How To Waterproof Under Pavement Vaults

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Delta Membranes being used to waterproof basement vault

How to waterproof under pavement vaults.

Waterproofing under pavement vaults is a popular way to add additional living and storage space to city centre properties. Choosing the right waterproofing system is key.

What are under pavement vaults?

Under pavement vaults are storage areas under the pavement and are common in Georgian, Regency and Victorian buildings in town and city centres.

The design of these vaults mean that they offer suffer from dampness and even flooding from moisture in the ground, rainwater as well as leaking water mains in the street.

Waterproofing of under pavement vaults.

As under pavement vaults are “existing structures” it is generally only possible to waterproofing them from the inside. External waterproofing is not possible as this would require access to the entire external structure including underneath the existing footings and floor slab. 

There are two methods of internal waterproofing that can be used that we will look at:

1: Internal Cementitious Barrier Systems – Type A Waterproofing.

2: Internal Cavity Drainage Waterproofing – Type C Waterproofing.

Before we look at these two different methods, we first need to think about the structure.

Vault Waterproofing

Structure of the vault:

The structure needs to act as the first line of defence from a waterproofing perspective, it needs to be strong enough to waterproof – this normally means:

1- The coal chute will need to be filled with concrete.

2- Voids in the structure will need to be filled.

3- Loose mortar will need to be repointed.

4- The floor slab may need to be upgraded – Example –  tiles or bricks on a dirt floor are not a suitable structure for waterproofing as they could easily move and degrade behind the waterproofing.

Basement Waterproofing Planning

Planning Consent and permission – IMPORTANT

Conversion and waterproofing of vaults may need prior approval by your local planning or conservation office. Please contact your local council planning department for advice.

Cementitious Tanking – Type A Waterproofing Option.

This option for waterproofing can be effective and has the bonus generally takes up the least  space. It can be very useful for dry storage areas or where insulation is not required.

Material costs for this type of waterproofing is generally quite low however it can be quite labour intensive to do correctly and easier to make mistakes if the specification is not correct.

Typical Specification

Inspect the vault and repair and structural issues (see above)

Bush hammer surfaces to provide a key for the waterproofing. Our friends at Grindblasters can help with surface preparation.

Repoint mortar courses using Koster Repair Mortar Plus

Form fillets at internal angles using Koster Repair Mortar Plus

Apply 2 Coats of Koster NB1 Grey.

Provide a key coat to the walls using Koster Restoration Key

Render Walls using Koster Restoration Plaster

Either: – Screed the floor – 50mm 

or apply 2 coats of Water Based Epoxy to provide a trafficable surface.

Note: These cement based waterproofing system will need at least 2 weeks to cure, during this time they will “sweat” as they hydrate.

Also Note: This is a barrier system – if you drill, chase of fix through the waterproofing it will fail!

Picture of Delta MS500

Internal Cavity Drainage – Type C Systems

Internal Cavity Drainage systems are also a great way of waterproofing an under pavement vault. They allow for easier inclusion of insulation, vapour barriers and services meaning that they are often used when converting under pavement vaults to habitable spaces or high end storage areas.

Typical Specification.

Surface Preparation – loose mortar should be removed but any bonded renders can remain in place. There is less requirement for surface preparation with cavity membrane systems.

Drainage – Cavity Membrane systems are drainage systems, they don’t stop any water coming through the structure behind so drainage discharge is required. 

Channels – typically perimeter drainage channels are used to take any water to the discharge point.

Discharge point – While it is possible to connect to a surface water drainage outlet in an external area, cavity drainage systems must not be connected to sewers due to the risk of sewer surcharge and smells. Often a groundwater pump system is required to provide best protection

Walls and Vaults – 

Delta MS500 membrane is used to waterproof the walls and vaults. When fixing into the vaults we recommend using Delta Plugs sealed with Delta Rope as these give a much better waterproof seal to the vault than the plugs with rubber grommets (such as Delta Qwikseal Plugs which are great for vertical walls). Joints between the membrane are sealed using Delta Tape.


After installing the drainage, Delta MS20 is used over the floor slab. The junction with the wall membrane is sealed using Delta Corner Strip.


The method to finish the walls and the vaults is plasterboard supported on timber battens.

For the floors a screed or T and G Chipboard can be used over the floor membrne.

Need help in selecting or installing the right waterproofing system?

Let us know our team of waterproofing experts will be happy to help.