Damp FAQ

Selected topics that have generated a lot of interest.
Posts: 1880
Joined: Fri 26th May, 2006 3:19 am
Location: Earth

Post by NT » Sat 21st Oct, 2006 9:45 am

First actions for damp properties

If your property is damp, there are several simple actions that can be taken first which may solve or improve the situation.

A good place to start in case of damp problems is to check your underfloor vents. Timber ground floors are normally suspended, and the exterior walls have airvent bricks to allow air exchange between the underfloor void and the outside world. This air exchange carries away water vapour. If the airbricks are blocked or buried, damp trouble is likely. Clear the holes in the bricks with a stick to ensure free airflow. If the vents are buried they must be dug clear and kept unobstructed. Sunken vents will require regular clearance of debris to maintain free airflow.

Rainwater goods.

Does a gutter overflow in heavy rain? If so, clean it out and check again. (Looking up during heavy rain is always fun.)

Does a downpipe discharge water onto a wall? If so, the pipe may be corroded through at the back, or partly blocked. Sometimes this corrosion can be difficult to see.

Are there open hoppers fitted? If so clear them out, as these are prone to accumulating materials causing a blockage or flow restriction.

Are your ground level drains taking away all water, or is standing water visible during heavy rain? If not draining freely, drains might need rodding or repair. There are also drain clearing chemicals such as caustic soda and sulphuric acid, but choose these with caution, as they can damage to some drain systems, and can be violent when mixed.

Also check for pipes that have moved slightly out of place, causing a leak where downpipe meets gutter, or they may discharge water that misses the hopper.

Ground levels:

Outdoor ground level should be at least 6" below interior floor level. Ground level too high is a common cause of penetrating damp at the base of walls. If the ground is too high, remove earth next to the wall to reestablish a healthy level.

Sun & wind:

Direct sunlight & free air movement both help to evaporate water. If your damp wall is shielded from the sun or free air access is obstructed, the obstructions may be removed if practical. Common problems here include plants, wood piles, binbags and junk.

Ground drainage:

Check the ground immediately next to the building drains properly in rain. If there is standing water, the remedy will depend on the details of the situation.

Heat & ventilation:

Buildings that have remained unoccupied and unheated are prone to becoming damp. Normal heating & ventilation may be all that's needed to resolve such situations.

Once all relevant first line actions have been carried out, the building will need time to dry and settle to its new moisture equilibrium level. Drying may take months.

Often first line actions are enough to resolve a damp problem, but in some cases they aren't. In these cases, further damp resolving measures which require significantly more work than the first line actions may be needed.
Last edited by NT on Tue 26th Jun, 2007 7:31 pm, edited 1 time in total.

Posts: 1880
Joined: Fri 26th May, 2006 3:19 am
Location: Earth

Post by NT » Sat 21st Oct, 2006 9:46 am

Second line actions for damp properties

If first line actions have not resolved your damp problem, and you've given the building enough time after such works to dry, then its time to move on to second line works. These are further damp resolving measures which require significantly more work than the first line actions.

Taking good advice before embarking on such measures is only sensible. This post is a description of measures used rather than any specific recommendation for your property.


Look up at the roof, ideally wih binoculars, and look for any slates or tiles missing or out of place. This does not mean look for holes, but rather look for a change in the normal pattern which would indicate a place only 1 tile/slate thick instead of 2. Any area 1 slate thick will let in water. A tile or slate that has slipped down lower than its normal position is also liable to cause water ingress, and fall out entirely.

Inspect the flashing or fillets.
Flashing is (usually) lead sheet fitted to direct water from walls and chimneys onto the roof, where it will run down the roof and drain away. Leadwork can be detached, broken or even missing.
Fillets are strips of mortar applied on top of the roof where it meets a wall or chimney, and do the same job as flashing. These are more prone to trouble because they are inflexible and the mortar has a limited life. Look for fillets that are broken or have pulled away slightly from the brickwork. Rain only requires a tiny gap to enter the building, so fillet trouble spotting is best done either close up or using binoculars.

Some roofs have valleys that also need inspecting, and clearing, repair or relining if necessary.

Rain water goods:

Check for missing gutters. All roof sections should be fully guttered. Absent sections mean water from the roof is discharged onto the ground at the foot of the building, soaking the ground and splashing walls. When necessary use matching guttering. Cast iron gutter is still available new. Wood gutters must be made from the right wood types to be durable.

Check where rain water discharges. If not fed to a mains drain it should be dumped at least 5m from the house, preferably further away.


Check condition of external pointing (the mortar joints between the bricks). If it is falling out in places, this is liable to permit penetrating damp to enter the wall. Before repointing, realise that renewing all the pointing with cement mortar can result in damage to the wall and exacerbate damp problems. Nearly all period properties should be repointed using lime mortar rather than cement.

Traditional Lime Mortar:
3 volumes sand, perferably sharp sand
1 volume lime putty

Biff's Instant Lime Mortar:
3 volumes sand, perferably sharp sand
1 volume bagged hydrated lime (dry)
add water to make a stiff mortar.

Bagged (hydrated) lime is available from builders merchants and many but not all diy sheds.
Lime putty can be purcahsed ready made, or made from dry bagged lime. Note that lime putty should be made before it is needed, as it is best stored before use.

Some houses may have mortar wih other ingredients added. Where these were used, replacement mortar should be made up to give the same apprearance. Mortar can be analysed.

Cement should not be added to lime mortar, as it is liable to cause premature failure, and reduces the mortar's ability to evaporate water.

Raking out whole walls and replacing all mortar is not generally recommended. Mortar need only be replaced where missing, broken, disintegrating, turning to sand, or at risk of falling out.

Exterior lime mortar should not be applied when there is a risk of frost or heavy rain, unless you're willing to cover the work temporarily with sacking etc.

Exterior cement render:

This is a common contributor to damp problems. Cracks allow rain water in, but the cement gives very little opportunity for evaporation, so wall water content gradually increases.

The only real solution to cement render is to remove it. However removal can sometimes be a problem, as soft some bricks and stones are liable to be broken when firmly adhering cement is removed. Use only the minimum amount of force needed to remove the cement, and if damage occurs, stop. Do not damage your brick or stone, as damaged bricks sometimes steadily deteriorate until they require replacement.

If cement can be removed without damage, this will make a real difference to the wall's damp handling abilities. The wall can either be left unrendered, or if an eyesore it can be lime rendered. Lime render allows evaporation to continue.

If the cement can't be removed without damage, it is an option to remove only areas where the cement is weaker or not properly attached. Find these areas by tapping the cement: a hollow sound indicates where the cement is not stuck to the underlying wall. Removed patches may be patched with lime render, giving patchwork evaporation areas. It is fair to say though that it may be difficult to hide such patchwork completely, as lime and cement renders are not identical in nature. Also there may be a risk of slight cracking at the boundaries due to differing thermal expansion coefficients.

Warning: Cement rendered earth walls can (rarely) reach the point where the earth inside has become wet enough to lose structural strength, resulting in the building being held up by the cement render. If this were to occur, removal of cement render could then cause a collapse. Since cement render on earth causes this risk, it is important that these houses have their render removed, but it must be done with care to avoid the risk described. Cement may be removed in areas, allowing enough time for the wall to dry before progressing to the next area. The earth wall material may be tested for strength to detect when risk is present, and when the building is sound enough to support itself without its render. If you have cement rendered earth you will need to take advice specific to your building.

Showers & Cooking:

These activities can produce large quantities of water vapour. Reducing vapour production from these sources can make a real difference.

Showers have 5 main solution options.

Closed shower cubicles reduce vapour release into the room.

Window ventilation exhausts vapour during and after a shower. This may be done by opening the window by hand, or fitting a locking mechanism that allows it to be locked a 1/4" - 1/2" open. This can often be done at minimal cost.

Extractor fans remove some damp air during showering. Adding an overrun timer causes them to continue extracting for some time after room use. The principal downside of fan use is wasted heat. There is also noise.

Dehumidifiers extract water vapour from the air. Although unit purchase cost is higher, fitting cost is zero and no heat is thrown out. This results in lower total cost. A humidistatic unit should be chosen, as non humidistatic units fail to respond to shower use and can overdry the air during drier times.

Cooking has 2 options for reducing vapour output:

1. Fit an exterior venting cooker hood. Interior venting recirculating hoods do not remove water vapour.

2. Use lids on pans when cooking, turning the heat down low. This makes heat setting more critical, glass lids or pans make setting the heat correctly much easier. It also saves money & energy, and reduces risks of burning food and fire.

Impermeable coatings & paints.

Damp proof paints, bitumen and proprietary wall coatings all stop evaporation, and thus raise water content in exterior walls. Purchasers of these products see their ability to stop rain penetration, but fail to realise the effect on the other side of the equation, evaporation, or fail to realise there even is an equilibrium situation in effect.

Impermeable paints such as masonry paint have a similar effect. Buyers should be aware that paints sold as breathable, microporous or similar terms are often mostly-impermeable paints, ie their level of permeability is often much too low to make them suitable for most period properties.

Removing these coatings enables walls to evaporate dry. How the coating is removed depends on the coating and wall type. Sand blasting is rarely appropriate.

Breathable interior paints:

Gypsum plaster is breathable, but most modern paints are low breathability. Interior emulsion paints reduce evaporation. Many lime based paints give the inside of a wall more opportunity to dry, but it is necessary to remove any existing impermeable paints first to gain this advantage.

If replastering, lime plaster has some advantages over gypsum, and is generally recommended for most period properties.

If these actions dont sort your damp problem out, either you've got an unusual situation, or a cause that has been missed or not correctly resolved.
Last edited by NT on Tue 26th Jun, 2007 7:45 pm, edited 1 time in total.

Posts: 1880
Joined: Fri 26th May, 2006 3:19 am
Location: Earth

Post by NT » Sat 21st Oct, 2006 9:54 am

Wall types

Solid vs cavity walls

Solid and cavity walls are both found in period properties, despite the often heard assumption that old houses used only solid walls. The 2 types of wall handle damp to some extent differently.

Cavity walls have 2 inner surfaces from which damp can evaporate, and are thus relatively tolerant of cement render, impermeable paints and wallcoatings, and other contributory factors to damp walls.

Solid walls however have no such hidden evaporation surface, their visible sides are the only places where damp can evaporate. Consequently it is more important with these walls to ensure that damp can evaporate, and inappropriate wall finishes or treatments can more easily cause damp troubles.

When assessing wall construction, it should be born in mind that many Victorian houses used several types of wall construction in one building.

4" walls

The majority of house walls are 9" brick, but various other types also exist. One of the more troublesome from a dampness point of view is 4" brick walls. 4" walls offer poor insulation, and are thus prone to winter condensation and ensuing damp trouble. Where 4" walls suffer from damp, insulation may help stop the problem, as well as provide a welcome reduction in heating costs.

The Damp Man
Posts: 3
Joined: Fri 20th Oct, 2006 7:53 am
Location: UK

Post by The Damp Man » Sat 21st Oct, 2006 10:01 am

NT Wrote......

The last and least desirable option is to have shorter showers or use the bath instead.

You couldn't make this up could you!

We're all still waiting for an answer to my previous posting NT!


Posts: 9402
Joined: Sun 6th Aug, 2006 4:04 pm
Location: Planet Earth

Post by Nemesis » Sat 21st Oct, 2006 10:25 am

Actually, we are all getting a tad impatient with you as

a) you have been asked to post anything which is a discussion as part of the other thread;

http://periodpropertyshop.co.uk/phpBB2/ ... php?t=6779

b) you seem to have ignored Biff's question.

This is a serious attempt to get some help to people, not an argument.

NT - I am sure this entire thing can be tidied up and reposted eventually, with extra stuff removed.

Posts: 89
Joined: Mon 14th Aug, 2006 5:07 pm
Location: shropshire, england


Post by bournonite » Sun 22nd Oct, 2006 12:05 am

Wouldnt this be FAR better served by putting a few decent pages on the website? You cant really do this in a forum - why not have a damp section (the mind boggles...) on the website itself, which can carry photos and diagrams, and be edited by Rob/Simon with input from all those who wish to contribute. If not, I'll happily put one together on our site and link to it - we have loads of relevant photos.. I know its a lot of work, but a forum isnt really the way to inform people - not quick and visual the way it should be... The forum has the advantage of geting loads of different inputs - but if they were collected and edited to produce some really good web pages, I think the overall benefit to the general public would be much greater...

I'll copy onto the other threads..


Posts: 336
Joined: Wed 14th Feb, 2007 9:06 am

Post by charlie-ia » Sun 24th Jun, 2007 11:49 am

Posts: 883
Joined: Mon 12th Feb, 2007 8:44 pm
Location: wenslydale

solid/ rock and rubble/ cavity walls

Post by chuckey » Wed 27th Jun, 2007 8:54 pm

An intermediate wall type of typical thickness 12 - 24" is the rock and rubble type. Here an outer and inner skin of dressed or partially dressed stone is used, with the cavity filled with rubble and often dirt. To add stability to the walls "through" stones are used either randomly or in courses. These go right the throught the walls. Often the outer layer of blocks are "weather or water shot". The stones are laid tilted forward so any rain penetrating runs outwards.
Rain penetration around windows and door opening is often caused by the lack of weather shooting from the side (slope is from in to out, no slope from side to side around openings)
Throughs may be badly set or porous.
Because of all the rubbish in the cavity any water ingress through the outer skin may cause random penetration to the inner skin.

Thought I would add this.

Posts: 9402
Joined: Sun 6th Aug, 2006 4:04 pm
Location: Planet Earth

Post by Nemesis » Wed 27th Jun, 2007 10:12 pm

In my experience the cause of this is defective mortar. Repointing (lime!) is the answer.

Posts: 883
Joined: Mon 12th Feb, 2007 8:44 pm
Location: wenslydale

Post by chuckey » Thu 28th Jun, 2007 6:34 pm

Not in my case.

Posts: 9402
Joined: Sun 6th Aug, 2006 4:04 pm
Location: Planet Earth

Post by Nemesis » Thu 28th Jun, 2007 6:38 pm


Trad buildings do tend to respond to trad repairs best.

Matt Green
Posts: 576
Joined: Wed 10th May, 2006 3:40 pm

Post by Matt Green » Fri 29th Jun, 2007 9:45 am

Seems pretty fair to me. Through stones and bricks bridging early cavity walls have been found to transmit water to the internal surface. Adding that to the damp FAQ doens't seem like a crime. It might be something to consider.

Frank isn't recommending what to do about it, so the traditional repair comment seems misplaced.

Clearing out blocked cavities at the base of walls, whilst a little invasive, can be a reasonable route to take if the problem warrants it. Fallen debris effectively turns a cavity wall into a solid one, and you could argue that reducing ground levels would reduce the risk of ground water ingress into the base of the wall I suppose.



Posts: 9402
Joined: Sun 6th Aug, 2006 4:04 pm
Location: Planet Earth

Post by Nemesis » Fri 29th Jun, 2007 9:53 am

No - it's just Frank is very fond of cement... :? so hope he isn't recommending that as a solution...

Posts: 3626
Joined: Wed 25th Oct, 2006 8:51 pm
Location: Cornwall

Post by MdB » Tue 14th Aug, 2007 1:15 pm

An interesting article that MDE found.
http://www.greensteps.co.uk/tmp/assets/ ... 050906.pdf

Posts: 9402
Joined: Sun 6th Aug, 2006 4:04 pm
Location: Planet Earth

Post by Nemesis » Tue 14th Aug, 2007 1:17 pm

Read the caveat on page ten, consider that 'lime' is many things.

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest