Why we validate gas protection when the sums look safe

Gas protection is validated because nobody can tell from a finished floor whether the membrane beneath it is complete. Validation is the independent check, made while the membrane is still visible, that it was installed as designed. It is the only point at which a missing collar or an open lap can be found and put right.

The case for it is not obvious from the numbers. A simple calculation suggests a room over a membrane with a 10 mm hole in it would never fill with gas. If one hole does so little, why inspect every lap, corner and pipe, and why hold up the screed until someone has signed the plot off?

This article works through the answer. In short, the calculation is right about the average day and says nothing about the bad ones. Validation is how we know the protection will be there on the bad ones, in a house where it can never be checked again.

What the sums say

On steady, average figures a 10 mm hole in a membrane looks harmless. Take a room 4 m by 3 m with a 2.4 m ceiling on a site classed as Characteristic Situation 3 (CS3). At the top of the CS3 range, the ground under that room gives off about 4.2 litres of gas an hour.

• Solid slab, room perfectly sealed, all the gas enters: 5% methane after about 14 days, and the room full after about 9.5 months.
• The same room with half an air change an hour: settles near 0.03%.
• Block and beam, vented void working: the room cannot exceed the void, which is designed to stay below about 1%.
• Block and beam, vents blocked: the void reaches 5% in about 21 hours.

Methane becomes explosive at 5%, so three of those four cases look comfortable. They rest on three assumptions: gas arrives at a steady rate, it mixes evenly through the room, and the ventilation works. None of them holds for the life of a house.

Gas does not arrive at a steady rate

The 4.2 litres an hour is an average, and ground gas comes in surges. Falling atmospheric pressure draws gas out of the ground. Rising groundwater pushes it up. New paving, a neighbouring development or a change in old mine workings can redirect it towards a building that was previously unaffected.

A site's classification also comes from a limited period of monitoring. The boreholes may never have been read on the worst day. The screening value carries a margin for that, and the protection is designed for the surge, not the average.

The explosion at Loscoe in Derbyshire in 1986 is the usual reference. Atmospheric pressure fell by 29 millibars in seven hours, landfill gas entered a bungalow and a boiler pilot light ignited it. The bungalow was destroyed, its three occupants were badly injured, and 55 households were moved into temporary accommodation.

Gas collects in small spaces

Gas entering through a floor does not spread evenly through a room. It gathers where it comes in and where air barely moves:

• cupboards and the space under the stairs
• boxed-in pipe runs and service ducts
• wall cavities and the void under a bath
• the sub-floor void itself

The same 4.2 litres an hour that takes 14 days to bring a sealed room to 5% takes about 6 hours in a sealed half-cubic-metre cupboard. Those are often the places with a boiler, a meter or a light switch, any of which can ignite it.

Ventilation you cannot count on

The comfortable figures assume half an air change an hour in the room and open air bricks under the floor. Neither is guaranteed.

• Airtight homes. New houses are built far tighter than older stock, so less fresh air is available to dilute anything coming up through the floor.
• Occupants. People close trickle vents, tape over draughts and leave a house shut up for weeks.
• Buried air bricks. Patios, raised beds, extensions and higher ground levels cover the vents that keep a sub-floor void clear.

Blocked vents change the picture quickly. A sealed void under the example room would reach 5% in about 21 hours and fill completely in about 18 days. From then on, every gap in the membrane is fed with near-neat gas instead of diluted air.

Carbon dioxide harms at low concentrations

Methane needs 5% and a spark. Carbon dioxide needs neither. UK workplace limits are 0.5% over a working day and 1.5% over 15 minutes, and a sealed room at the top of CS3 would pass the first of those in about 34 hours.

Gorebridge in Midlothian shows what that means in practice. Sixty-four council homes were built in 2009 without gas membranes. In September 2013 carbon dioxide from old coal workings was found seeping in, and six people were taken to hospital with suspected poisoning.

After nine months of investigation the council decided that evacuation and demolition were necessary. Demolition began in March 2016, with the rebuild estimated at £12 million.

Real defects are bigger than one hole

A single 10 mm hole is the mild case. It has an open area of about 80 square millimetres. One metre of lap that has lifted by a millimetre has 1,000, more than twelve times as much.

The defects that matter on site are usually of that second kind:

• laps left untaped or unwelded along part of their length
• service pipes with no collar, or a collar sealed to the pipe and not to the sheet
• corners cut and left open where three faces meet
• tears made by later trades and covered before anyone looks

That is why the detailing matters and why it is checked. A membrane can only be inspected while it is visible, so verification to CIRIA C735 happens before the floor is covered. The record of that inspection is the only evidence the barrier was ever complete.

Layers, so that no single failure is enough

BS 8485 does not rely on any one measure. It scores the floor structure, the ventilation and the membrane separately, and requires a total that rises with the gas regime and the type of building.

• Floor structure resists gas passing through. It fails through cracks, joints and service holes, and block and beam offers little resistance.
• The ventilated void dilutes gas and carries it away before it reaches the membrane. It fails when air bricks are blocked or buried.
• The membrane seals the floor and its junctions with the walls. It fails through open laps, missing collars and damage by later trades.

The sums in the first section showed the principle at work. A hole in the membrane over a working void was harmless. The same hole over a blocked void, under an airtight room, was not.

Each layer is there to cover for another one failing. Over 60 years or more, at least one of them usually will.

One chance to get it right

The membrane goes in at one point in the build and cannot be fitted properly afterwards. Once the screed is down and the walls are up, a missing or failed barrier means lifting the floor, or worse.

Gorebridge puts a figure on "worse": £12 million for 64 homes is nearly £190,000 a home, before counting the years residents spent displaced.

The rules reflect that. On affected sites, Building Regulations and planning conditions require gas protection, and warranty providers such as NHBC will not cover a plot without evidence that it was installed and verified.

So the answer to the question is that the average day was never the design case. Protection is fitted for the surge, the closed cupboard, the buried air brick and the lap nobody checked, in a house that has to stay safe for as long as it stands.

One chance to get it right. Do the right thing.

About the figures

The fill times and concentrations in this article are rough estimates for illustration, based on a 28,800 litre room, a 1,800 litre void and 4.2 litres of gas an hour. They are not a risk assessment for any site.

Sources

• Ground Gas: The Lessons From Loscoe, Today's Conveyancer
• Newbyres Crescent homes demolished years after gas scare, Edinburgh Evening News
• General hazards of carbon dioxide, Health and Safety Executive
• An Introduction to Verification of Gas Protection Measures, CL:AIRE, for CIRIA C735
• BS 8485:2015+A1:2019 and CIRIA C665, for the gas regime and scoring approach