Rain Entering Open Chimney Pots: Why Flues Get Damp

Why an open pot lets rain in at all

A chimney pot is a tube with an open top, and rain falls straight down a tube. Even a tall narrow pot catches whatever is blown across it in a shower, and rain driven at an angle by wind runs down the inside wall of the flue instead of dropping clear. Over a year, a surprising volume of water enters through a pot that has no cover at all.

Most of it does little harm. The flue is lined with clay or rendered brick that is built to take a certain amount of moisture, and the air moving up and down the flue carries much of it away again. A flue that is open at the top and has some air entering at the bottom behaves like a slow breathing system, drying out in dry spells and taking on moisture in wet ones. In that sense damp in a flue is partly a normal condition of the structure, not a defect to be cured.

The trouble starts when the water arrives faster than the flue can dry, or when it has somewhere harmful to go. That is the point where the question moves on from why rain gets in to where it ends up, and that is what the rest of this guide covers.

How much moisture is normal

There is no number that separates normal from abnormal, and it would be wrong to pretend otherwise. A better test is behaviour over time. A flue that is damp after heavy rain and dry a few days later is doing what flues do. A flue that stays wet through dry weather, or one whose dampness appears on the chimney breast in rooms below, is telling you the moisture is not clearing.

Ventilation is the main difference between the two. Air movement through the flue carries moisture out, and the warmth of a stack in sunshine helps. Blocking both ends of a redundant flue traps damp air inside, which is one reason a flue that was tightly sealed years ago often turns out to be wetter than one left open. The better approach to a flue no longer used is to cap the pot and keep a path for air through it, as explained in Capping a Flue You No Longer Use, and Why Sealing It Solid Causes Damp.

Different flues in the same stack can behave differently, and it is worth remembering that when one breast is wet and its neighbour is bone dry. A flue on a cold north-facing side may stay damp longer than one on a sunny side, and a flue next to a heated room may dry faster. That is one reason a stack with several pots can show damp in one part of the house and not another. Seasons matter too: a wet autumn after a dry summer often brings damp patches that fade by the following spring without anyone touching the roof.

What damp damage looks like

From inside the house, damp from a flue usually shows on the chimney breast. Brown or yellowish stains that spread slowly, plaster that blisters or goes powdery, wallpaper that lifts, and a musty or sooty smell in wet weather are the usual signs. White salt crystals on the surface are another clue, because moisture moving through brick and mortar carries salts out with it. Tar-like staining in older flues is a separate matter, left from earlier burning, and can come through as brown marks once the walls get wet. Damp on a Chimney Breast: Leak, Condensation or Salts? describes these signs in more detail.

From outside, the signs are different. Mortar that is soft or crumbling around the pots, bricks with flaking faces near the top of the stack, algae and moss on the head, and white or green staining running down the sides all point to a stack that is holding water instead of shedding it. Frost makes everything worse, because water trapped in brick and mortar expands when it freezes and breaks the surface apart.

Staining that comes and goes with the weather tells a different story from staining that only ever spreads. None of these signs prove that rain is entering through the pot, though. The same marks are caused by failed leadwork, a cracked flaunching or a gutter overflowing against the stack. It is easy to blame the pot when the real source is elsewhere. Is It the Chimney? Working Out Where the Water Is Getting In helps separate the possible causes before anyone pays for the wrong repair.

Open, capped or guarded: what suits which flue

A flue that is in use needs an open top, because the appliance below depends on it to carry gases away. Rain entering that flue is dealt with by the way the appliance and flue are made, and the pot and any terminal should stay clear. Anything affecting a working flue, including terminals, liners, and testing an appliance connected to it, belongs to a HETAS or Gas Safe registered installer or a sweep, and not to outside chimney work.

A flue that is no longer used is a different case. A ventilated cap keeps most of the rain out while leaving a gap for air, and it also keeps birds and debris from getting in. A bird guard stops nesting but does little for rain. A full cap with no ventilation can make a damp flue worse, so the type of cover and the way it is fixed both matter. Choices and trade-offs are laid out in Chimney Pots, Cowls and Bird Guards: Which Do You Actually Need?.

Birds are worth a mention here too, because nesting material in an open pot soaks up water and holds it against the flue wall long after the rain has stopped. Whatever is fitted, the stack around the pot has to be in good order. A sound flaunching, firm pointing and tight leadwork do far more to protect a chimney from damp than any cover on the top. We deal with these on the outside only, and our chimney repairs in Wallasey page describes the tall multi-pot stacks we see in Wallasey. A free written quotation will set out which items look needed.