water volume at that point even greater, The actual amount of water will depend upon how much roof area is being drained by the gutter. If a stop-end were to be installed, a down-pipe would be needed to drain the water out of the gutter.
     It is possible that a single down-pipe could take away half of the water falling on a simple gable to gable roof, which could easily exceed 100m2 (depending upon the size of the building). The size of the outlet from the rain water pipe is always less than a valley for the same water volume as water drops quicker down a vertical pipe than down an inclined valley.  Where a down-pipe discharges water from a high level roof onto a low level roof the volumes of water can be the same or greater than from an inclined valley, but out through a smaller aperture, making the water faster and more concentrated, making the risk of flooding the side laps and interlocks even greater. The only advantage may be the introduction of a shoe that turns the water to flow directly down the roof.

But if you watch water flowing down a pipe it spins down the inner surface of the pipe and sprays out at the bottom in all directions, once the volume increases above a certain level.  
     Down-pipes can be relocated more readily than an inclined valley, and could be run parallel to the lower level roof along a side abutment wall, or into the head of a lower inclined valley, rather than onto the lower level roof. They can be connected directly into a low level down-pipe, rather than onto the roof, or into the low level gutter, which may be working at full capacity. For some reason designers either forget to show the position of down-pipes on elevation drawings, or go for symmetry, rather than calculate where the down-pipe will perform its function best; so compounding the drainage problems. Once the underground drainage layout has been determined, the cost of moving a low level down-pipe is disproportionate to the cost of amending the guttering system.

Overflow pipes
In many instances external overflow pipes are eliminated to stop water wastage, but where they do occur they can become another source of localised water that when at full flow, can fall onto the roof below adding to the overall water discharge, and result in either a limescale deposit on the roof covering, or the erosion of the surface of the roof covering if left unchecked for many weeks, months, or years.

Conclusion In most cases the more complicated the shape of the roof, the more valleys and roof slopes there will be at different levels. While this makes the building look interesting, it increases the risk of water discharging from either a high level inclined valley, or gutter, onto a lower level roof. Once the designer has determined the roof shape, rafter pitch and roof covering, then there is little the roofing contractor can do to prevent some of the problems associated with excess water flow happening. In most cases the underlay is stopping water getting into the building, but if left unchecked will rot away and a few years after completion, water ingress will appear and people will be looking to determine who to blame – not you I hope.

Tips

• Wherever possible, try and avoid designing a roof where water is discharged back onto a roof, especially at low rafter pitches.
• If there is a situation where there is an excessive water flow onto a lower level roof, report it in writing to the specifier for their consideration.
• Return to site when the building is finished to record what has changed, or been added since the roof was installed, and determine what effect these changes will have had on the performance of the roof.