Foundations — strip foundations and slab-on-grade

Frost depth and frost protection · reinforcement · concrete volume · regulations and structural design

Foundation design is essential for a building that can resist loads, moisture and ground movement over time. Incorrect depth, poor drainage, insufficient sub-base compaction or inadequate reinforcement can lead to settlement, cracking and moisture damage. This guide explains the main principles of strip foundations and slab-on-grade construction, with practical rules of thumb, concrete volume calculations and clear limits on what must be established by design.

Important: This page is general guidance only. Foundation type, sizing, reinforcement, frost protection and build-up must always be assessed against soil conditions, structural loads, local code requirements and documented system guidance. Where in doubt, consult a structural engineer, geotechnical engineer or the local authority.

Foundation types

Strip foundation

A strip foundation transfers loads from load-bearing walls into the ground along a continuous concrete strip. It is common in houses, garages, extensions and other structures with linear wall loads.

Slab-on-grade

A slab-on-grade distributes loads over the entire ground-bearing floor area. It is widely used for garages, outbuildings and residential buildings, typically together with a capillary break layer, polyethylene membrane, insulation and a frost-protected perimeter solution.

Pile foundation

Pile foundations are used where the soil has low bearing capacity, such as soft clay, peat or loose fill. This always requires geotechnical input and structural design.

Pad foundation / isolated footing

Pad foundations support individual posts or columns. They are often used for decks, sheds and light structures where a full perimeter foundation is unnecessary.

Frost depth and frost protection

Foundations must be designed so the structure is protected against frost heave in accordance with applicable regulations and local ground conditions. In practice, this often means founding below frost depth or using a documented frost-protected foundation system. Indicative Norwegian reference ranges:

Region	Indicative frost depth
Southern Norway / coast	900–1,200 mm
Eastern Norway inland	1,500–1,800 mm
Central Norway	1,500–2,000 mm
Northern Norway	2,000–3,000 mm
Alpine / highly exposed areas	up to 4,000 mm

Note: Local factors such as snow cover, drainage, groundwater, wind exposure and terrain can affect frost conditions. Always verify the local requirement and use documented frost-protected solutions where relevant.

There is no universal frost depth for all locations. Frost protection must be assessed using local climate data, soil conditions and the selected foundation system.

Cross-section of a strip foundation with reinforcement, gravel bed and frost-safe placement

Strip foundations — typical build-up

Typical residential sizing

Foundation width is determined by design; typical small residential solutions are often around 400–600 mm
Foundation depth (section height) is determined by design; many simple residential cases are often around 300–400 mm
Loads, soil bearing capacity, drainage level and the wall build-up above the foundation must all be considered together
The top of the foundation is often kept somewhat above finished ground level to reduce splash water and moisture risk

Concrete grade

A common concrete class for small residential foundations is C25/30
For frost exposure or more aggressive ground conditions, a higher or otherwise more suitable class such as C30/37 may be relevant if justified by design or supplier guidance
Exposure class, air entrainment, cover and curing conditions must be assessed together

Reinforcement

Example of typical reinforcement for simple residential strip foundations:

Longitudinal bars: often 2 × ø12 mm top and 2 × ø12 mm bottom
Stirrups / links: often ø8 mm at 300 mm
Concrete cover must suit the exposure; exterior situations often use at least 50 mm
Bars must be supported on plastic chairs or bar supports so the reinforcement stays in position during the pour

Note: Reinforcement must be designed as part of the structural design where required. The values above are indicative only and are not a substitute for engineering.

💡 Plan pipe sleeves and service penetrations before the pour. Cast-in sleeves are usually easier, cleaner and more robust than drilling openings later.

Slab-on-grade

Typical build-up

Compacted capillary break layer of clean gravel or crushed stone
Polyethylene membrane / damp-proof membrane above the capillary break layer
Insulation where required by the building use and energy strategy
Reinforced concrete slab with a suitable perimeter and frost protection solution

Typical slab thicknesses

Garages and outbuildings are designed according to load and soil conditions; often around 150–200 mm
Residential slabs are determined by design; often around 250–300 mm including full build-up
The capillary break layer is often around 150–200 mm, but must suit the site and the chosen system

Membrane, insulation and joints

The membrane below the slab must be selected and installed according to a documented system and supplier guidance
Insulation thickness depends on energy requirements, building use and the selected construction
Reinforcement mesh and crack-control joints must be adapted to slab size, geometry and loading
Movement joints and panel layout should be designed to manage shrinkage and thermal movement in a controlled way

Step-by-step — strip foundation

Excavate to the designed depth with enough space for the capillary break layer and working room.
Lay and compact gravel using a plate compactor or rammer until the sub-base is level and stable.
Install blinding or levelling layer if required to provide a clean and stable base for reinforcement and formwork.
Install sleeves and penetrations before reinforcement and concreting.
Place reinforcement with the correct supports, position and cover.
Erect and brace formwork so it can resist fresh-concrete pressure and hold the intended geometry.
Pour and vibrate the concrete in accordance with the supplier's recommendations to reduce air voids and ensure full compaction.
Protect and cure the concrete against drying, rain and frost during the curing phase.
Do not load the foundation too early; follow the design, temperature conditions and concrete supplier guidance.

💡 In low temperatures, winter concreting measures may be required. Concrete that freezes during early curing can suffer major strength loss and durability problems.

Concrete volume — calculation

Volume for a strip foundation is calculated as: V = length × width × depth

Example: 4 sides each 10 m, width 0.5 m, depth 0.35 m:
V = (4 × 10) × 0.5 × 0.35 = 7.0 m³
With 10% added for waste and variation: 7.7 m³

It is common to add about 10% for waste, residual concrete and site variation
Ready-mix is often most suitable for larger pours with good access
Bagged concrete may be practical for small volumes or difficult access
The delivery method should be selected based on volume, site logistics and available labour

Reference table

Parameter	Indicative value
Frost depth — Southern Norway coast	900–1,200 mm
Frost depth — Eastern Norway inland	1,500–1,800 mm
Frost depth — Northern Norway	2,000–3,000 mm
Foundation width (typical residential)	typically 400–600 mm
Foundation depth (typical residential)	typically 300–400 mm
Common concrete class	C25/30
Typical longitudinal bars	often 2×ø12 mm top + 2×ø12 mm bottom
Typical stirrups	often ø8 mm at 300 mm
Exterior concrete cover	typically at least 50 mm
Slab-on-grade — garage	typically 150–200 mm
Slab-on-grade — residential	typically 250–300 mm
Membrane under slab	polyethylene membrane in a documented system
Capillary break layer	typically 150–200 mm
Waste allowance	about 10%
Approx. yield per 25 kg bag	about 0.0125 m³

Common mistakes

✗Founding above the frost depth — in cold climates the minimum is 80–100 cm below ground; frost heave will lift and crack the foundation
✗No drainage around the foundation — water collects against the basement wall and causes rising damp inside
✗Pouring on ground that has not been assessed for bearing capacity — differential settlement cracks the structure
✗Reinforcement without enough cover against soil (min. 40 mm) — corrosion sets in within 10–20 years
✗No capillary break layer (gravel or crushed stone) below a ground slab — moisture rises through the slab
✗Backfill not properly compacted — settlement after pouring creates uneven loading and cracking
✗Loading too early after pouring — concrete needs at least 7 days curing time before taking normal structural loads

Calculate your materials

Use the concrete calculator to estimate volume, bag count or ready-mix quantity:

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Volume, bagged concrete and ready-mix with waste allowance

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Frequently asked questions

How deep do foundations need to be?

Foundations must extend below the local frost depth. In the South: 12–18". Mid-Atlantic: 24–36". Northern states: 42–60". Always check local code.

What is the difference between a strip footing and a slab foundation?

Strip footings run only under load-bearing walls. A slab-on-grade is a continuous concrete pad under the entire building — better on expansive soils and provides a finished floor.

Do all foundations need rebar?

Rebar is required in most residential footings and slabs by code. Minimum is #4 rebar placed in the lower third of the footing. Always verify with local building requirements.

References

→ NGU — Norwegian Geological Survey: geological maps and ground information
→ SINTEF Byggforskserien — foundation guidance for small buildings
→ IRC 2021 — International Residential Code (foundation and footing sections)
→ Local building authority — structural safety, permits and frost protection requirements for your jurisdiction

Values in this guide are indicative and simplified. Actual design must be based on soil conditions, loads, local frost protection requirements and documented system solutions. For permit-controlled or load-bearing work, always involve a qualified professional. Last reviewed: May 2026