Property Repair Systems

How to Stop Damp on Your Internal Walls

This project is sponsored by Property Repair Systems

Summary: The different causes of damp patches, condensation and damp patches on internal walls and how to stop damp forming on your walls once and for all. Find out what causes damp patches including issues such as cracked mortar, roof tiles, damaged guttering or lead flashing and find out how to fix them once and for all.

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In this project we’ll go through the various different causes of damp patches and condensation on your internal walls, as well as explaining how to definitively stop damp entering your walls in the first place. Damp patches on internal walls of your home can be a real pain, not only due to the fact you get ugly looking marks on your walls but also the fact that moisture and damp patches on walls can also produce mould. This can in turn be detrimental to your health, as mould spores have been linked to quite a few issues such as asthma and allergies - so it’s not just your home that will suffer in terms of damage.

It’s also a good point to note that if you do have damp on your internal walls then the fact that it is there will make a room feel colder and more uncomfortable to be in – damp walls lose heat three times faster than dry walls.

The reason for you getting damp on your inside walls can be various and in most situations the cause of the internal damp will either come from the top of the house (roof area) or from the bottom (ground area), however there are occasions when the damp can come from the walls themselves.

We will now go through the most common causes of damp appearing on internal walls and also cover some methods for stopping the damp from entering your walls.

First Assess Whether Your Damp Wall Area is Actually Damp

The first task you will have to carry out is to assess the affected wall area to establish how damp it actually is – this can be done two ways:

  • Back of Hand – Although not the most accurate way to measure damp on your walls it will give you a good idea of how bad the damp actually is. Touch the back of your hand against the damp area and gauge the level of dampness. Now touch the back of your hand against a dry area on the wall and compare the two. This will give you an idea as to how bad the damp on your wall actually is
  • Damp Meter – These are handy little gadgets that feature two prongs on the base and a level indicator on the front. Stick the two prongs into your damp wall surface and leave for a while. The level indicator on the front will then display the percentage of damp within the wall’s surface.

Another handy instrument is a Relative Humidity Hygrometer. Position it in a room and the reading on the front will give you the moisture content level inside the room space. This is quite useful in situations when you are experiencing heavy condensation on your walls and will allow you to get an initial reading and then monitor the drop in moisture as you implement fixes to resolve any damp issues. More information on testing your walls is available on PRS’ website, along with a range of damp testing solutions available to buy.

If you do detect damp, don’t rush to treat it without finding the cause first, as this greatly affects what you need to do to treat your damp effectively and what tools and materials you’ll need to buy.

How To Find the Cause of Your Internal Damp and How To Fix it

Penetrating Damp

Cracks in walls and Mortar or Render – Whether this is the cause of your internal damp issue or not, this is still a problem!! If you have cracked mortar in your walls or chimney stack then water and moisture has the potential to enter your property and cause untold damp and rot damage.

Brick wall with mortar joints missing

Mortar joints missing from a brick wall can allow water to get in and make internal walls damp

If it is allowed to sit on timber such as roof timbers, floor joists etc…. then these can rot out and cost a small fortune to put right.

Check all of your outside walls (using a decent pair of binoculars for the higher areas - these can be hired if necessary) for cracks in the mortar joints of your brickwork, chimney pots and chimney stack.

If you do find any cracks or faults in your mortar then they will need repairing. Rake out the joints using a plugging chisel or small bolster to a depth of around 10 – 15mm. Using a suitable mortar mix (information for this can be found in our Mortar Mixes project) re-point the damaged mortar joints with new mortar.

In the case of broken/cracked mortar around chimney stacks and pots this will also need to be replaced. Carefully chisel off the damaged areas and replace with the aforementioned mortar mix, using the Mortar Mixes project above for more information.

If you have cracks in your render then this can be a little trickier. In the case of large cracks these will need plugging asap!! If moisture is allowed to sit behind a rendered surface then it will gradually erode the inner surface away and create further cracks and eventually render will start falling off, creating a larger and more expensive repair job.

Cracked render allowing water into walls

Cracks in your render can allow water to enter your walls, creating damp patches

In the majority of cases render repair can be quite a time consuming job so for an interim fix you can seal over the crack with a silicone based sealer. This is also the case for larger cracks where you can seal the crack void and surrounding area with a silicone sealer.

Once you have some time to correctly repair the crack you can simply remove the silicone filler and proceed with the repair.

In respect to the correct repair method, any cracks should be filled with a render mix of the same grade and consistency as the current render on the wall (premixed render or render patching compound can be purchased for this job. Products include K-Rend and Euromix. We have never used these products so cannot confirm how competent they are).

Fill any cracks with your mortar or pre-mixed mortar and allow to dry for the required time. To ensure that no moisture is allowed to penetrate your repaired area it’s also worth considering covering the area with a sealer or waterproofing agent.

If you have coloured render then this is another element that needs to be considered. You will have to try and create a matching colour to patch any cracks. As you can imagine this can be really tricky so be prepared for the fact that the repaired area is going to stand out. One possible solution for this is to then paint your property using a suitable exterior render paint or cement based paint. Use our project on matching mortar to get the correct colour

Roof Tiles and Flashing – As with the above, broken or ineffective flashing (the lead sheeting that covers the joint between your roof and wall and chimney stack and roof etc….) can cause water ingress which results in damp patches on your internal walls. Using a decent pair of binoculars (either borrow or hire) check all the flashings around your roof and chimney stack for cracks and separation from the adjoining surface. In the case that you find any flashing faults then this will need to be repaired. Ideally any cracked flashings need to be replaced with new flashings. Lead is very expensive and this can be a pricy job, but ultimately flashing replacement is the best course of action.

Check lead flashing for any missing mortar joints

Check lead flashing for and splits and cracks allowing water to penetrate

In the case that you don’t have the funds to do a replacement right away then there are alternatives available. Flashing Tape is a fairly good short term fix for sealing cracks. It’s effectively a waterproof tape with an adhesive underside that you can stick over the crack, sealing it over. We have used a few of these in the past and found Sylglas to be one of the better ones. See our project on lead flashing for more information on this.

In the case of cracked and broken tiles these will also need to be replaced as any cracks present can allow water to penetrate into your roof area, run down rafters and down into cavities and cause all manner of damp problems on internal walls.

Change broken roof tiles immediately to stop water entering

Changing your broken roof tiles immediately can ensure that water doesn’t enter your property

The type of tile (clay, slate, concrete etc….) will dictate how this is done. In most cases the tile will be nailed to the roofing battens below and, sometimes on old properties,  fixed in place with mortar  so you will need to break the nail off using a suitable slate ripper or hacksaw blade to cut the nail off or chip off the mortar with a suitable hammer and chisel. With the nail(s) removed slide the tile out and replace with a new tile.

Once replaced fix the tile in place, or in the case of slates, instead of nailing back in (as you will be unable to access suitable nailing points) cut a suitable length lead strip (about an inch longer than the length of the tile when positioned in place) and nail to the roofing timber. Lead strip or galvanised metal is better to ensure that it does not rust.

Push the tile up into place and ensure that it is sitting flush with the surrounding tiles. Now bend the protruding edge of the piece of metal back over the top of the tile so that it now holds the tile in place.Take a look at our project on roof tiling to understand more about how a roof works

Guttering – The purpose of your guttering is to ensure that any excess water that runs off of your roof is channelled away from the walls or your property and allowed to drain in a suitable area.

If the guttering becomes ineffective water can then run down the outside walls of your home and if this is allowed to consistently happen then eventually the walls themselves will suck the water in by capillary action causing damp patches on your internal walls.

Using a pair of decent binoculars check all of your guttering for defects – cracks, splits, leaking joints, holes in down pipes, broken brackets etc…. If you find any of these then get them fixed as soon as possible.

Blocked guttering can cause water to overflow

Blocked guttering can cause water to overflow and make your walls and roof damp

Any of the aforementioned issues are quite easy to fix but will in most cases involve working at heights so get a professional in if you are not comfortable doing so. See our project on installing plastic gutters to see how your guttering should work.

Service Fixings – Most objects that are fixed to the internal walls of your property are done so using screws, bolts and other such items. In some cases these fixings protrude through the inner walls and into the cavity void. If there is any damp present within the cavity area (leaks from above, cracks in mortar etc….) then this water and moisture has a direct route into your wall if it is allowed to settle on the part of the fixing that is present in the cavity. This moisture and water can then cause damp to appear on your internal walls. You can get guidance on fixing to masonry on this project page.

Drill the correct length hole when fixing to external walls

Be careful to drill the correct length hole when fixing to external walls

To resolve these issues remove any fixings that enter the cavity area and seal any holes left with a suitable waterproof filler e.g. mortar with waterproofing agent, silicone etc…. This should then prevent any further damp issues appearing on your internal walls. You can find out how to apply silicone sealant here.

In the event that you cannot remove the objects then make sure that the area around the object is sealed tight. Use a flexible filler to do so such as silicone as rigid fillers can crack with the movement of your home and changing temperature.

Using a silicone gun to waterproof windows and doors

You can use a silicone gun to waterproof around window and door frames

Rising Damp - This can be a tricky problem to both diagnose and cure. Many people and some experts diagnose damp issues as rising damp when they are in fact nothing to do with it at all.

Classically, rising damp is damp that rises up through the ground into the brickwork or stonework of a wall through capillary action (the act of the water molecules sticking to the walls surface and surface tension holding the molecules together and helping them move up the wall inside the pores of the bricks). To stop rising damp being an issue some houses have Damp Proof Courses (DPCs), which is a waterproof barrier inserted into a house’s brickwork all around the property at about 6 inches from the ground.

It is believed by some experts that rising damp does not exist at all as technically walls constructed with engineering bricks should be impervious to capillary action due to the density of the brick material itself.

It is also argued that many Victorian properties were constructed with no damp proof courses for the reason that rising damp did not exist. However many of the construction materials used at that time such as lime mortar, lime plaster etc…. are very breathable in nature and any moisture that happened to rise up from the ground, in this case, would then be allowed to naturally evaporate through the breathable surfaces. In light of this some houses from this era were constructed with a slate damp proof course.

With modern techniques, houses and buildings are constructed using materials that should be impervious to water and moisture e.g. bricks fired at a high temperature, Portland cement, Gypsum plaster, plastic damp proof course and concrete slab membrane etc…. In the case that moisture does penetrate – say the damp proof course, then as it works its way up the wall there is very little scope for it naturally evaporate due to the moisture resistant qualities of the materials involved. It is thought that at this point rising damp can occur.

Engineering bricks used as a damp proof course

Engineering bricks can be used to build a damp proof course

In the event that your current damp proof course has failed or has been breached (via an attached structure stretching across the DPC and providing the damp an alternative pathway around the DPC) a good option for repair is inserting another damp proof course, most likely by injecting a chemical mixture into your internal and exterior wall. This may sound like a fairly intensive job and you would be right but it can be done at a DIY level by anyone with a little savvy.

The basic principle in terms of DPC injection is that you drill holes in the external wall at the level you want the new DPC to appear at (6 inches above ground level) and then you inject the chemical cream DPC in using a suitable application gun (e.g. a skeleton gun). The holes themselves should be around 12mm in diameter and around 20mm deep at centres of 125mm (this may vary depending on the manufacturer of the DPC – more information on chemical DPC injection is available here.

Once the external walls are treated you then have to treat the internal walls using the same process. In terms of the holes you have now drilled in your walls, with external holes these need to be left for as long as you can, at least 2 weeks to ensure correct drying. For internal walls these also need to be left for a minimum of 2 weeks (preferably longer). Once you have ensured that the liquid filled holes that make up the DPC are dry you can then fill the surface of the holes you have drilled. Use a suitable mortar mix with a waterproofer added and ensure that you push the mixture well into the holes to ensure they are properly filled.

The chemical DPC itself is just a waterproof chemical similar to the likes of silicone or other waterproofing chemicals. When it is pumped into the hole you have drilled, the bricks absorb the substance (as they are porous) and this essentially makes them waterproof – the chemical bonds to itself within the brick to form a waterproof layer.

In most cases the chemical DPC comes in tubes similar to those used for silicone or decorators caulk. You can also get chemical DPC injection machines. These are similar to pumps that then pump the chemical agent into the wall. The applicator guns are a little easier to use if you are not experienced in using the injection machines.

In respect to chemical damp proof courses there are quite a few on the market and as with most products some are better than others. Examples of products available include Permaguard, Twistfix etc…. Historically, we tend to use a product called Quick Cream as we have found this to be about the best and it’s BBA (British Board of Agrement) approved.

Cold Spots Giving Rise to Condensation and Salts

These issues are arguably one of the most common causes of damp on walls. Condensation forms when warm moist air within a room touches a cold internal wall or surface. It then cools rapidly and then condenses back into water. This condensation then sits on the internal wall’s surface and creates damp patches on the wall.

Now that we know how the condensation forms we have to find what is causing the cold spots that are causing the condensation to appear. This can be caused by several defects. Firstly, are there any cracks in the mortar on the external wall around the area where the damp is appearing? This could be letting draughts into the cavity and cooling the associated area on the internal wall, stopping it from warming up with the rest of the room. If there are cracks in any mortar then this will need repairing using the methods outlined above.

Is the condensation appearing on a chimney breast? If so then this example of condensation could be caused by a poorly ventilated chimney breast. If you have bricked up fireplaces or a capped chimney stack then this may be preventing any cold air that is trapped in the chimney void from escaping and thus keeping the area at a consistently cooler temperature than the surrounding air. In this case, again, moisture can then condense on the chimney breast surface and cause damp on your wall.

In this case, the ventilation provided by the open fireplaces and the act of fires drawing air up the chimney stack is no longer present and air can become static so we will have to introduce some new ventilation in order to get the air flowing again and the air warmed up. Air bricks can be inserted on the ground level around 300mm above floor level and also towards the top of the chimney breast on the external wall. This will then introduce an air flow and prevent static cold air sitting within the void and keeping the chimney area cold.

Keep chimney pots vented to prevent damp

Keep chimney pots vented and allow air to circulate to prevent damp by using chimney cowls

The wall around your chimney can also be prone to suffering from Hygroscopic Chimney Salts. Salts and minerals released from fuels burnt on a fire can settle on the inside of the chimney breast and over time seep into and through a wall onto the internal wall surface. Tars and deposits that are also present on the inside of a chimney breast can also be drawn through along with the salts. The salts themselves appear as a white salt-like build up while stains from tars and deposits appear as a yellow/brown stain.

You may be thinking the salts are damp and make the affected area damp when they are present, this is not the case. The salts themselves are hygroscopic in nature which means that they attract moisture from the internal air. This moisture then settles and creates damp on the internal walls.

In terms of a fix – the likelihood that any plastered surfaces have survived this are pretty slim. If it can be caught in the very early stages and dried out then there is the chance that you can simply treat the area with a salt neutraliser, allow it to dry and then redecorate but in all honesty the correct fix would be to remove all of the plaster from the damaged areas right back to the walls surface and then to treat the wall itself with salt neutraliser to prevent any further salt ingress. You can then replaster and redecorate with confidence knowing that future damage has been prevented.

Use salt neutraliser to prevent salts penetrating and making walls damp

You can apply salt neutraliser to your walls to stop salts penetrating the internal surface of your wall

The salt neutraliser is a substance removes the salts and then prevents any further salt formation. There are quite a few different salt neutralisers available online or in your local DIY store but we use the salt neutraliser from Property Repair Systems as we have found this to be the best of the bunch - some of the other brands produce inconsistent results.

In some instances salt deposits can be very heavy and to provide even more protection and further minimise the chances of salts forming and causing damp to appear on your internal walls you may want to add a membrane to your wall before replastering. After you have treated the walls surface with salt neutraliser you can then fix a membrane over the area. The membranes themselves usually come on a roll (similar to wallpaper) and feature a dimpled surface. There’s a guide to the different types of membrane available as well as a chance to buy them here.

Fitting a mesh membrane stops salts from appearing in your wall’s surface

Fit a mesh membrane before replastering to prevent salts returning and causing damp on your internal walls

If you need any help finding the causes of condensation and damp patches on your internal walls and would like further information on how to stop your walls getting damp please contact PRS. We have worked with them on many projects over the years and they are experts in this field. If you’d like more information on how to insert a new Damp Proof Course why not visit our How to Fix Rising Damp project page.

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