Substructure and ground floor slab: damp, radon and insulation

The substructure is everything below the first floor level that you will never see again: the foundation walls, the drains that run under the house, the damp and radon protection, the insulation and the concrete slab. It is a short stage on site, often one to three weeks, but it hides more than any other. Below: what happens, what to ask for and what to photograph before the slab covers it.

What happens, in order

1. Foundation walls. Blockwork or concrete rises from the foundation to just above ground level. Cavity walls need the cavity filled or the lower courses built to the designer’s detail.
2. Underground drainage. Foul and surface water pipes are laid, with falls, bedding and inspection chambers. Pipes that pass through the foundation walls need sleeves or flexible joints so that settlement does not crack them.
3. Hardcore. Clean, compacted stone fills the footprint to the level the engineer or designer specified.
4. Blinding. A thin layer of sand or fine stone protects the membranes from sharp stone.
5. Radon and damp-proof membrane. A sheet laid across the floor and lapped up the walls, with taped or welded joints.
6. Insulation. Rigid board goes under or over the slab, depending on the design.
7. Reinforcement and services. Mesh, underfloor heating pipes, and ducts for electrics and water are fixed in place.
8. The pour. Concrete is placed, levelled and finished. It must cure, which means keeping it damp and protected, before it takes load.
9. Damp-proof course. The DPC is bedded in the walls above ground level and lapped to the membrane.

Who does what

The builder or the groundworkers do the work. Your designer specified the layers; your engineer has specified the concrete and any reinforcement. Your assigned certifier inspects at the points listed in the inspection plan. If your lender needs a stage inspection (often described as “foundations” or “ground floor level”), the lender’s inspector looks separately. See the assigned certifier and staged drawdown.

Radon: ask this before the slab goes in

Radon is a radioactive gas that forms in rocks and soils and can enter a house through cracks in floors and gaps around pipes and cables. The Environmental Protection Agency (EPA) says it is a cause of lung cancer in Ireland, and its map shows High Radon Areas, where more than 10% of homes are predicted to exceed the national reference level. Radon barriers and sumps are far cheaper to build into a new slab than to add afterwards.

Ask your designer, before the slab:

• Which radon measures apply to my site, and where are they on the drawings?
• Is there a sump, and has the pipe from it been left in a place where a fan could be added?
• Is the membrane taped at every joint, around every service pipe and up to the DPC?
• Who will inspect it before the concrete goes on, and how will I see the record?

After the house is finished, test for radon. A test is the only way to know your level. The EPA page explains how.

Damp and the damp-proof course

Water gets into a building where a barrier has a gap. The usual gaps are:

• A membrane torn by a boot, a barrow or a rebar chair.
• A service pipe pushed through the membrane after it was laid, with no collar and no tape.
• A DPC that is not lapped to the membrane at corners or openings.
• Mortar dropped into a cavity that bridges it and carries water across.

Photograph the membrane before the reinforcement and insulation go on. Photograph the DPC at a door threshold, which is the highest-risk point. Ask that nobody walks on the membrane in boots and that repairs are taped, not left.

Floor insulation

The ground floor loses heat through its whole area. How much insulation and where it sits depends on the floor build-up in the drawings and the energy calculation behind the house’s Building Energy Rating (BER). Common options are rigid board under the slab, over the slab beneath a screed, or a mix. If you have underfloor heating, the insulation under the pipes matters twice: it reduces heat loss and makes the floor respond faster. Check:

• The board is the thickness and type in the specification.
• Joints are tight, and any gaps at the edges are filled and taped.
• Edge strips (a band of insulation at the wall edge) are in place so that the screed does not touch the wall.
• If you use a timber ground floor, ventilation of the void is designed in.

See nZEB and Part L for the energy targets the floor helps to meet.

Underfloor services

Once the slab is poured, a pipe in the wrong place needs a cutter. Before the pour check:

• Foul and surface water drains are tested or CCTV-surveyed if the specification says so, and the inspection chambers are where the drainage plan has them. See site works and services.
• Sleeves and ducts for incoming water, electricity, telecoms, and a future heat pump, solar cable or ventilation duct.
• Underfloor heating pipes are pressure-tested, with the gauge reading and a photograph on record.
• Tank, sump and wastewater pipes that cross the footprint are in the right place and cased where the design says so.

Pouring and curing

The slab is poured in one day where possible, with the concrete supplier’s docket kept. Concrete reaches design strength over weeks, but it can be built on much sooner. It should not dry too quickly in hot, windy weather or freeze in the first days; ask your builder how it will be protected. A power-floated finish is common when the slab is the finished floor surface. Where a screed will go over it, a rougher surface is fine.

A sand and cement screed needs far longer to dry than most people expect. The usual rule of thumb is about a day per millimetre of thickness up to 50 mm, and longer beyond that, but your screed supplier’s guidance for your product is the one to follow. Flooring that goes over it, particularly timber and vinyl, needs a moisture reading below the flooring manufacturer’s limit. See plastering, screed and drying out.

What goes wrong, and what it costs

• Concrete poured before the membrane inspection. There is no way to check it afterwards without breaking out. Ask for a “do not pour until released” instruction to the builder.
• Drains not tested before they are covered. A cracked pipe under a slab is a major repair.
• Missing sleeves. Core drilling through the slab later is possible but messy, and it can breach the membrane.
• Radon sump left out. Adding mitigation later costs far more than building it in.
• Insulation boards left open to rain. Wet boards or a flooded slab base can trap moisture.
• The slab is the wrong level. A finished floor level that does not match the drawings affects thresholds, steps and the planning permission. Check it with a level before the walls go up.

Payments, site risks and cover

Lenders often tie a stage payment to the foundations or the ground floor slab. Confirm the stage name, who certifies it and what evidence they need before you tell your builder it can be invoiced. The stage payment planner shows the effect on your cash.

At this stage the site is open and the materials are valuable: reinforcement, insulation and fuel. Storm damage is rare in a slab, but water can flood excavations, and a pile of block or insulation can be stolen or blown away. Check that your contract works and liability cover is already in force and what its excesses are. If you are managing the build, look at insurance when you project manage, and when you are ready use the cover checker to see the types of cover that suit your build.

Before the walls go up

• Photographs of drains, membrane, insulation and reinforcement before the pour.
• The inspection record from your certifier or engineer.
• A level check on the slab and the DPC.
• The next stage: walls up to wall plate.