Blocked gullies and grids
The commonest cause is also the least dramatic. A yard gully or a driveway channel fills with leaves, moss, grit washed off a slabbed area and general silt, and the trap underneath silts up until only a trickle gets through. In dry weather nothing shows. In heavy rain the surface water has nowhere to go and ponds against the building.
Lift the grid, scoop the silt out by hand and flush the trap through with a hose. If the water still will not go away, the pipe below the gully is restricted and needs jetting rather than cleaning.
A surcharging sewer
Where surface water and foul water share a combined sewer, and much of older central Scotland does, a heavy storm can fill the public sewer faster than it can discharge. The level rises, and when it rises above your connection the flow backs up into your private drainage. You see all chambers full to the same level, water coming up rather than draining down, and everything clearing on its own within an hour or two of the rain stopping.
That pattern is not a private blockage and jetting will not fix it. It is worth reporting to Scottish Water, because repeated surcharging on the same street is recorded and is what drives network improvement work. In the meantime a non return valve on your connection is the practical defence for a property that floods repeatedly.
A failed or undersized soakaway
Properties away from a surface water sewer often discharge roof water to a soakaway, essentially a stone filled pit that lets water disperse into the ground. Soakaways silt up over twenty or thirty years, and heavy clay subsoil, which is widespread across West Lothian and Lanarkshire, slows dispersal even when the pit is sound.
The symptom is a downpipe that backs up and overflows at the shoe, or a permanently boggy patch of garden in the same place every winter. The remedy is either cleaning and re-forming the existing soakaway or installing a modern crated system with proper silt trapping ahead of it.
Partial collapse and root ingress
A pipe with a partial collapse or a root mat still passes normal daily flow, which is why nothing seems wrong most of the time. Storm flow arrives at several times that rate, meets the restriction and backs up. This is the fault most often missed, because by the time anyone attends the rain has stopped and the drain runs perfectly.
The way to catch it is a CCTV survey rather than a visit during a storm. The camera shows the defect and its distance from the chamber whether or not it is raining, and that turns a seasonal mystery into a measured repair.
Telling them apart
- Only surface water affected, foul drainage fine: gully, channel or soakaway.
- Everything full to one level and clearing after the rain: surcharging sewer.
- Same downpipe overflowing every time: blocked gully or failed soakaway on that side.
- Foul chambers backing up in rain only: partial restriction in your run, worth a camera.
- Neighbours affected at the same time: network issue rather than a private fault.
What we do about it
We jet gullies, channels and surface water runs to full bore, clear silt traps, and survey the run afterwards so you can see it is actually clear rather than just flowing on the day. Where a soakaway has failed we can excavate and rebuild it, and where a partial collapse is the cause we can line or replace the section.
Rainfall problems are worth dealing with in a dry spell rather than during the next storm, when everyone in the area is ringing at once.
Getting ahead of the next storm
Rainfall faults are best dealt with in a dry week. Everything is accessible, nothing is under water, and you are not waiting behind every other property in the street that flooded on the same night. A seasonal clear of the gullies, channels and silt traps takes under an hour on a typical house.
If your property has flooded more than once in the same place, ask for the surface water run to be jetted and cameraed rather than just cleaned at the top. We record the inspection and give you the footage, which is also the evidence an insurer will want if the flooding recurs.
Questions we get asked
Is rainwater flooding my responsibility?
Private drainage on your land is yours. The public sewer and the road gullies are not, and repeated street flooding should be reported so it is recorded.
Can I connect a downpipe to the foul drain?
Not where a surface water system exists. Adding roof water to a foul run overloads it and is the reason many properties flood in a storm.
How long does a soakaway last?
A well built soakaway with silt trapping can last decades, but on heavy clay and without maintenance twenty years is a realistic life.
Will a non return valve stop backflow?
It stops sewage entering your property from a surcharging public sewer, but it does need occasional inspection to remain reliable.