Damp-Proof Course and Foundation Waterproofing: Stop Ground Moisture at the Plinth

Damp-proof course: what it is and where it goes

A damp-proof course, or DPC, is a continuous waterproof band built into every wall at plinth level. Brick, mortar and concrete are full of fine pores that draw ground moisture upward. The damp proof course is the one layer that water cannot climb past. Below it sit the footings and foundation walls, which stay in contact with wet soil for the life of the building and are coated before the soil goes back. It is a small job at plinth stage, and it decides whether the walls above stay dry. It is also the hardest waterproofing to add later. Five checks tell you what your building needs.

Find the right DPC and foundation waterproofing route

Match your building to one route. Each route says what is done and takes you to its section.

Build it once, then keep it clear. A damp-proof course rarely fails on its own. It is beaten by soil, paving, plaster and floors that are later raised past it.

Damp-proof course finder

Answer three questions for a starting recommendation, then confirm it with an Armstrong expert.

DPC in construction: laying the damp-proof course at plinth level

In new construction the damp-proof course is laid on top of the plinth masonry or plinth beam, before the walls go up. The usual form in Indian practice is a layer of rich cement mortar or cement concrete made watertight with an integral waterproofing compound. The DPC chemical is that compound. Armstrong makes three that suit the job.

All three suit a damp-proof course. They differ in dosage and form, not in fitness for the job. Pick the one that fits the site, or ask an Armstrong expert to choose with you.

1. 1
   Fix the DPC level

   Set the DPC at plinth level, at least 150 mm above the highest finished ground or paving that will ever sit against the wall. Decide the final outside levels first, because the paving comes later and is what usually bridges the DPC.Why: Raised paving or soil above the DPC carries water around it. Setting the level against the highest final ground stops that.
2. 2
   Prepare the bed

   Level the top of the plinth masonry or plinth beam. Remove all loose mortar, dust and laitance, and dampen the surface so it does not pull water out of the DPC mix.Why: A dry, dusty bed pulls water out of the DPC mix and leaves a weak, porous layer.
3. 3
   Dose the mix

   Add the integral compound to the gauging water for each bag of cement: CONPROOF LW 200 at 200 ml, CONPROOF No. 1 at 100 ml, or CONPROOF 500 at 500 ml. Keep the water low. Extra water makes a porous DPC.Why: The compound only works at its stated dose, and extra water makes the DPC porous. Accurate dose and low water give a dense course.
4. 4
   Lay the DPC to full width and thickness

   IS 3067:1988 describes a damp-proof course of cement mortar 1:4 or cement concrete 1:2:4, 12 mm thick, with a waterproofing compound to IS 2645. Many site specifications use a thicker concrete course. Whatever is specified, lay it across the full thickness of the wall, not set back from either face.Why: A thin or narrow DPC leaves moisture a path round its edge. Full width and thickness close every path.
5. 5
   Make it continuous

   Run the DPC through every wall, outside and inside, with no gaps at corners, junctions or door openings. Where floor levels differ on the two sides of a wall, a vertical DPC on the wall face joins the two horizontal courses.Why: A DPC with a gap at a corner or door opening lets damp straight up. Every wall, inside and out, is joined into one barrier.
6. 6
   Compact and finish it

   Compact the course fully and finish it level. A honeycombed or cracked DPC lets moisture through.Why: A honeycombed or cracked DPC is a sieve. Full compaction closes the voids.
7. 7
   Cure it

   Cure for at least 7 days before building on it.Why: Cement only gains strength and density while it is kept moist. A DPC that dries in the first days stays porous.
8. 8
   Add a bituminous coat where it is specified

   Some specifications call for a bituminous coat over the cured DPC. RAINSEAL BMW 250 is the Armstrong water-based bituminous coating, listed for new and old masonry and concrete. It is applied in two coats and can be thinned with 20% water. Where mortar is to follow, fine dry sand is sprinkled on the final coat while it is wet, to give the mortar a key.Why: The bituminous coat is a second line of defence over the cured course. It seals any pores the mortar left.
9. 9
   Keep it clear as the building goes up

   Stop the outside plaster at the DPC line. Keep fill, paving and garden levels below it. Set the inside floor so it does not bridge above it.Why: A DPC works only while nothing bridges it. Plaster, paving and floors built over it undo the work.

The same compounds go into the plinth plaster and the wall plaster above. For the rest of the building, see new construction waterproofing and waterproof concrete.

Foundation waterproofing: coat footings and foundation walls before backfilling

The damp-proof course protects the wall above the plinth. The footing and foundation wall below it sit in damp soil permanently. Coating them keeps ground moisture and soil salts out of the concrete and masonry and reduces the load on the DPC. The soil side can only be reached before the trench is backfilled, so this is done at foundation stage or not at all.

The cementitious coat goes on first and the protective coats go over it. Pick the products that fit the ground conditions, or ask an Armstrong expert to choose with you.

1. 1
   Clean the surface

   Remove soil, laitance, release agent and all loose material from the faces to be coated.Why: The coating bonds only to clean, sound concrete or masonry, not to soil and release oil.
2. 2
   Repair honeycombs and holes

   Fill honeycombs, voids, tie-rod holes and gaps around pipes with a polymer putty of ROOFBOND SBR 401, cement and graded sand in a 1:1:1 ratio, as in the Armstrong retaining wall procedure.Why: Holes and voids are leak paths straight to the inside, and a coating bridging over a void is unsupported.
3. 3
   Plaster masonry foundations

   Brick or stone foundation walls are plastered first, with a compound such as CONPROOF 500 in the plaster, to give the coating an even surface.Why: Rough brick and stone take coatings unevenly. A plaster coat gives an even face for the coating to bond to.
4. 4
   Apply the cementitious coat

   Apply ROOFCOAT FLEXI 100 or ROOFBOND RFX PLUS 210 as a continuous, uniform coating over the whole face, in two coats. ROOFCOAT FLEXI 100 is mixed at 1 litre to 2 kg of cement and 200 ml of water.Why: The cementitious coat is the first waterproof layer. Two coats close the pinholes a single coat leaves.
5. 5
   Apply the protective coats

   Apply two coats of RAINSEAL BMW 250 diluted with 20% clean water. Each coat dries in 40 to 50 minutes. Let the first dry before the second. Where the site engineer specifies it, the Armstrong procedure allows two coats of ROOFEX 2K 711 as the alternate.Why: The bituminous coats resist the dilute salts and acids in soil and add a second membrane over the first.
6. 6
   Carry the coating up to the DPC

   Take the coating over the top of the footing and up the foundation wall so that it meets the damp-proof course, leaving no uncoated strip between them.Why: An uncoated strip between coating and DPC is where ground moisture climbs into the wall.
7. 7
   Protect it, then backfill

   Protect the cured coating with a mortar screed or protection board before the soil goes back. Backfill carefully, with no sharp rubble thrown against the wall.Why: Backfilled soil and rubble scrape and puncture a coating. A protection layer keeps it whole for the life of the building.

Basements, retaining walls and high ground water

Where the wall holds back soil or water, the job is bigger than a damp-proof course. For rooms below ground see basement waterproofing. For walls that hold back earth see retaining wall waterproofing. For pits see lift pit waterproofing.

What bridges a DPC: rising damp in a house that already has one

Most houses with a damp band at the base of the wall do have a damp-proof course. It has been bridged. Over the years soil, paving, plaster and floors get raised until they give water a way around it. Clearing the bridge is a repair, not a rebuild, and it is always tried before anyone cuts into the wall.

1. 1
   Find the DPC line

   Locate it outside and inside and mark it right round the affected walls.Why: Every later check depends on knowing exactly where the DPC runs, inside and outside.
2. 2
   Check every bridge in the table

   More than one is common. Check the whole length of the wall, not only where the damp shows.Why: Damp usually has more than one route. Clearing only one leaves the band where it was.
3. 3
   Rule out rain and leaks

   A band near the floor can also come from rain splash or plumbing. The checks are on the wall dampness treatment page.Why: A damp band near the floor can come from rain splash or a leaking pipe. Treating it as rising damp wastes the work.
4. 4
   Clear the bridges

   Lower the soil and paving, correct the slope and cut back the plaster. Remove all loose and salt-damaged material from the cut.Why: Removing the bridge is often the whole cure, and it is far simpler than cutting a new DPC.
5. 5
   Give the wall time

   A wall dries slowly once the bridge is gone. Watch the damp band over the following dry months before deciding on more work.Why: Masonry holds water and dries over months, not days. Judging the result too early leads to unnecessary work.
6. 6
   Treat the salts and redecorate

   Once the wall is dry, brush the salts off dry and prime with RAPIDSTRONG SALT FLUSHER before repainting. The full sequence is on the efflorescence treatment page.Why: The salts stay in the wall after the source is gone. Brushing them off and priming stops them breaking the new paint.

Damp-proof course in an existing wall: adding a DPC to an old house

This is the extreme case and the last resort. A new damp-proof course is cut into an existing wall only when the wall has no DPC, or the DPC has plainly failed, and bridging, rain and leaks have all been ruled out. It can be done without rebuilding the wall. It does mean cutting into it, so it is done in short sections and, in a load-bearing wall, with an engineer's advice.

1. 1
   Confirm it is needed

   Work through the bridging checks first. Cut a DPC only where one is missing or has failed.Why: Cutting a DPC means cutting into the wall. It is done only where one is missing or has failed.
2. 2
   Mark the line

   Mark a continuous level line along a mortar bed joint, at least 150 mm above the ground outside and below the inside floor finish where possible.Why: A continuous level line along one bed joint makes the new DPC continuous and keeps the wall evenly supported.
3. 3
   Remove plaster along the line

   Take off the plaster on the line so the joint can be seen. Remove all loose and salt-damaged plaster with it.Why: The joint must be seen to be cut cleanly, and the salt-laden plaster would contaminate the new work.
4. 4
   Cut the chase in short alternate sections

   Cut along the bed joint in short lengths, leaving the lengths between untouched to carry the wall.Why: Cutting the whole length at once would take support from the wall above. Alternate short sections keep it carrying load.
5. 5
   Clean the chase

   Remove all loose mortar and dust. The paste needs a clean, firm surface to bond to.Why: The paste bonds to clean, firm masonry, not to dust.
6. 6
   Pack it solid with RAINSEAL BXL

   Press the paste firmly into the chase with a trowel or putty knife so there are no voids, and finish it flush.Why: A void in the packed chase is a path for water. Solid packing gives a continuous barrier.
7. 7
   Cut and pack the sections between

   Only after the first sections are filled, cut and pack the remaining lengths so the new DPC is continuous, with each section joined to the next.Why: The first sections carry the wall while the gaps between are cut. Joining each section to the next makes the barrier continuous.
8. 8
   Make good and let the wall dry

   Make good the wall face. Leave the wall above to dry before redecorating.Why: The wall above is wet and must dry before redecoration, or the new paint blisters.
9. 9
   Replace the salt-damaged plaster

   Plaster that has carried rising damp is full of salt. The stripping, replastering with CONPROOF 500 or CONPROOF LW 200 over a bond coat of ROOFBOND SBR 401, and the salt barrier of RAPIDSTRONG SALT FLUSHER are set out on the efflorescence treatment page.Why: Plaster that has carried rising damp is full of salt, and fresh paint over it fails. It comes off and is replaced with a waterproof mix.

DPC chemical and foundation coating options compared

Every Armstrong product used at the damp-proof course and below it, with the job each one does and its stated figures. Where more than one suits a job, pick the one that fits the site, and an Armstrong expert will help you choose.

DPC in new construction and DPC in an existing wall

Damp-proof course chemicals and products by stage

How much DPC compound and foundation coating for a house

A worked example for a house with 150 running ft of wall at plinth level, 230 mm thick, and a foundation wall face 3 ft deep coated on the soil side. Figures come from each product's stated coverage and dosage. Rough masonry takes more. An Armstrong expert confirms the final quantity after seeing the site.

Standards for damp-proof courses and integral compounds

• IS 3067:1988 is the Indian code of practice for general design details and preparatory work for damp-proofing and waterproofing of buildings. It describes a damp-proof course in cement mortar 1:4 or cement concrete 1:2:4, 12 mm thick, with a waterproofing compound to IS 2645.
• IS 2645:2003 is the Indian standard for integral waterproofing compounds for cement mortar and concrete. CONPROOF LW 200 and CONPROOF 500 state compliance with IS 2645:2003, and CONPROOF No. 1 with IS 2645.
• CONPROOF LW 200 states water permeability below 15%, against the 50% the standard allows, and chloride below 0.1%. CONPROOF 500 states nil chloride.
• RAINSEAL BMW 250 states 500% elongation, water absorption of 0.5 to 0.8% and a temperature range of -5°C to +85°C.
• RAINSEAL BXL has no numeric test figures on file, so none are quoted here.

Damp-proof course and foundation coating calculator

Enter the wall length, wall thickness and foundation depth to see the DPC area and the coating needed. Quantities only.

Seven mistakes that defeat a damp-proof course

1. Leaving the DPC out to save a day. It cannot be added later without cutting the walls.
2. Setting it too low. A DPC at ground level is bridged by the first layer of paving.
3. Stopping it short of the wall faces or at door openings. Any gap is a path for moisture.
4. Laying a wet, porous mix without a waterproofing compound. Plain mortar is not a barrier.
5. Plastering over the DPC line outside. The plaster carries water past it.
6. Raising soil, paving or floors above it later. This is the commonest cause of rising damp in houses that have a DPC.
7. Backfilling against uncoated footings and foundation walls. The soil side can never be reached again.

Damp-proof course questions and answers