Intermediate

Foundation Waterproofing: Bitumen Coating & Drainage

14 min read

A damp foundation means a sick house: rising damp in the walls, efflorescence, mould in the crawlspace, distorted interior finishes… and at worst, weakening of the concrete through carbonation. Foundation waterproofing is not optional: as soon as your wall is in contact with the ground, it must be protected. And that protection is never just a coating — it is a three-layer system: impermeabilisation, mechanical protection and water removal. This article details each level, the products to choose, and the application method step by step.

FOUNDATION WATERPROOFING — FULL SECTION Bitumen coating + drainage membrane + perimeter drain Finished ground level NATURAL SOIL BELOW-GRADE WALL FOOTING SLAB / CRAWLSPACE DRY INSIDE Rain Water flow Buried depth LEGEND 1 Bitumen coating (2-4 mm) Applied on clean dry wall 2 Delta-MS membrane Mechanical protection + drainage 3 Gravel 20/40 mm Vertical drainage, 30-50 cm wide 4 Anti-clogging geotextile Wraps gravel and drain 5 Perforated drain diam. 100 1% slope, external outflow Waterproofing ALONE is never enough — must always be paired with drainage build-yourself-a-house.com

Why a foundation needs to be waterproofed

A concrete wall or hollow concrete formwork block (ICF) wall is porous. While it is exposed to air, moisture evaporates as fast as it penetrates. But as soon as it is buried — strip footing, basement wall, crawlspace wall, below-grade wall — the balance reverses: the soil retains water and hydrostatic pressure pushes moisture inward by capillary action.

Without protection, you risk:

  • Rising damp in the walls of the floor above (staining, efflorescence, peeling render)
  • Mould in the crawlspace or basement (odours, fungi, contaminated air)
  • Concrete degradation through accelerated carbonation and reinforcement corrosion
  • Deterioration of insulation in contact with the wall (loss of thermal performance)
  • Direct water infiltration in saturated ground conditions (flooded basement)

Warning — Many self-builders think a simple coat of bitumen paint is enough because “the soil drains well”. That is false. Even sandy soil can become saturated during a heavy rainfall event, and the groundwater table can rise well above the level recorded during the soil survey. Protect systematically, even on a “dry” plot.

The 3 levels of foundation protection

A proper foundation waterproofing system combines three complementary functions. Remove any one of the three and the whole system loses its effectiveness.

Level Role Typical product
1. Impermeabilisation Watertight barrier bonded to the wall Bitumen coating (emulsion, KMB paste, membrane)
2. Mechanical protection Prevents soil and stones from puncturing the coating Dimpled drainage membrane (Delta-MS, Fondaline)
3. Drainage Removes water before it can exert pressure on the wall Gravel + perimeter drain + geotextile

Best practice — A bitumen coating alone (without drainage) is useless on clay soil: water accumulates against the wall and eventually finds a crack. Conversely, drainage without waterproofing is insufficient during prolonged rainfall. All three functions are inseparable.

Choosing your bitumen coating

Question

Three main product families exist, from the most economical to the most effective.

3 TYPES OF BITUMEN COATINGS FOR FOUNDATIONS Comparison — budget, thickness and performance EMULSION Cold-applied bitumen BITUMEN Thickness 0.3 - 0.8 mm Application Roller/brush Price m2 3 to 6 EUR Drying 6-24 h / coat Durability 10-15 years RECOMMENDED USE Detached house moderately damp soil THICK COATING Polymer-bitumen paste (KMB) KMB Thickness 3 - 4 mm Application Trowel / brush Price m2 10 to 18 EUR Drying 2-3 days total Durability 25-40 years RECOMMENDED USE Habitable basement, wet or clay soil MEMBRANE Self-adhesive or torch-applied Thickness 1.5 - 4 mm Application Glued or torch-applied Price m2 18 to 35 EUR Drying Instant Durability 40+ years RECOMMENDED USE Groundwater table, heavily water-laden soil build-yourself-a-house.com

Cold-applied bitumen emulsion

This is the entry-level product, sold in 5 to 25 kg cans (brands Siplast, Soprema, Axton). It is applied by roller or brush in two coats, like a thick paint.

  • Final thickness: 0.3 to 0.8 mm (thin — this is its main weakness)
  • Price: €3 to €6/m² (material only)
  • Strengths: low cost, easy application, compatible with damp substrates for SBS versions
  • Limitations: durability 10–15 years, susceptible to micro-cracks in the substrate, limited protection against water under pressure

Thick bitumen coating (KMB)

KMB (Kunststoffmodifizierte Bitumendickbeschichtung, or polymer-modified thick bitumen coating) is the modern European standard. It is supplied as a paste applied by trowel or brush, in 2 thick passes (2 mm + 2 mm).

  • Final thickness: 3 to 4 mm (10 times thicker than an emulsion)
  • Price: €10 to €18/m²
  • Strengths: high elasticity, bridges micro-cracks, durability 25–40 years, compatible with water-repellent treatments
  • Limitations: higher cost, drying time 2 to 3 days, frost-sensitive during curing

This is the best compromise for a self-build detached house with a habitable basement or damp soil. Brands: Weber (Weberdry PP), Sika (Sika Igolflex), Parexlanko.

Self-adhesive or torch-applied bitumen membrane

Pre-formed membranes offer maximum waterproofing. Two variants:

  • Self-adhesive (type Soprema Alpal, Siplast Paradiene): rolled out and bonded directly onto the primed substrate
  • Torch-applied: used for tanking and basements subject to high hydrostatic pressure

Thickness 1.5 to 4 mm, price €18 to €35/m² excluding installation. Reserved for heavily water-laden soils or proximity to a groundwater table.

Tip — For 90% of detached house projects with a crawlspace or small basement, KMB coating is the right choice. Emulsion should be reserved for dry, well-drained soils (sand, gravel). Torch-applied membrane is only relevant if your G2AVP soil survey revealed a groundwater table less than 2 m below slab level.

How to choose based on ground conditions

flowchart TD A{What type of soil?} -->|Sandy or gravelly| B{Buried depth?} A -->|Silty or clay| C{Risk of groundwater?} A -->|Expansive clay| D[KMB coating + reinforced drainage] B -->|Less than 0.6 m| E[2-coat emulsion
Simple drainage] B -->|0.6 to 1.5 m| F[KMB coating
Delta-MS + drain] C -->|Groundwater below 2 m| G[KMB coating
Full drainage] C -->|Groundwater within 2 m| H[Torch-applied membrane
Tanking] style A fill:#0F4C81,stroke:#0F4C81,color:#fff style B fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style C fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style D fill:#F58220,stroke:#F58220,color:#fff style E fill:#56C6A9,stroke:#56C6A9,color:#fff style F fill:#56C6A9,stroke:#56C6A9,color:#fff style G fill:#F58220,stroke:#F58220,color:#fff style H fill:#CD212A,stroke:#CD212A,color:#fff

Application: step by step

Here is the complete method for the most common self-build scenario: below-grade wall in hollow concrete formwork blocks on a strip footing, buried depth 1 m, silty-clay soil.

Step 1: Prepare the surfaces

The bitumen coating only adheres to a clean, sound, dry and even substrate. Never skip this step.

  1. Wait a minimum of 21 days after casting the wall (or the ring beam) to allow the concrete to release its construction moisture
  2. Remove concrete fins, mortar splashes and laitance projections with a hammer and chisel
  3. Fill insert holes and honeycombing with a fibre-reinforced repair mortar
  4. Form a corner fillet at all sharp arrises (footing/wall junction) with a 45° mortar fillet — bitumen cannot follow 90° angles
  5. Dust down with a brush and industrial vacuum

Warning — Never apply bitumen coating on a damp substrate (unless the product is specifically rated “compatible with green concrete”). Even a wall that “looks dry” can contain 5 to 8% residual moisture — enough to cause blistering under the bitumen layer. Test with a pin-type moisture meter: result must be < 4% for most coatings.

Step 2: Apply the bonding primer

The bitumen primer (or “pore filler”) creates the chemical bond between the concrete and the coating.

  1. Stir the can thoroughly, dilute if necessary according to the technical data sheet (often 20–30% water)
  2. Apply with a long-pile roller or wide brush over the entire surface to be waterproofed
  3. Observe the waiting time (2 to 6 h depending on product) before the main coating
  4. Re-treat absorbent areas (the primer must form a uniform film, not just wet the concrete)

Typical consumption: 150 to 300 g/m² depending on substrate porosity.

Step 3: Apply the bitumen coating (KMB)

Tip

KMB is applied in 2 crossed passes.

  1. Apply the first pass with a notched trowel (4–6 mm teeth) in vertical bands, consumption ~2 kg/m²
  2. Immediately smooth with a flat stainless steel trowel to even out
  3. Embed a fibreglass reinforcement mesh (4×4 mm mesh) over angles and concrete/block joints — essential at sensitive junctions
  4. Wait for the tack-free drying (4 to 12 h depending on temperature, never below 5°C)
  5. Apply the second crossed pass (horizontal direction), consumption ~2 kg/m²
  6. Check final thickness with a wet film gauge: minimum 3 mm dry

Best practice — Bring the coating down to the top face of the footing and turn it up 5 to 10 cm above finished external ground level. This “high capillary break” prevents water from bypassing the waterproofing from above. At the bottom, the junction between footing and wall must be reinforced with a corner fillet and mesh.

Step 4: Fix the drainage membrane (Delta-MS)

The coating alone will not withstand backfill — stones and roots will eventually puncture it. The dimpled drainage membrane is its physical protection, and it also channels water downward.

  1. Wait for the coating to dry completely (minimum 3 days, ideally 7)
  2. Unroll the membrane dimples facing the wall (towards the waterproofing), smooth face towards the soil
  3. Fix at the top with a fixing profile nailed into the concrete (every 25 cm)
  4. Never nail through the main field — perforations would pass through the waterproofing
  5. Overlap vertical runs by at least 10 cm and horizontal runs by 20 cm
  6. Bring the membrane down to the drain (do not tuck it under the drain)

Tip — The Delta-MS membrane comes in two thicknesses: 8 mm (standard) and 20 mm (heavy-duty). For a wall buried more than 1.50 m, or if the backfill contains a lot of angular stones, use the 20 mm version. Extra cost: only €2 to €3/m².

Step 5: Install the perimeter drain

The drain collects water that has passed through the gravel and discharges it away from the building.

  1. Dig a trench at the base of the footing, 30 to 50 cm wide, 10 to 15 cm deep below the underside of the footing
  2. Line the bottom and sides with geotextile (200 g/m² grade, brand Afitex or equivalent) — leave 1 m overhang to fold over after backfill
  3. Lay a 5 cm bed of 20/40 mm gravel
  4. Unroll the perforated PVC drain Ø 100 mm (perforations facing down)
  5. Maintain a minimum fall of 1 cm/m towards the outfall (soakaway, inspection chamber, surface water sewer)
  6. Surround the drain with 20/40 mm rounded gravel (not crushed aggregate — sharp edges can block the perforations)
  7. Fill with gravel to 30 cm below finished level
  8. Fold the geotextile over in a parcel wrap (30 cm overlap)
  9. Backfill with topsoil or a filter material

Warning — The drain must have a gravity outfall (inspection chamber, infiltration basin, deep soakaway, surface water sewer). A drain that cannot empty becomes a reservoir — the exact opposite of what you want. If your plot is flat, plan a soakaway of at least 1 m³ at the lowest point, or connect to the surface water sewer with planning consent.

Common special cases

Party wall or wall against an existing structure

It is impossible to waterproof the outside once the neighbour has built. Two solutions:

  • Interior tanking: waterproof render (type Weber.tec Superflex, Sika 107) applied on the dry side. Effective but invasive (the interior face must be exposed)
  • Resin injection into the wall to create a horizontal damp-proof course (specialist work, €80–€150/m)

Service pipe penetrations through the wall

This is the classic weak point. Every drainage or surface water pipe or electrical conduit penetration must be:

  1. Sleeved in the concrete (PVC duct projecting 3–5 cm on both sides)
  2. Sealed with a bitumen-polymer sealant/adhesive or a swelling joint (brand Hilti CP 611A or equivalent)
  3. Covered by the bitumen coating with reinforcement mesh over 10 cm around the penetration

Junction with the horizontal DPM (capillary break)

The vertical waterproofing of the buried wall must connect to the horizontal DPM between the footing and the first course of masonry (bituminous felt type Ipanol, or EPDM DPC strip). Otherwise moisture rises through the junction.

Classic mistakes to avoid

Mistake Consequence Remedy
Coating on green concrete Blistering, delamination Wait 21 days min + moisture meter
Single coat only Insufficient thickness, cracking Always 2 crossed passes
Sharp arrises not filleted Cracking at the corner 45° mortar corner fillet
No drainage membrane Puncturing, direct backfill on coating Delta-MS as standard
Drain without fall Standing water, ineffective Minimum 1% fall to outfall
Drain without geotextile Silting within 2–3 years 200 g/m² geotextile wrap
Pipe penetrations not sealed Localised water infiltration Sleeve + sealant + mesh reinforcement
No high capillary break Run-off water bypasses waterproofing Bring coating 5–10 cm above finished ground

Standards and regulations

  • DTU 20.1 (masonry of small elements) — requires protection of buried wall faces
  • DTU 14.1 (tanking works) — applies when the wall is subject to permanent water pressure
  • NF P 84-204 (DTU 43.1) — reference standard for bituminous systems
  • Fascicule 67 — titre III: application of polymer-bitumen waterproofing
  • CSTB technical approvals: certified products are covered by the decennial guarantee

Tip — Keep the delivery notes, technical data sheets and a few photos of the key stages (prepared surface, primer, first pass, second pass, membrane fitted, drain in place). In the event of a claim, these documents will prove compliance with the DTU to your insurer. Without them, your dommages-ouvrage policy may refuse to cover the loss.

Total cost for 40 m run (100 m² house footprint, 1 m buried depth)

That is 40 m² of wall to waterproof and drain.

Item Quantity Unit price Total
KMB coating (2 passes, ~4 kg/m²) 160 kg €4 to €7/kg €640 to €1,120
Bitumen primer 10 kg €5 to €8/kg €50 to €80
Reinforcement mesh (corners) 50 m² €3 to €5/m² €150 to €250
Delta-MS membrane 8 mm 45 m² €6 to €10/m² €270 to €450
Top fixing profile 40 m €2 to €3/m €80 to €120
Perforated drain Ø 100 45 m €1.50 to €3/m €70 to €135
Geotextile 200 g/m² 60 m² €1 to €2/m² €60 to €120
20/40 mm rounded gravel 8 m³ €35 to €50/m³ €280 to €400
Total materials     €1,600 to €2,675

As a self-build, allow 3 to 5 days for two people to complete everything (preparation, coating, drainage, backfill). A contractor would charge €4,500 to €7,000 for the same work — the saving is substantial.

Waterproofing fits between building the below-grade wall and backfilling the foundations.

  1. D+0: casting the basement walls
  2. D+21: surface preparation (dust, corner fillets, filling holes)
  3. D+21: primer
  4. D+22: first coat + mesh at corners
  5. D+23: second crossed coat
  6. D+26: fitting the Delta-MS membrane
  7. D+27–28: drain + geotextile + gravel
  8. D+29: backfill and compaction in 30 cm layers

Checklist: foundation waterproofing and drainage

  • Basement walls cured for at least 21 days
  • Surfaces dusted, fins removed
  • Arrises filleted with 45° mortar corner fillets
  • Holes and honeycombing filled
  • Substrate moisture content < 4%
  • Bonding primer applied uniformly
  • First KMB pass (2 mm) + mesh at corners
  • Second crossed pass after drying
  • Final thickness checked with wet film gauge (3 mm min)
  • Pipe penetrations sleeved + sealed + reinforced
  • High capillary break: coating turned up 5–10 cm above finished ground
  • Delta-MS membrane fitted dimples towards wall
  • Overlaps: 10 cm vertical, 20 cm horizontal
  • Top fixing profile nailed at head only
  • Drain trench dug below footing
  • 200 g/m² geotextile laid in trench
  • Ø 100 drain, perforations down, 1% fall
  • Gravity outfall connected (chamber, soakaway, surface water sewer)
  • 20/40 mm rounded gravel around drain
  • Geotextile folded over in parcel wrap
  • Backfill in 30 cm compacted layers
  • Photos and technical data sheets archived for insurer