Increasing house purchase and rental prices might prevent you or your family members from moving from the family home. But, how do you provide extra living space?
Converting your basement into another living area is a relatively low-cost solution compared to moving house. However, before moving Grandma into it, the room must be watertight and comply with the UK Building Regulations.
The average cost of waterproofing a basement is £100-£125/m2, although it can be as low as £40/m2 or as high as £275/m2. So, to treat a standard basement measuring 5m x 4m x 3m high (total 74m2) costs £7,400-£9,250, on average. However, this amount might vary significantly across the range, depending on the contractor and other factors we’ll discuss later.
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This guide discusses the potential benefits of waterproofing (also known as tanking) your basement, how to do it, the price and those factors affecting it, and the regulations you must comply with. Finally, we’ll discuss how you find a qualified contractor to do the work.
How Much Does Basement Waterproofing Cost?*
Use the basement waterproofing prices below as an approximate cost calculator to compare with your basement or cellar dimensions. We only consider walls and floor, as the ceiling is already protected from dampness.
|Basement dimensions||Surface area||Min-Max range||Average range|
|5m x 4m x 3m high||74m2||£2,960-£20,350||£7,400-£9,250|
|6m x 5m x 3m high||96m2||£3,840-£26,400||£9,600-£12,000|
|7m x 6m x 3m high||120m2||£4,800-£33,000||£12,000-£15,000|
*We compiled these figures from various sources. Every property and its basement is unique. Therefore, only use the estimated amounts when searching for a reliable contractor. Moreover, if you want accurate quotations, ask a reputable contractor or use the form on this page. Note, these prices disregard major structural building work such as underpinnings, or other operations that will turn the basement into finished living accommodation with lighting and central heating.
As we mentioned above, there will be other expenditures such as:
- Surveys are usually free-of-charge. However, if there’s a lot of work involved, the contractor will charge £100-£200 to ensure your commitment.
- Often, you need a dehumidifier to speed up the drying process. These cost £60-£90/week to hire.
- If you intend to waterproof the room yourself, suitable sealers cost £30-£100 per 5L.
- Waterproofing the room’s interior is only part of the solution. You must also reduce the amount of water pooling outside. Therefore, upgrade house gutters, downpipes and divert rainwater away from the walls to relieve the hydrostatic pressure.
- Before you cover them up, check the foundations aren’t damaged.
- Ensure the ground around the house is adequately graded away from the property.
- Check for leaking water pipes and repair or replace them.
- You probably need to remove waste. Usually, hiring waste skips is the best option and costs from £110 depending on their size and the amount of waste to remove.
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11 Basement Waterproofing Price Factors
Let’s consider the main factors affecting the cost of waterproofing a basement.
1. Will the room be habitable?
If you intend to use the basement as a habitable room, you must comply with the Building Regulations. These rules affect how the contractor waterproofs the basement and prepares for additional work. Therefore, they affect the total cost of conversion. We will talk about Building Regulations later in this guide.
2. Water table
Suppose part or all of the basement extends below the surrounding water table. In that case, hydrostatic pressure makes waterproofing the room an arduous task without using additional methods. Sometimes, the contractor installs cavity drainage channels, sumps, water pumps and extra outside drainage, which will incur additional expenditure. In contrast, if the room is above the water table, all that’s needed is a cementitious tanking system which will cost much less.
3. Waterproofing membrane
Any waterproofing system needs a tanking membrane to protect the walls and floor from dampness. The amount of surface area and the membrane’s quality add to the project’s total costs.
You might need the project to be completed by a specific date, or the project’s complexity might substantially increase. In both these cases, the labour costs rise so that you can meet the deadline.
5. Additional finishing work
In most cases, tanking a basement is merely the initial phase before converting the space into a habitable room. At the very least, the waterproofing method must be designed to consider the planned construction. And, at most, you must budget for the additional construction and finishing work. For example, levelling floors and screeding, plastering, electrical fittings, plumbing and drainage, heating, insulation and partition walls. Depending on the requirements, expect to increase the project’s costs by several thousand pounds.
6. Basement size
Larger basements will cost more to waterproof than smaller basements. The average cost of tanking is £100-£125/m2. Therefore, a basement with a floor 5m x 4m and walls 3m high will cost, on average, £7,400-£9,300. In comparison, a large basement with a floor of 7m x 6m and walls 3m high will cost £12,000-£15,000.
7. The shape of your basement
Most basements follow the house’s footprint and can be a simple rectangle or comprise a more complex shape. Furthermore, some basements have vaulted, curved, or pitched ceilings that further complicate the waterproofing process. Complex shapes such as these require more meticulous work. Therefore, the labour costs will be higher.
8. How are the existing walls and floor constructed?
The construction method of the basement and the surrounding ground conditions will determine the suitable waterproofing method and which products to use.
9. Fully or partially underground?
If the basement is partially underground, it may be possible to remove part of the external wall to provide better access. Constructing an exterior door will make the finished basement conversion attractive and useful. Furthermore, if some of the basement’s walls are above ground, you reduce waterproofing costs.
10. Preparation work
Before starting the project, you must have the basement surveyed by a waterproofing specialist. Some won’t charge for the survey, or they might include it in the overall quotation. However, a survey report takes time and uses specialist skills, and the customer might not be genuinely committed to the project. Therefore, some contractors charge a nominal fee of £100-£200 to cover costs. Furthermore, a comprehensive basement survey will also highlight structural repairs, the water table height, and additional work to complete before waterproofing. This report is valuable, even if you eventually choose another contractor.
Some areas of the UK have a higher cost of living than others. Therefore, the labour charges will be correspondingly higher. The most expensive region to live in is Southeast England, which can have labour rates 10-15% higher than other regions. Moreover, many contractors stay in their areas and don’t venture out. But, if you insist on a waterproofing specialist from another location, expect to pay travelling expenses, and the contractor’s going rate, even if it’s higher than in your area.
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How To Waterproof A Basement
There are many skills involved in tanking a basement, and it’s helpful to show the customer how it’s done. Although these might seem simple steps to the layman, the contractor has spent many years learning the trade, so don’t expect to learn the job by reading this or watching a YouTube video.
We’ll assume that the basement doesn’t require any structural repairs or significant alterations.
- Check there’s an entirely unbroken and stable surface. If there are any cracks, eroded mortar joints or gaps in the floor surface or corners, repair them with a sand/cement grout or concrete. When dry, remove all traces of dust and cover the surface with a water-based rubberised bitumen emulsion paint (RBE). Then, continue painting the bitumen about 100mm up the wall, using the manufacturer’s instructions as a guide to the number of coats. Finally, lay about 50mm of sand/cement screed on top of the final coat, ensuring the bitumen is visible on the walls.
- An alternative method is to place about 50mm of soft sand onto the basement floor and tamp it down, removing any cavities. Then, place a sheet of polythene damp-proof membrane onto the sand, continuing the sheet at least 100mm up the wall, neatly folding the corners, and hold the sheet in place with flashing tape. If there are joints in the damp-proof membrane, fold the two edges together, lay them flat on the ground and seal the edge with flashing tape. Finally, cover the sheet with about 50mm of sand/cement screed.
Before starting, check for damage to the masonry walls. Also, repoint eroded cement joints in brickwork.
- If you’ve used polythene to waterproof the floor, cover the join between plastic and wall with flashing tape around the border.
- With eroded pointing, apply 20mm of render onto the rough surface, wait until dry and cover with the required number of RBE coats.
- If the pointing is good on masonry walls, brush clean the surface to remove dust and other debris. Then, use RBE to seal the surface. Firstly, use a diluted coat as a primer, followed by the recommended number of coats. Before the final bitumen coat has dried, throw sand onto it using a shovel. The sand will stick to the bitumen and give a key for the render coat.
- Apply 20mm sand/cement render to the bitumen/sand surface.
Note: If the outside water table’s hydrostatic pressure forces the RBE and render away from the wall, you must apply extra support by using reinforcing galvanised wire mesh fixed to the wall.
If the wall is wet, RBE won’t stick to the wall. Therefore, use a thick studded plastic sheet fixed to the wall. The studs provide a ventilating air gap for unrestricted airflow. Finally, apply render to the studded plastic sheet.
Remember, the success of different methods outlined here depends on many factors that only a waterproofing specialist will know and understand. Carry out trials with each method before choosing the best option.
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Can I waterproof my basement myself?
Only attempt waterproofing your own basement if you are skilled in the methods outlined previously and understand waterproofing technology.
If you want to do the job without professional help, the DIY materials cost about £20/m2. Some of the typical components you need will include:
- Block brush for RBE application – £3.
- Waterproof tanking membrane – £45/5-Litre.
- Remmers MB 2K (Multi Tight 2K) (RBE) – £199.60/25Kg.
- Damp-proof membrane 4m x 25m – £79.
- Studded plastic tanking membrane 20mm x 2m x 20m – £297.
What’s the basement waterproofing lifespan?
The lifespan of basement waterproofing depends on the following:
- Quality of products.
- Climate conditions.
- The hydrostatic pressure of the surrounding water table.
- The condition of the existing basement walls.
However, the average waterproofed basement lasts around ten years. But, a good quality initial installation can last much longer.
If you have a basement wall shared with a neighbour and you have to alter it. For example, underpinning or strengthening foundations before waterproofing the walls, you must conform to the Party Wall etc. Act 1996. To help you comply with the legislation, the UK government publishes guidance.
Usually, you won’t need permission to waterproof a basement. However, you might require approval to convert it into a habitable room if:
- It’s separate from the house.
- You intend to change its use, for example, from residential to commercial or industrial use.
- The building is listed.
- You intend to change its outside appearance. For example, when adding external windows or doors.
Most people decide to waterproof their basement with the long-term intention of turning it into a habitable room. Therefore, even if you initially use the space for storage, it’s good to plan for human habitation. So, comply with the Building Regulations. If you have any doubts, contact your local authority.
Generally, the basement must comply with the following regulations to be habitable.
- Part A. The basement’s foundations, load-bearing and party walls must be structurally sound.
- Part B. All construction materials must have 30-minutes fire resistance. Furthermore, provide at least two means of escape in case of fire.
- Part C. The basement conversion must prevent the ingress of water and other soil contaminants.
- Part D. Use only approved non-toxic waterproofing materials. Also, follow the manufacturer’s instructions.
- Part F. The basement must have ventilation to remove excess moisture and provide fresh air to the inhabitants.
- Part L. Thermally insulate the basement to the required standard.
- Regulation 7. All materials and workmanship must be of good quality.
There are other regulations dealing with plumbing, drainage, soundproofing and electricity. But these depend on the basement’s usage.
Final Hiring Checklist
Unfortunately, the construction industry is full of contractors who know a bit of this-and-that, but aren’t specialists. A project such as waterproofing needs thoroughly trained and qualified contractors who know what they’re doing and are abreast of the latest waterproofing technology. Therefore, there are several points to check before you choose a contractor.
- Can they self-certify their work under the Building Regulations, and are they members of a Competent Person’s Scheme?
- Is he a locally established contractor or a well-known national waterproofing company? Avoid those contractors who have minimal history.
- Has the contractor a list of references from previously satisfied customers? Follow them up and find out the contractor’s professionalism.
- Does the contractor have Public Liability Insurance and Employers Liability Insurance?
- Are they a member of one of the recognised waterproofing trade associations?
- Furthermore, they might also belong to:
Waterproofing your basement or cellar is the first step to having additional living space in your home and often costs less than moving house or building an extension. But, to get the project done correctly, you need a skilled contractor specialising in waterproofing.
Complete the form on this page, and we’ll ensure you receive up to four local basement waterproofing quotes.
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