Log Cabin Foundations: Choosing The Best Base For Your Cabin

Deciding upon the foundation to be used for your log cabin is an important decision.

Log Cabin foundations are not the same as simple shed foundations.

Foundations are used to transfer the weight and load of your cabin safely through the ground. Consequently, your log home should have a strong and durable base to be built upon.

A foundation is used for strength and stability; a good foundation will protect your cabin from:

• Subsidence – where the soil is moving away from your cabin over time and removes the surrounding foundation. Subsidence occurs over a long time, but, it’s easy to spot in its most severe forms as typically the cabin will be lopsided.
• Settlement – where the weight and load of the cabin force soil to move away, causing the cabin to sink into the ground. Settlement typically occurs early in the cabin’s life.

Keep reading to discover the different types of foundations to use, when to use them, and how to build them for your new log home.

Handy guide to selecting a foundation type

Just because your cabin’s foundation won’t be seen after the cabin is built – this does not mean it isn’t important!

The foundation you decide to build for your cabin will be influenced by:

• Your site’s rock and soil type (Rock, Chalk, Gravel, Sand, Clay or Peat)
• The size of your log cabin
• Water tables
• Rock and gravel contours
• Drainage design
• Site ground (topography)
• Construction budget

Remember, throughout the design phase of your construction, to not over-engineer your foundation.

Most advice given for foundations relates to brick and mortar homes, which require a far more substantial foundation than a log cabin or timber frame building.

This results in unnecessary costs for your cabin construction project and wasted materials.

As a result, this post features shallow foundation types: strip, pad, and raft foundations.

More advanced foundations such as deep foundations: pillars, reinforced wide strip and piles aren’t typically required for traditional log cabin homes.

Raft Foundation (Concrete Base)

A raft foundation is a very simple foundation technique where a raft of concrete is laid underneath your entire log cabin.

The raft foundation is used to spread the entire base of the cabin’s load over all of the site’s floor area.

A raft foundation is cheaper and quicker than a strip foundation as it requires less raw materials and excavation.

Raft foundations are typically laid on hardcore, such as stone or broken bricks – which don’t absorb water, to protect your cabin from movements in the ground.

When should you use this technique?

• Typically, raft foundations are laid on soft grounds (such as soft clay) to help spread the load of your cabin. – this is because soft grounds have a lower bearing capacity.
• Raft foundations are suited to poor ground conditions where traditional foundations (e.g. strip foundation) wouldn’t transfer the cabin’s load properly.

Top-Tip
Calculate the volume of concrete required for your raft foundation by calculating:

(Length x Width x Depth “Block Thickness”) *1.3

Example for a 900FT² cabin:

(9.15m x 9.15m x 0.1m) * 1.3 = 8.36m³ * 1.3 = 10.89m³

How to build a concrete base raft foundation

1. To start with, mark all four edges of the cabin’s perimeter with pegs. Add 6” to your cabin’s outside perimeter when marking out:
   

   e.g. A 30FT x 30FT cabin needs a 31FT x 31FT foundation.

   Once you’ve marked all four edges you can use Pythagoras’ theorem to make sure the cabin’s foundation is square. Alternatively, an easier way is to measure both diagonals of your foundation and if they both measure the same – your foundation is square.

2. Excavate all of the ground within your pegs to a depth of 80mm to 100mm. Fill this area with a layer of hardcore and then use a plate compactor to make sure the hardcore is compact:
   
   Optional – Use steel spacers (known as “rebar”) for further reinforcement. This is typically used in strip foundations or for larger cabins. If you’re interested in this look at costs to build a log cabin under the site foundation heading.
3. Around the outside perimeter of the foundation, you will need to lay wooden formwork to act as a shutter to hold the concrete in.
4. Mix and pour the concrete using the suggested mix strength below. When mixing the concrete try not to add too much water at once and weaken the mix.

   Mix Strength
   Building Standards 5328 (BS-5328) recommend a strength of C30:1 part cement, 2 part sand and 3 parts hardcore.

   Rule of thumb: 1 man can mix 1m³ per day

   Mixing and wheel barrowing concrete can become very physically challenging. My advice is, anything over 2m³ per day and you should be looking to purchase ready-made concrete.

5. Level the concrete with a long straight piece of wood, resting each end on the formwork, to get a level finish.

Top-tip for a stronger foundation

For a solid base, instead of using formwork and wood to shutter the concrete base, you can use blockwork. This is a common trick used for heavier cabins to give the base more rigidity.

Strip Foundation Base

A strip foundation is the most common foundation technique used for constructing traditional brick and mortar homes.

A strip foundation is a continuous ‘strip’ of concrete underneath your cabin’s load-bearing timber. This strip then acts as a level standing which you can use to lay your logs on.

A strip foundation can be used when your cabin’s site has got a strong soil base and no evidence of standing water or waterlogging. The bottom of the strip foundation should rest on a compact soil base.

The strip foundation is the second cheapest form of foundation that you can use for your cabin.

When should you use this technique?

• A strip foundation should be used when you have clay on site.
• A strip foundation should only be used for log cabins being based on dry-sites, without standing water.
• Strip foundations are a shallow form of foundation so should really only be used to support single story cabins – unless you chose to make a deep strip foundation (similar to the one you would use for a brick and mortar house).

Top-tip

The starting point for any base or foundation is a level and dry site.

• If your site isn’t level then you will need more shuttering and concrete to get a finished level which will cost more money.
• If your site isn’t dry then the concrete won’t cure properly.

How to build a concrete base raft foundation

1. To start with, using your floor plan design for your log cabin, identify the entire load bearing timbers. Then, using pegs and string, mark out where these timbers will run parallel to the site’s ground.
2. Once you have marked the strip foundation out, excavate all of the ground within your pegs to a depth of 600mm with a width of 300mm. The width of 300mm will make sure that 12” logs will sit well on the foundation.

   Top-tip
   Ensure the trench’s edges are level both horizontally and vertically.

3. [Optional] Lay down concrete spacers and rebar in the bottom of the trench – steel reinforcement mesh, which provides extra strength for the foundation.
4. Following the same steps in the raft foundation, mix and pour the concrete to a depth of 150mm.
   

   Keep both eyes on the weather: If it is too hot (95 Fahrenheit or above) then the concrete will set too fast and become weaker. If it is too cold (35 Fahrenheit or below) then the water in the concrete mix can freeze and crack causing the mix to not cure properly.

5. Once the concrete has set, using engineering blocks, build a wall up to your finished ground level.
   

Variations of a strip foundation

A trench fill foundation is a common adaption of a strip foundation. A trench fill foundation is deeper than a strip foundation, but, also thinner.

The trench is then filled with concrete as opposed to part concrete and part engineering brick.

Typically, trench fill is used for clay soil types where a deep trench is required.

Pad Foundation (Known as concrete piers)

A pad foundation carries a single-point load from your cabin. Typically the four corners of your cabin will rest on a concrete pad foundation – so each of the four concrete pads will be underneath the corner of your log cabin.

Pad foundations are typically square and made from concrete reinforced with rebar (“steel members”).

The pad then transfers the load from your log cabin home into the surrounding ground.

The size of your pad foundation can be increased to further distribute your cabin’s load over a wider space (think of the raft foundation above).

Top-tip
Pad foundations are sometimes referred to as ‘pad-stone foundations’ or ‘concrete piers’.

The pad foundation is the cheapest form of foundation that you can use for your log cabin.

When should you use this technique?

• A pad foundation should be used when you have an existing strong soil base for your cabin’s site.

As the construction process for a pad foundation installation is very similar to a raft, it’s worth reading the construction process for raft foundations. The differences are as opposed to excavating and concreting the entire base you limit to smaller concrete blocks.

Best advice to remember:

• A foundation is used for strength and stability; a good foundation will protect your cabin from subsidence and settlement
• Throughout the design phase of your construction do not over-engineer your foundation
• If you are new to construction then read Building and Construction Handbook – Roy Chudley to make sure you properly calculate the weight of your cabin’s timber, loadings and the foundation required

I hope you have found this post on cabin foundations informative and helpful. You should now be able to pick the right foundation for your log cabin home!

Make sure you review the best tips and advice summary above to avoid making any easy mistakes and, where possible, consult a structural engineer to verify your foundation choice.

Have you used one of the above foundation types or something different for your cabin? Let us know below.