Waterproofing a house: roofs, bathrooms, walls and damp

Water follows gravity and finds any gap. It also travels along surfaces, so the stain is often far from the source. Waterproofing is the work of giving water no way in, and a way out.

Most leaks come from details, not from the membrane: a flat spot, a blocked drain, a short upstand, a pipe through the slab without a sleeve. Settle these in the drawings.

A flat roof is a shallow tray that must shed water, resist sun and survive being walked on. Good roofs are made of layers, each doing one job, and each checked before the next covers it.

1. 1

   Prepare the slab

   The slab surface is cleaned of dust, oil, loose material and any projecting wire. Cracks are cut open and filled with a suitable repair material. Honeycombed areas are repaired. The slab should be fully cured and dry enough for the product to bond; follow the manufacturer’s data sheet.

2. 2

   Form the slope

   A screed or traditional brick-bat coba lays the fall to the outlets. Check it with a level and a string line. Pour water after it sets (the bucket test, below) and mark any puddle. Fix the slope now. It is very costly later.

3. 3

   Fit the outlets

   Outlets are fixed with a flange that the membrane sits on. Check that outlets are at the lowest points and are not blocked by debris, and that pipes through the roof are sleeved.

4. 4

   Form the corners and upstands

   Where the roof meets the parapet or any wall, the corner is rounded or chamfered so the membrane does not crack, and the membrane runs up the wall (commonly at least 150 mm above the finished roof level), finished into a groove or under a coping or flashing.

5. 5

   Apply primer and the membrane

   Apply the primer if the system needs one, then the membrane in the thickness and number of coats or layers the manufacturer states, with the stated overlaps, and treat joints, outlets and upstands with extra care. Do not apply in rain, on a wet slab, or in the hottest part of the day if the data sheet forbids it.

6. 6

   Test it by flooding

   Before any protection goes on, block the outlets and flood the roof to a shallow depth, held in by temporary dams at doorways. Keep it for 24 to 48 hours, mark the water level, and check the ceiling below for damp. Any leak is found and repaired now, while the membrane is exposed.

7. 7

   Add the protective layer

   A screed, tiles or a cement finish on top shields the membrane from sun and from feet. Leave expansion joints at the spacing the specification gives and fill them with sealant.

8. 8

   Check again at the first rain

   Look at the ceilings, the parapet and the outlets in the first heavy rain. Keep the photographs.

The bucket test, before the membrane

1. Pour a bucket of water on the roof at several points and watch where it flows.
2. It should run to the outlets within a short time. Any place where it pools for more than a few minutes is a flat spot.
3. Mark the puddles with chalk and have the contractor correct them before the membrane is laid.

Bathroom leaks are usually the most expensive to fix, because tiles must be broken to reach them. The work is cheap at the time and costly later, so do it carefully and test it.

1. 1

   Set the floor level and the slope

   The bathroom slab is sunk below the neighbouring floor, or has a threshold, so water cannot run out. The screed falls toward the drain at about 1 in 50 to 1 in 100. Check it with a level.

2. 2

   Fit and sleeve the pipes and drains

   Floor drains, waste pipes and water pipes are fixed before waterproofing, with sleeves through the slab. Pressure-test the supply pipes before they are covered. See the inspection guide.

3. 3

   Treat the corners and joints

   The joint between floor and wall is treated with a fillet or a reinforcing tape. Pipe penetrations and the drain flange are sealed with the product the system specifies.

4. 4

   Apply the waterproofing

   Apply the coating over the whole floor and turn it up the walls: commonly at least 150 mm, and higher around the shower, where the full height of the wetted wall should be covered. Apply in the number of coats stated and let each cure.

5. 5

   Flood test for 24 to 48 hours

   Plug the drain, fill the floor to a few centimetres, mark the level and leave it. Check the ceiling below for damp, and the walls beside. A drop in level or a damp patch means a leak.

6. 6

   Protect it, then tile

   Add the protective screed, then the tiles with adhesive, then grout. Use waterproof grout or sealant at the floor-wall joint and around fixtures.

Rising damp

Ground moisture rises into brick and plaster by capillary action, leaving flaking paint, peeling plaster and white salt along the lower wall. A damp-proof course (DPC) at plinth level is the barrier. It is easy to add during construction and nearly impossible to add properly afterwards.

External walls

• Plaster. Use the specified mix and thickness, and cure it. Cracks at the joint between column and brick are reduced by a strip of mesh across the junction before plastering.
• Paint. Use a weather-resistant exterior paint over a primer, applied to dry plaster. Interior paint outside peels quickly.
• Windows and doors. Sills slope outward and have a drip groove underneath. Frames are sealed to the wall with sealant, not only plaster.
• Projections. Sunshades, copings and cornices need a drip groove on the underside, so water drips clear of the wall.
• Penetrations. Holes for AC pipes, cables and pipes are sleeved and sealed on the outside.

Basements, retaining walls and underground tanks

Anything below ground is under pressure from water in the soil, so it needs a tanking system on the outside face (a membrane or coating protected by a layer), a drainage layer where the engineer specifies, and care at joints and pipe entries. The engineer designs this with the structure. Underground water tanks and septic tanks are tested by filling them before they are used.

Water often enters at one point and shows up at another, because it runs along slabs, pipes and cracks. Repairing where the stain is usually fails. The method is to isolate each possible source and test it.

1. 1

   Map the stain

   Draw a plan of where and when it appears: only after rain, only after a bathroom is used, or all the time. Photograph it with the date. Mark the edges of a stain with a pencil and date to see if it grows.

2. 2

   Decide if it is rainwater or plumbing

   A stain that appears only during or after rain points to the roof, walls or windows. A stain that appears regardless of rain points to plumbing. To check, close the main water valve for a few hours when nobody is using water and see if the water meter or the stain still moves.

3. 3

   Inspect above the stain

   Walk the roof or the floor above. Look for a blocked outlet, ponding, a cracked parapet, a failed joint at the wall, a missing sealant at a pipe and cracked tiles in a bathroom.

4. 4

   Test one area at a time

   Flood or hose one suspect area for a few hours (a roof section, a bathroom floor, a balcony), then check below. Wait a day, let it dry and test the next area. Testing everything together tells you nothing.

5. 5

   Measure moisture

   A moisture meter shows how wet a wall is, how far the damp extends and whether it is drying. It is cheap and helps stop you repainting a wet wall.

6. 6

   Get an engineer for structural or hidden leaks

   A leak in a slab, a wall with wide cracks, or damp that you cannot trace needs a professional to find the source.

What to put in the BOQ

Maintenance before the monsoon