Old House Reconstruction Log: Demolishing Old Walls and Building New Masonry Surfaces

Because a bad neighbor illegally built an apartment next door, the wall shared with them had been seeping water and growing efflorescence for years, which had long been unbearable. So, starting at the end of August, on my mother’s orders, began nearly a month of days in which one troublesome task after another came pouring in.

Why Does Efflorescence Occur?

Efflorescence (also called white bloom or saltpetering) is a common wall problem in old Taiwanese houses, and its cause is actually not complicated: moisture + free calcium in the cement.

When a wall is damp for a long time, water seeps into the cement mortar layer, dissolves the calcium ions within, and as the water evaporates it is carried to the wall surface, where it reacts with carbon dioxide in the air to form calcium carbonate crystals—producing white powdery crystals on the wall. Over time, the paint bulges and peels, and mold spots even grow.

To cure efflorescence at the root, the key has never been “scrape off the surface and repaint,” but rather cutting off the water source. As long as the source of the seepage remains, the efflorescence will recur again and again. This is also the fundamental reason why this old house reconstruction absolutely had to knock down the old walls and redo the masonry.

The Two Stages of Masonry Work: Scratch Coat and Finish Coat

The masonry portion of general construction work is divided into two stages: the scratch coat and the finish coat.

Scratch Coat (Rough Base)

When applying a scratch coat to an RC wall or brick wall, a 1:3 mix of cement and sand is used (with an appropriate amount of water). Because the scratch coat is applied relatively thick, in addition to forming the base layer (a waterproof layer can be applied separately on the surface), it can also be used to level the wall surface through the placement of “standard dabs” (also called screed markers or mochi).

The purpose of the standard dabs is to first establish several thickness reference points on the wall, and then the craftsman pulls lines and trowels flat according to these reference points to ensure the flatness of the entire wall. This is a very critical step in masonry work—whether the wall is flat or not depends entirely on this step.

And the scratch coat, as the name suggests, is just a base layer! The rough surface can then be finished with cement rendering or tiled over.

Finish Coat (“Xi-a-gei”)

As for cement rendering (which traditional masonry craftsmen pronounce “xi-a-gei”), a 1:1 mix of cement and sand is used. Because the sand content is lower, the finished surface is finer and flatter.

The key to the finish coat is “wrapping up”: the surface must be troweled flat and fine, with no obvious seams or bubbles. This process tests the craftsman’s skill—done well, the wall feels smooth as a mirror; done poorly, it leaves wavy marks.

Generally, masonry work can wrap up after the finish coat is done, and then it’s the painter’s turn to come in, first applying putty, sanding flat, and then applying the topcoat.

The Importance of the Waterproof Layer

In this project, the waterproofing treatment of the wall was specially reinforced. Since the root of the problem came from seepage through the neighboring wall, after the scratch coat was completed and before the finish coat, an additional waterproof layer was applied to the wall to form a barrier against moisture.

The principle for applying the waterproof layer is “from the outside in, blocking layer by layer”:

• Treat the exterior wall first to block rainwater from seeping in directly.
• Add another waterproof layer to the interior wall as a second line of defense.
• Corners and joints need special reinforcement, as these are the places most prone to seepage.

The Bad Neighbor’s Illegal Construction History

The bad neighbor had already secretly built up the building when I was still a child. People in that era were more simple and honest (the bad neighbor being the exception—an exception since his grandfather’s generation, who swindled quite a bit of land through his work as a land registration agent and became a local rich man), and were more tolerant of their reckless, arbitrary behavior. Times have changed, and now I really don’t know what can be done about them.

As for why I say it was “secretly built,” the reason is that the area they initially built was, say, only 40 ping, and the documents they applied to the government for were also for 40 ping, and when the officials came to inspect it was still 40 ping—but after the inspection was completed, they inexplicably built another 20 ping forward, making it more than 1.5 times larger.

This tactic of “build according to the plans first, then do a second round of construction after inspection” was not uncommon in the early years when illegal construction was rampant. By the time anyone wants to pursue it years later, the statute of limitations has often long passed, or it’s difficult to prove, leaving one able only to sigh in frustration.

Construction Defects Discovered During Demolition

Moreover, their old house shared a wall with ours, and during the demolition they knocked several holes in our wall. When I inspected a few days ago while the red brick wall was exposed, I discovered that they had never properly dealt with those holes at all—some holes even had entire pieces of wood rotted inside, and the exterior wall wasn’t sealed with cement either, so you could see straight into their wall.

No wonder it had been seeping water and growing efflorescence for years. With such sloppy construction quality on the neighboring wall, moisture naturally rushes right in, and no matter how much waterproofing you do on your own wall, it’s probably just treating the symptoms, not the root cause.

A lesson can also be learned from this case: the construction quality of a shared wall is often beyond your control. When buying an old house or a house connected to a neighboring one, it’s best to first understand the condition of the neighboring wall; otherwise, dealing with seepage and efflorescence problems later will cost both money and effort.

Summary

Old house reconstruction involves not only your own masonry work but may also involve wall problems shared with neighbors. This process of demolishing the old walls and redoing the masonry not only taught me the difference between the scratch coat and finish coat and the principles of applying a waterproof layer, but also unexpectedly revealed the construction defects of the neighboring wall over the years.

The way to cure efflorescence at the root is always the four words “cut off the water source.” Scraping and repainting the surface only treats the symptoms; only by addressing the wall structure and waterproof layer can the problem truly be solved. I can only say that when you encounter such a bad neighbor, apart from doing your own wall as thoroughly as possible, there really seems to be no better solution.