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waterproofing of an interior garage wall

How to Waterproof a Garage


Waterproofing a Garage

Whether you have a garage adjoining your house or an external garage, there's every chance you will have experienced some degree of dampness on the internal walls. This can occur, or become much worse, following construction work by neighbours with a higher land level or if you build behind your garage. It's usually at this point that people look at their options when it comes to waterproofing a garage.

Waterproofing is not considered a necessity in most cases when constructing a garage. This is because garages are usually single skin buildings that are not designed to be an habitable dry environment. However, with space a premium, many of us want to use our garages as much as possible and be safe in the knowledge that anything we store there will remain dry.

One of the most cost effective methods for waterproofing garage walls is by using tanking slurry to prevent dampness. Tanking is commonly used in cellars and basements to prevent groundwater entering, however, tanking slurry is also highly effective at preventing damp walls in a garage. It is the cheapest and most effective method of garage waterproofing.

If completed correctly, this method will allow you to protect the contents of the garage, or you may want to use the garage for storage, even a living or office space. In this simple no nonsense guide, we take you through all of the stage of waterproofing a garage with tanking slurry.


How to Waterproof a Garage with Tanking Slurry 


About Tanking Slurry 

The most effective and economical way of waterproofing a garage is with tanking slurry. Occasionally this product is referred to as cementitious tanking. Tanking slurry is a specially formulated mixture designed to stop water ingress. It either comes pre-mixed or as a powder to mixed on site with clean water. The combination of Portland cements, aggregates. chemical modifiers and acrylic polymer in PermaSEAL tanking slurry combine to block the passage of water into your garage.  

1. Preparing Your Garage Walls

You will need to prepare the walls of the garage before applying the tanking slurry. Make sure the walls are free from any coatings, paints, plaster, renders and dust. What you want is the original brick or blockwork, fully exposed and clean. If you come across any cracks or holes fill them with PermaSEAL Fillet Seal.

 wet wall in garage

Identify any loose mortar joints and look to make good as per below image

 dubbing out garage wall

Make good any repairs - we recommend PermaSEAL Fillet Sealant

2. Mixing the Tanking Slurry 

Most tanking slurries come as a powder and require mixing on site. It's important to protect yourself when mixing tanking slurry by covering exposed skin and wearing a facemask and goggles. Always follow the manufacturer’s instructions closely and only mix enough tanking for 30 - 45 minutes as the product will become unworkable after that time.

PermaSEAL Tanking is supplied in 20kg buckets. You should start by adding the required quantity of clean water to a suitable mixing vessel. Slowly add the powder to the water whilst continuously mixing. We recommend mechanical mixing at a slow speed with a high torque drill and a plaster mixer. Mixing should be continued for three minutes after all the powder has been added to the mixing water to obtain a batter-like consistency. Allow the material to stand for one minute before remixing and then applying. 

 mixing tanking slurry garage waterproofing

3. Wetting out the Walls

Tanking products are designed for application onto damp substrates. If your garage walls are relatively dry, you should wet out the substrate fully with clean water, making sure it is damp but with no standing or surface water before applying the tanking.

4. Wall Floor Joint

Where the wall and floor meet is often prone to leakage and therefore a potential weak point. We recommend creating a reinforced fillet joint. To do this, you should chase out the floor to a minimum of 20mm x 20mm at the wall floor joint, cutting into the wall if possible. Flush out the chase and remove any debris.

Whilst this is still damp, apply one coat of tanking slurry 100mm up the wall and 100mm across the floor. Whilst the slurry is still tacky, apply PermaSEAL Fillet Seal over the tanking slurry and into the chase creating a cove from the wall to the floor. 

You do not necessarily need to waterproof your garage floor. However, if you wish to do so then follow the method outlined in our Cellar Tanking guide

grey tanking slurry permaseal

5. The First Coat

Tanking slurry requires at least two coats. The first coat should be applied directly to the brick or blockwork by brush in a horizontal direction. Normally we recommend applying the slurry up to one metre high as this will combat most rising damp scenarios. Also, If the wall is retaining earth then we suggest the tanking is taken up to one metre above ground level. You should also go down and across the wall floor joint and apply the slurry 100mm onto the floor as per the below image.

PermaSEAL cellar tanking comes in two colours – grey and white. As you can see from this series of photographs, this allows you to use a different colour for each coat so that you can clearly see where you are painting. This ensures that when you apply a second coat you cover all areas.

6. The Second Coat

It can take anything between 2 – 24 hours for the garage walls to be ready for a second coat. The first coat of slurry should be touch dry and able to support a second coat without pulling off. Don't wait too long though before applying the second coat - this needs to be applied within 24 hours, even if the substrate still looks damp. 

Apply the second coat over the top, using vertical brush strokes. The wall floor joint should also receive a second coat.

waterproofing a garage grey tanking


7. Curing 

After a day or two, the tanking on the walls will cure. It does so at different rates with wetter parts of the walls taking longer to fully cure.

You may notice moisture as the slurry cures. This is called ‘sweating’ and is nothing to worry about. 

tanking slurry drying phase


 8. Finishing

24 hours after applying your tanking slurry to the walls and floor of the garage, you can render over the tanking surface with a breathable render. Even if the tanking looks damp or darker in certain areas a render can still be put over the surface as the whole area will dry together.

As with most tanking slurries, PermaSEAL Tanking needs to be covered by at least a 10mm coat of three parts sharp washed sand to one part cement or PermaSEAL Renovating Plaster. After this render coat has been applied and allowed to dry, dot and dab plasterboard or a multi finish can be used. We do not recommend using paint directly on the tanking. 

render bead on tanking

A render bead has been added to provide a consistent depth and a neat finish

Costs of Waterproofing a Garage

The cost of tanking a garage depends on the size of your garage and the building material. However, using the tanking method outlined in this guide, waterproofing an average-sized (4 metres wide by 6 metres long) garage costs around £150. 

Tanking costs are commonly calculated on a metre squared basis. PermaSEAL tanking slurry comes in 20kg buckets with coverage of between 6.5m² to 8m² unit depending on the state of the substrate.

Our grey tanking slurry is available at £29.50 excluding VAT. If we take the lowest coverage estimation of 6.5m² per bucket, then the cost of tanking a garage works out at £9.22 per m² for two coats. Two walls of 6m tanked up to one metre in height and one wall at 4m tanked up to a height of one metre would require five buckets of slurry at a cost of £147.50 excluding VAT. There are some additional costs such as the fillet seal but all-in-all, it's a cheap way to waterproof a garage.   

If you are planning on converting the garage into a living space, you may want to consider using a damp proofing membrane system or a cavity drainage membrane system. This has its own associated costs that are considerably higher than tanking.

Permagard – Garage Waterproofing Products at Low Trade Prices

If you have any unanswered questions around waterproofing a garage or the tanking process then call our technical team for free advice on 0117 9823282. To find out more about cavity drainage membranes and how they work then feel free to contact us too. We are more than happy to share our 30 years+ of waterproofing knowledge.

Find out How to Tank a Cellar or check out our guide to garage conversions.

Related Products 

PermaSEAL Tanking Slurry in White

Permagard Cementitious Tanking products

PermaSEAL Fillet Seal

Professional Tanking Brush


Many thanks to Richard Uphill for providing the images for this guide.