Crack Where the New Floor Meets the Old House? Why It Leaks

You added a floor to the old house a few years ago, or you bought a house where the previous owner did. Now a thin crack runs along the wall or ceiling where the old part meets the new part, and after heavy rain a damp line follows it. The painter fills it, repaints it, and it is back by next monsoon. That is not bad paint. That is a joint doing what joints do.

Why the old-new junction keeps opening

New concrete and masonry are not the same as ten-year-old concrete and masonry. New work shrinks as it cures and dries. Old work has already settled. Add temperature swings between a hot afternoon and a cool night and the two parts move by tiny, different amounts. The weakest line, the joint between them, takes the strain.

Plaster is stiff. It cannot stretch across a moving joint, so it cracks along the line. Rain finds the crack, soaks into the wall behind it and shows up inside as a damp streak, flaking paint or a white salt trail. Filling the crack with putty only gives the water a fresh surface to push off. The joint moves again and the filler splits.

Where to look

• The outer wall: a vertical or stepped crack where the old wall ends and the extension begins.
• The terrace of the old portion: where the old roof slab meets the wall of the new floor above it. Water collects at this base if the slope is poor.
• The ceiling below: a straight stain running along one line, not a round patch, usually points to a joint.

A quick check after the next rain

1. Mark the crack ends. Draw a pencil line across each end of the crack and write the date. If the crack runs past the mark, it is still moving.
2. Measure its width. Hairline and steady is usually surface movement. A crack you can slide a coin into, or one wider at the top than the bottom, needs an engineer to look at it first.
3. Look at the terrace above. Check whether water stands at the base of the wall, or whether the old drain outlet is blocked. Our guide on the parapet wall base explains this junction.
4. Tap the plaster. A hollow sound around the crack means the plaster has lifted and has to be cut back, not patched.

What a lasting fix looks like

If the crack is a stable movement joint, the aim is not to lock it shut. A rigid fill will crack again. The usual approach is to cut the crack into a groove, clean it out, and seal it with a flexible material that can stretch and recover. Then the wall around it gets a waterproof coating, and the terrace junction above is dealt with, because that is where most of the water comes from.

The order matters. Stop the water at the top, treat the joint, let the wall dry, then repaint. Our wall dryness test shows when it is safe to paint. If the fix is done in the wrong order, the paint peels again within a season and you are back to square one. We covered the same trap in why waterproofing fails.

What you can do this week

• Clear the terrace drain near the junction so water does not stand against the wall.
• Do not repaint over a damp line. It will blister.
• Keep a dated photo log of the crack. It helps whoever inspects it.

Where LoklFix fits

We trace whether the water is coming through the joint, the terrace above or a plumbing line, and we say so plainly. For stable joints we do the groove, flexible seal and wall treatment. For anything that looks structural, we tell you to bring an engineer in first. Related: seepage fixing.

Frequently asked questions