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.

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:

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.

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

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
• Radon / ground-gas barrier and sub-slab venting where the site or local code requires it — a damp-proof membrane is not automatically a radon barrier, so use a documented radon system if radon is a risk in your area
• 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

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

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

Common mistakes

• ✗Founding above the local frost depth — frost heave will lift and crack the foundation. There is no universal minimum depth; use your local frost depth (from local code / frost-index data) or a documented frost-protected design rather than a fixed figure
• ✗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: