The vast majority (think 90%) of the rat population lives in the sewer systems of towns and cities.
Since these were built in the Victorian era they were very quickly populated by rats as they provide the ideal habitat.
Think of all of the food you scrape off your plate or push down the kitchen sink every day – this all ends up in the sewers.
Think of all the hot showers and hot taps you run every day – that all ends up in the sewers (sewers are typically 3-7 degrees C warmer than surface temperatures as a result).
In short, it’s a sheltered, warm and a very food-rich place – and pitch black too which suits their way of life as nocturnal creatures.
There is plenty of space to eat and breed and there are no humans (or other predators) to persecute them.
Because it’s even warmer and drier (building fabrics rarely drop below 15 deg C). Most invaders will be nesting females - cavity wall insulation and loft insulation provides ideal nesting sites and often they can access foods within the living areas too.
Structural timbers, wires and plastic pipes also provide material to gnaw on which is otherwise hard to find within sewers – gnawing is an essential activity for rats (and all other rodents) as it keeps their teeth in check.
The sewers contain the rat population and that rat population likes to nest and rest within houses.
The bit that connects houses to sewers are the drains – and this is where the vast majority of entry routes arise.
Most common scenarios are as follows:
This happens when a downstairs toilet or external soil vent pipe (SVP) has its position moved but the old pipe is not capped off.
Period properties with modern extensions are classic cases for this one as the original ‘outhouse’ or cast iron SVP is demolished to make way for the extension - the open pipe left behind is simply built over or primitively sealed using expanding foam or a rubble sack.
Some Victorian properties pass their waste under the footprint of the house via a clay pipe.
These clay pipes can crack, collapse or have sections come apart allowing rats to emerge under the floor (but the waste can still flow through OK).
Inevitable where a modern extension has been added or a modern soil vent pipe (SVP) introduced.
Clay pipes had ½” thick walls compared to the 4mm thick walls of modern PVC pipes – this means their overall dimensions are closer to 5” than the 4” of PVC.
A rubber flexible coupling is typically used to join the two pipes together which literally represents nothing more than a rubber sock and 2x hose clips.
Rats can chew through these like butter and they’ll often come apart during slab settlement.
Modern plastic pipes are made from PVC of around 4mm wall thickness – provided rats can access a leading edge, they can chew through these very easily.
Leading edges arise where 2 sections of pipe are not fully fitted or where 87.5 degree bends are introduced.
Particularly vulnerable are the ‘Quickfit’ waste pipes which are of a concertina design and very thin walled to maintain flexibility – these virtually encourage rats to chew through!
Up to around the 1960’s, these were usually cast iron and extended up above the roof line so as to disperse foul smells.
Cast iron corrodes quite heavily internally (as this is the unpainted part) and this corrosion allows rats to grip and climb up the inside.
They can then emerge out the top of the SVP and enter roof voids under the tiles above the gutter or other such weak spots.
Comparatively rare this one but definitely one to consider.
Guttering downpipes carry ‘white waste’ (rainwater) rather than the ‘grey waste’ (foul water) that goes into sewers.
Despite modern building regs stipulating that white waste should not be connected to grey waste drains, builders still do it.
Then, where these gutters are short – for example on single storey extensions – rats are able to climb up the insides and emerge into the gutters whereupon they can access the flat roof cavity via soffit defects.
Because gutter pipes are so thin (68mm) the rats don’t need to grip the smooth insides – they simply push themselves against the inside of the pipe and wriggle up in a similar undulating movement to a snake.
More common than you might think but nowhere near as common as the first four scenarios.
Surface rat activity can develop where food is available through bird tables, dog/cat food left outside, compost heaps, uncontained refuse etc.
If an opportunity presents itself for rats to gain access to a building fabric, then they will enter for the same reasons sewer rats do – it’s drier and warmer than outside.
Most common scenarios are as follows:
Air bricks can be cast iron, ceramic or plastic and are there to ventilate wall cavities and floor cavities and so keep damp issues at bay.
Sometimes these air bricks are damaged or have pipes/cables passed through them – defects such as these allow rats to access the wall cavities or floor cavities and then activity establishes within the building fabric.
Plastic air bricks are the most high risk – these can be chewed through by rats overnight.
Water pipes often pass through walls to service outside taps or carry waste water to outside gulleys.
If the walls are cavity walls and the gap around the pipe is more than 10mm, then this could be a potential entry point to the building fabric.
Modern buildings have external gas and electrical meters that then pass a feed into the building via a hole in the wall.
Again where cavity walls exist and the gap around the feed is more than 10mm, then this could be a potential entry point.
A common problem for extensions and for uPVC doors in particular – there is often a gap between the brickwork and step as the builder will always make a doorway larger than the door to make fitment easier.
Expanding foam is used to fill the gaps but this is very quickly chewed through enabling surface rat activity to access floor voids and wall voids.
The problem with living in towns and cities is a that a fully detached existence is rare.
Most houses will be within a terraced row or block of flats – semi-detached at best.
This means you will have a shared building fabric via cavity walls, roof voids or enclosed service ducts and rats entering the building fabric of your neighbour can travel across.
This is a particular headache as the route of entry is within the neighbours property and therefore you need them to be proactive in resolving this (not always the case).