ᐅ Sealing the Foundation and Backfilling: Proper Preparation of a Residential House and Garage
Created on: 15 Nov 2025 22:16
M
magicmaxI am currently in the floor screed heating phase of my new build. Since there is little to do inside at the moment, I want to progress with the work outside.
My house does not have a basement. The ground slab is edged on the gable side with Bachl XPS 300 C SF perimeter insulation boards. My garage, which is attached to the house, is adjacent to the house wall. The garage consists of the first two rows of concrete masonry units, above which lies a floating, ground and polished concrete slab as the garage floor.
My earthworks contractor now wants to prepare the site: backfilling the driveway and terrace area as well as establishing the planned ground level behind the house. The planned finished ground level at the garage is about 1.5 brick courses above the concrete masonry units, and at the house approximately at the level of the top edge of the ground slab plus half a brick course (see photo).
I now have several questions regarding waterproofing and the base area:
As a client, I am still quite inexperienced in this area. My earthworks contractor is mainly focused on the backfilling and excavator work, so I would like to get expert advice before starting to avoid potential mistakes and prevent later rework.
Thank you very much!
My house does not have a basement. The ground slab is edged on the gable side with Bachl XPS 300 C SF perimeter insulation boards. My garage, which is attached to the house, is adjacent to the house wall. The garage consists of the first two rows of concrete masonry units, above which lies a floating, ground and polished concrete slab as the garage floor.
My earthworks contractor now wants to prepare the site: backfilling the driveway and terrace area as well as establishing the planned ground level behind the house. The planned finished ground level at the garage is about 1.5 brick courses above the concrete masonry units, and at the house approximately at the level of the top edge of the ground slab plus half a brick course (see photo).
I now have several questions regarding waterproofing and the base area:
- Base area waterproofing: How should the base area, which will be backfilled, be properly waterproofed?
- XPS boards: Do the existing perimeter insulation boards need any special treatment, or is a standard dimpled membrane sufficient?
- Garage: What is the correct procedure here? Is bitumen coating enough, or should a slurry waterproofing plus dimpled membrane be applied?
- Connection to base plaster: How should the connection to the later base plaster be correctly executed?
- Garage insulation: Does it make sense to add polystyrene insulation to the garage to insulate this area, even though the garage is currently uninsulated?
As a client, I am still quite inexperienced in this area. My earthworks contractor is mainly focused on the backfilling and excavator work, so I would like to get expert advice before starting to avoid potential mistakes and prevent later rework.
Thank you very much!
Hi Max,
There are several critical points in your project that you should carefully plan before excavation begins. In particular, the base area, garage, and perimeter insulation carry common risks that can become very costly if water or moisture penetrates later on.
Base area waterproofing: For a new build, the base area that is backfilled must be completely protected against rising or lateral moisture. Typically, this involves both horizontal and vertical waterproofing layers. Horizontally, directly on the concrete slab, a high-quality bitumen membrane or a flexible sealing slurry is sufficient. Vertically on the base, a combination of sealing slurry and a dimpled membrane is recommended. It is important that the waterproofing extends at least up to the planned ground level or slightly above to prevent moisture buildup.
XPS panels: Your perimeter insulation is generally pressure-resistant and moisture-proof. It is sufficient to protect it additionally with a dimpled membrane that prevents direct contact with soil or gravel and protects against mechanical stress from backfill material. No additional bitumen treatment is necessary as the panels are inherently water-resistant. Just ensure that the dimpled membrane fits tightly without gaps or voids.
Garage: The execution of the garage slab is crucial. Since the garage is attached to the house, the vertical concrete surface should be sealed against rising moisture. A dimpled membrane plus sealing slurry is a reliable solution. A simple bitumen coating often does not suffice where soil contact occurs, since mechanical damage from soil, gravel, or tools can compromise it. Make sure the waterproofing transitions seamlessly into the base area of the garage.
Connection to base plaster: The plaster should not be in direct contact with soil. The proper connection is at ground level plus 10–15cm (4–6 inches) clearance above the soil. The waterproofing must be installed first to prevent water from entering between plaster and waterproofing. Special plaster connection profiles exist that provide both decoupling and a clean edge.
Garage insulation: Additional insulation for the garage only makes sense if you plan to heat the garage or thermally decouple the garage wall. Otherwise, waterproofing is sufficient, as heat loss here is usually minimal. Styrofoam could introduce moisture risks if the waterproofing isn’t flawless.
My tip: Ask your civil engineer or site contractor to show you a plan detailing how the earthworks, waterproofing layers, dimpled membranes, and grading relate to each other. Especially at the interfaces between the house/garage and base/ground, a pre-construction inspection with a structural engineer or waterproofing specialist is worth it. It is often only on site that you realize where voids, slopes, or connections might cause problems.
Good luck!
There are several critical points in your project that you should carefully plan before excavation begins. In particular, the base area, garage, and perimeter insulation carry common risks that can become very costly if water or moisture penetrates later on.
Base area waterproofing: For a new build, the base area that is backfilled must be completely protected against rising or lateral moisture. Typically, this involves both horizontal and vertical waterproofing layers. Horizontally, directly on the concrete slab, a high-quality bitumen membrane or a flexible sealing slurry is sufficient. Vertically on the base, a combination of sealing slurry and a dimpled membrane is recommended. It is important that the waterproofing extends at least up to the planned ground level or slightly above to prevent moisture buildup.
XPS panels: Your perimeter insulation is generally pressure-resistant and moisture-proof. It is sufficient to protect it additionally with a dimpled membrane that prevents direct contact with soil or gravel and protects against mechanical stress from backfill material. No additional bitumen treatment is necessary as the panels are inherently water-resistant. Just ensure that the dimpled membrane fits tightly without gaps or voids.
Garage: The execution of the garage slab is crucial. Since the garage is attached to the house, the vertical concrete surface should be sealed against rising moisture. A dimpled membrane plus sealing slurry is a reliable solution. A simple bitumen coating often does not suffice where soil contact occurs, since mechanical damage from soil, gravel, or tools can compromise it. Make sure the waterproofing transitions seamlessly into the base area of the garage.
Connection to base plaster: The plaster should not be in direct contact with soil. The proper connection is at ground level plus 10–15cm (4–6 inches) clearance above the soil. The waterproofing must be installed first to prevent water from entering between plaster and waterproofing. Special plaster connection profiles exist that provide both decoupling and a clean edge.
Garage insulation: Additional insulation for the garage only makes sense if you plan to heat the garage or thermally decouple the garage wall. Otherwise, waterproofing is sufficient, as heat loss here is usually minimal. Styrofoam could introduce moisture risks if the waterproofing isn’t flawless.
My tip: Ask your civil engineer or site contractor to show you a plan detailing how the earthworks, waterproofing layers, dimpled membranes, and grading relate to each other. Especially at the interfaces between the house/garage and base/ground, a pre-construction inspection with a structural engineer or waterproofing specialist is worth it. It is often only on site that you realize where voids, slopes, or connections might cause problems.
Good luck!
M
MachsSelbst16 Nov 2025 22:30magicmax schrieb:
(...)
As a client, I am still quite inexperienced in this subject. (...)Then have the civil engineer or another specialized company carry it out. If the civil engineer can’t do it, you should ask what their actual profession is. This should be their daily business.
Because this kind of work is not trivial and carries a huge risk of damage if done incorrectly... and this particular spot is especially bad for practicing...
By the way, there is no picture...
Apart from that, right now is really not a good time for any kind of sealing or waterproofing, whether it’s slurry waterproofing, thick bitumen coating, PU sealant, etc. It’s too cold and too damp.
Yes, except for two-component coatings, of course. But as a layperson, you’ll be far too slow for those... trust me.
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