Module 1 · Moisture, Air and Materials

1.3 Capillarity and Pore Structure

9 min read

Liquid water moves through building materials by capillary suction, and the physics is unintuitive: finer pores pull harder and lift higher, but transmit more slowly. That single trade-off explains a great deal of what you will see on site — why a hard engineering brick can be wetter than the soft stock above it, why a render coat changes where a wall dries, and why a mortar joint and its brick can be in completely different states.

Learning objectives

By the end of this lesson you will be able to:

Explain capillary rise in terms of pore radius, surface tension and contact angle.
Predict how a change of material or a mortar joint will alter a moisture path.
Distinguish capillary saturation from full saturation and explain why it matters.
Account for pore structure when choosing where and how deep to sample.

Key terms

Capillary suction: The negative pressure generated where a wetting liquid meets a narrow pore, drawing water into and along the pore network.
Capillary rise: The equilibrium height to which capillary suction can lift water against gravity. Inversely proportional to pore radius.
Capillary saturation: The moisture content reached by capillary uptake alone, typically 70–90% of the material's total pore volume. The remainder is air that capillarity cannot displace.
Pore connectivity: Whether pores form a continuous network. Isolated pores hold water but do not transmit it, so porosity alone does not predict transport.

The height a capillary can lift water is inversely proportional to its radius. A 1mm pore lifts about 15mm. A 0.01mm pore lifts about 1.5m. A 0.001mm pore, in theory, lifts 15m.

But the rate at which water moves through a pore scales with roughly the square of the radius, so the fine pore that lifts 1.5m does it slowly, and the coarse pore that lifts 15mm does it fast. Real masonry contains a distribution of pore sizes, and the moisture profile you measure is the sum of that distribution's competing behaviours.

The practical upshot is the one that catches people out: 'more porous' does not mean 'wetter'. A very open material — a coarse aggregate, a well-graded gravel — has large pores that barely lift at all. That is why a rubble or gravel layer works as a capillary break: not because water cannot pass, but because the pores are too large to pull it upward.

What actually decides where water goes

Pore radius distribution — sets both the lift and the rate. Fine pores lift high and slowly; coarse pores lift little and quickly.
Connectivity — a material with high porosity but isolated pores stores water without transmitting it. Autoclaved aerated concrete is the classic example.
Contact angle — whether the pore surface is wetted at all. Water-repellent treatments work by changing this, not by blocking the pore.
The presence of salts — dissolved salts change surface tension and, once crystallised, change the pore geometry itself. Covered in 1.4.

Note what is absent from that list: the material's name. 'Brick' spans a range of pore structures wider than the gap between brick and stone, which is why manufacturer data and, where it matters, testing beat assumptions.

Interfaces are where things get interesting

Water crossing from one material to another does not simply continue. At the interface, the suction of the receiving material competes with the suction of the material the water is in.

Put a fine-pored material against a coarse-pored one and water will move from coarse to fine, because the fine pores pull harder — even downwards, and even against gravity over short distances. Put a coarse layer under a fine one and the water stops at the boundary, because the coarse pores cannot generate the suction to pull it in.

This is the mechanism behind several things you will meet: why a dense cement render can hold water against masonry rather than letting it out, why a soft lime mortar acts as a sacrificial path that concentrates evaporation and salt deposition in the joint, and why a physical damp-proof course works at all.

It also explains a common misreading. Finding the mortar joints wetter than the bricks does not mean water is 'travelling in the joints' — it can equally mean the joints are where the wall is successfully evaporating.

Where to sample, and why depth changes the answer

A wall being wetted from one side and drying from the other has a moisture gradient through its thickness. Take a sample from the first 10mm and you are measuring the drying face, which tells you about the room, not about the wall.

Sample at multiple depths — typically 25%, 50% and 75% of the wall thickness — and record which is which. A profile is evidence; a single number is an anecdote.
Discard the first few millimetres of drillings. Surface material is contaminated by the finish and by frictional heating from the drill.
Drill slowly, with a sharp bit. Heat from a blunt bit at speed drives moisture out of the sample before it reaches the tube.
Sample the brick and the mortar separately and label them. They are different materials with different pore structures and will legitimately give different answers.

The shape of the profile is the diagnostic, not the magnitude. Wet at depth and dry at the face means water is arriving from behind or below. Wet at the face and dry at depth means it is arriving from the room. That distinction is the difference between a construction defect and a condensation problem, and no surface reading can make it.

Common mistakes to avoid

Assuming a more porous material carries water further. Coarse pores lift less, which is exactly why gravel makes a capillary break.
Reading wetter mortar joints as proof water is travelling up them, when the joints may be where the wall is evaporating.
Taking a single sample at one depth and reporting it as the wall's moisture content. Without a profile there is no direction, and without direction there is no diagnosis.
Drilling fast with a blunt bit and then wondering why depth samples read low. The heat dries the sample before it is collected.
Mixing brick and mortar drillings into one sample. The result belongs to neither material and cannot be compared with any published figure.

Lesson summary

Capillary behaviour is set by pore radius, connectivity and contact angle — not by how porous a material looks or feels. Fine pores lift high and transmit slowly, coarse pores the reverse, and interfaces between the two are where water gets held, diverted or stopped. On site the consequence is about method: sample at several depths, keep brick and mortar apart, and read the shape of the profile rather than the size of any single number, because the shape is what tells you which direction the water is coming from.

Next: hygroscopic salts — the moisture a wall holds because of what is dissolved in it, not because water is still arriving.