Moisture barrier and blinding concrete: the two layers that protect foundations from below

After excavation and termite treatment, and before any footing steel is fixed, come two layers most owners barely notice: the polyethylene sheet and the blinding concrete. Both are thin, both end up buried for good, yet how well they are done affects how well your concrete and steel are protected from moisture and salts, and how accurately the footings themselves are built. Here is what each layer does, how to lay it properly, the usual mistakes, and what to check before the rebar goes in.

Where these layers fit in the sequence

The below-ground sequence we follow is:

1. Excavate to founding level, trim and compact the base.
2. Treat the soil against termites with approved products, as explained in termite treatment before foundations.
3. Lay the polyethylene sheet against moisture.
4. Pour the blinding concrete on top.
5. Mark the footing grid on the blinding, then fix formwork and reinforcement.

Details vary with the drawings and specification. On some raft and basement projects the engineer adds a further waterproofing membrane, such as a bituminous sheet, on top of the blinding before the reinforcement. The principle is the same: keep the structural concrete separated from the ground and its moisture.

The polyethylene sheet: what it does

The "nylon" you will hear about on Saudi sites is polyethylene sheeting, supplied in wide rolls. Under foundations it does three jobs.

It stops rising damp. Soil holds moisture, which can rise into concrete by capillary action, bringing dissolved salts such as sulphates and chlorides with it. Over the long term these are among the worst enemies of concrete and reinforcement.

It keeps the mix water in the concrete. Dry ground, common in Riyadh, sucks water out of fresh concrete poured directly on it, weakening the bottom layer. The sheet prevents that.

It separates treated soil from the concrete, keeping the termite-treated layer in place and the concrete clean.

Laying it properly

Keep boots and equipment off the sheet as much as possible, and lay temporary boards where workers must cross.

Blinding concrete: what and why

Blinding is a thin layer of plain, unreinforced concrete poured over the polyethylene under footings and ground beams, or across the whole footprint for a raft. It is typically around 5 to 10 cm thick as specified by the engineer, with a leaner mix than the footings because it carries no structural load. It gives you:

• A clean, level working surface. Steel fixers work on a firm, flat base instead of loose soil or a shifting sheet.
• Accurate setting out. Footing grid lines and edges are marked directly on the blinding, reducing position and size errors.
• Reliable concrete cover. Spacers sit on a hard surface instead of sinking into soil, so the concrete cover under the steel, the protective layer between bar and surface, stays as designed.
• Protection for the sheet during steel fixing and carpentry.
• Clean footing concrete, with no soil mixed in during the pour.

Pour it slightly wider than each footing so formwork can sit on it firmly, set its top level accurately because it fixes the underside of the footing, finish it flat, keep it damp, and let it firm up before work starts on it.

Protection doesn't stop at the blinding

The sheet and blinding protect the underside of the foundations. The buried sides and tops need their own protection:

• coat footings, necks and ground beams with bitumen or a product the engineer specifies, once the concrete has dried and before backfilling;
• pay extra attention to corners and joints;
• don't backfill before the coating has cured, and don't use fill with sharp stones that can damage it.

Where there is a basement or high groundwater, waterproofing becomes a full system designed by the engineer, which we handle under Infrastructure Works.

Common mistakes on site

• Blinding on uncompacted soil, which cracks or settles and throws out the footing levels.
• Torn or unlapped sheet, rolls laid side by side with no overlap or punctured by stones and boots.
• Skipping the blinding to save time, fixing steel straight onto the sheet or soil. The usual result: sunken spacers, steel touching soil and missing cover.
• Uneven blinding, so some bars sit closer to the bottom than others.
• A long gap between spraying and laying the sheet, leaving treated soil exposed to sun, wind and dust.

Owner's pre-rebar checklist

• Was the soil treated for termites before the sheet went down?
• Is the sheet intact, with clear overlaps between rolls?
• Is the blinding level and wider than each footing?
• Are the grid lines marked on the blinding and checked against the drawing?
• Are spacers sitting on firm blinding under the steel?
• Do you have photos of each layer before it was covered?

How Marafi does it

These two layers are a fixed part of how we work. After spraying approved termite treatment, we lay the polyethylene sheet carefully to prevent moisture, with proper laps and every tear patched, then pour blinding concrete to an accurate level and a flat finish, protecting the foundations and the life of the building. We then set out the grid and build the footings precisely, following the engineering drawings, and send owners photos of each stage before it is covered.

More on this work at Foundations & Footings, and on what comes next at Reinforced Concrete Works. For how the foundation type itself is chosen, read foundation types and Riyadh soil, and see the whole sequence on How we build.

Summary

• The polyethylene sheet stops rising moisture and salts and keeps mix water in the concrete.
• Blinding gives a clean, level base for accurate setting out and proper cover.
• Do it right: compacted base, intact sheet with good laps, flat blinding at the right level.
• Buried concrete needs bitumen or a specified coating before backfilling.
• Ask for photos of every layer before it disappears.

Planning foundations in Riyadh? Contact us to arrange a site visit and drawing review, and we will send an itemised proposal by stage.

Frequently asked questions