Help needed wet patch on chimney wall

gally74

hello all,

please help me with this if ye can, i have a damp patch that comes and goes with wet weather, it comes on a chimney about 5 foot up on the second floor, it also shows itself on the inner leaf block work beside the chimney, it seems to be the same thing,

ive inspected th elad work and all seems ok, new build last year, any idea;s the chippe suggests that it could be a cracked liner or cement briding across the 100 mm gap in the wall,

im tempted to get a camera to look down the cavity from the attick?

anyone ever seen this before,

Avns1s

I have had something similar. Turns out its the white plaster on the chimney which is absorbant of water. A chimney has no cavity, sorry to disagree with your chippie unless I have read your post wrong. Once water gets through the plaster, it will enter the blockwork and run downward and out eventually.

The solution, apply Thomsons water seal to the chimney outside obviously. It'll last about 2 years and then you will need to do it again. I put it on with a pump up garden sprayer. Takes more product but far easier than messing with a brush.

Of course, you're problem might be different but if its a bare plaster chimney and no obvious crack, then I would try this.

Let us know how you get on with it.

gally74

Avns1s wrote: »

I have had something similar. Turns out its the white plaster on the chimney which is absorbant of water. A chimney has no cavity, sorry to disagree with your chippie unless I have read your post wrong. Once water gets through the plaster, it will enter the blockwork and run downward and out eventually.

The solution, apply Thomsons water seal to the chimney outside obviously. It'll last about 2 years and then you will need to do it again. I put it on with a pump up garden sprayer. Takes more product but far easier than messing with a brush.

Of course, you're problem might be different but if its a bare plaster chimney and no obvious crack, then I would try this.

Let us know how you get on with it.

hello anvs1s

i have two chimneys in the house the other one is fine,

the cavity is the cavity wall beside the chimney, ie. there is a bit of damp approx 12 inches in a line on the cavity wall and the rest a blob on the side of the chimney, the plaster seemed sound but i did go for through precast chimney things, rather than shutter them, the brickey then used mortar to seal in the chuimney cowl,

any idea;s and thanks for the info,

joolsveer

I had a similar problem. It was the flashing around the chimney. The builders had used aluminium where lead would have done a better job (according to another builder).

gally74

has anyone ever heard of damp coming from the outside wall across the cavity if a lump of cement was on a wall tie etc?

mossfort

dampness can carry across on wall tie if the outer leaf of blocwork is a few mm higher than the inner leaf and if the cavity is full of mortar on the wall ties.
maybe the bricklayer didnt fit a dpc under the precast cap on the chimney this prevents water from soaking through and flowing down in the chimney fill.
or maybe he left out the aeroboard behind the chimney . we used to use fireproof fibreglass instead of aeroboard behind chimney breasts in external walls because the heat from the chimney could damage the aeroboard.

RKQ

Sounds like a lead flashing problem. Check lead flashings especially at back of chimney ie highest point chimney meets roof. If it were a brick chimney then I'd say it was a problem with the lead tray.

Waterseal will help if the plaster is porous, allowing water penetration of the chimney blockwork. We have a good history of applying Render (external plaster) unfortunately this tends to make us depend on the Render, which is not always water repellant.

Wind driven rain will easily cross a poorly installed wall tie. Especially if external leaf is a few mm higher than internal leaf.

gally74

hello all, thanks for all the help,

ive checked the chimney inside the attick and all seems good and dry, if it was the lead would i not see dampnees underneath the lead,

im worried about the dpc under the chiney sill thou, dont think one was used

RKQ

gally74 wrote: »

ive checked the chimney inside the attick and all seems good and dry, if it was the lead would i not see dampnees underneath the lead,

Check the lead Outside where the chimney meets the roof. Better still get a Roofer to take a look.

Water travels and appears at the easies exit - water on the chimney breast, as you describe, usually indicates poorly installed flashing.

jumpstart

gally74 wrote: »

hello anvs1s

i have two chimneys in the house the other one is fine,

the cavity is the cavity wall beside the chimney, ie. there is a bit of damp approx 12 inches in a line on the cavity wall and the rest a blob on the side of the chimney, the plaster seemed sound but i did go for through precast chimney things, rather than shutter them, the brickey then used mortar to seal in the chuimney cowl,

any idea;s and thanks for the info,

this is a problem i have encountered for years .As a roofer.Several times i have had to go back to houses i have done with same complaint ,e.g chimmney leaking.
Always same 2 problems mostely occor. With precast chimmney caps ,one should always lay a square of 1000 gauge polythene on top around chim pot and then 2*1 shutter about 3 inches larger than the existing hole . fill with concrete ,when dry mastic around chimmney pot.
Blocklayers always use mortar to fill arond chimmney caps ,which is too weak. Water soaks down through cap ,wets chimmney fill,and comes out the side of chimmney.
The second problem is plastering. plasters usually use stop bead around lead.nailed on with washer steel nails. no matter how far down on lead they put it ,if its not nailed high enough on stop bead it will leak.Meaning they nail stop bead too close to the botton .
Lastely if lead is leaking ,it will leak just below roof at chimmney, and you will see wet spot. If damp spot on side of chimmney more than likely fill had got wet
Hope this helps

onq

gally74 wrote: »

has anyone ever heard of damp coming from the outside wall across the cavity if a lump of cement was on a wall tie etc?

I traced two such occurrences in an office building on the keys once.

Characterised by a circular patch on the plaster on the inner face of the inner leaf.

ONQ.