Log Cabin Base: Strip Foundations and a Timber Frame That Lasts

A log cabin lasts as long as its base keeps the timber dry. We built ours on concrete strip foundations with a treated timber base frame on top. The base is perfectly level, which makes every later step easier, and the timber never sits in the wet: air circulates on all sides, so the wood can always dry out. After more than 20 years our cabin still shows no rot or weather damage on the base. The same base also works for a log cabin kit – just build the frame to the dimensions of your kit.

Digging the trenches

First, dig the foundation trenches. A good spade usually does the job; in stony ground you will need a pick. The best tool we found was an old two-sided hoe – pointed on one side, wide on the other – which loosens the soil and lifts it out of the trench at the same time.

Take the positions of the trenches from your foundation plan and set the exact line with the batter boards. Work carefully: the timber frame will later overhang the foundations by about 1.5–2 cm (3/4 in), so the foundations must not stick out beyond it anywhere.

We were lucky to have firm clay soil, so the sides of the trenches stayed straight and we did not need formwork below ground. If your trench walls crumble – because of rubble or gravel in the ground, for example – use formwork boards that reach right down to the bottom of the trench. How deep the foundations need to be depends on your soil and on the frost depth in your area.

Pouring the strip foundations

1. Mix semi-dry concrete. We mixed the concrete in a cement mixer only until it was earth-moist and placed it in the trench in layers of about 15 cm (6 in). Boards along both sides of the trench stop soil from falling in.
2. Compact each layer. We used a small garden rake to compact the concrete layer by layer.
3. Add reinforcement if needed. Depending on the ground, place reinforcing steel in the lower third and later in the upper third. We used 12 mm (1/2 in) bars – three at the bottom, three at the top. The steel must always be fully surrounded by concrete. A structural engineer can tell you what your foundations need.
4. Build the formwork above ground. Once the concrete reaches ground level, set up formwork for the visible part. Align it exactly with a string stretched across the batter boards – that string also gives you the finished height.
5. Brace the formwork. Do not underestimate the pressure of wet concrete. We held the boards with clamps; a spacer block between them stops the clamps from pulling the formwork too tight.
6. Strike off. When the concrete reaches the top of the formwork, level the surface with a straight piece of timber.

Curing

After 1–2 days, depending on the weather, the formwork can come off. We poured our four foundation strips on four consecutive days, so we could reuse the same formwork boards each time. The fourth strip had a bigger height difference between the ground and the top of the foundation – old boards lying around came in handy.

The foundations only reach full load-bearing strength after about three weeks, but we could start on the timber base frame before that.

The timber base frame

Every timber eventually fails – even oak – if it gets wet and cannot dry out. The base frame is built so that this never happens.

Damp-proof layer and overhang

We first covered the foundations with bitumen felt to stop rising damp from reaching the timber. The timber frame overhangs the edges of the foundations on all sides by about 1.5–2 cm (3/4 in). Rainwater running down the wall then drips off the timber instead of creeping in between the timber and the felt.

Cutting the joints

We cut the 8 × 16 cm (approx. 3 × 6 in) bearers to length, marked the crossing points and cut halving joints, each to exactly half the depth of the timber. Clamping the bearers side by side made it easy to transfer the marks to all of them at once. We normally cut joints with a plunge saw and a chisel, but our plunge saw only cuts 50 mm (2 in) deep, so here a chainsaw did the rough work.

Treating the timber

We laid the bearers on trestles under cover and lightly sanded them with a random orbital sander. Sanded timber takes the stain more evenly, uses less of it and sheds water better. We also treated the inside faces of every joint – better safe than sorry. What has worked well for us is a three-coat system: a wood preservative primer against moisture, blue stain, rot and insects, followed by two coats of a water-based wood stain.

Assembling the frame

Once dry, the cross bearers went onto the strip foundations, on the bitumen felt, and the long bearers were slotted into the joints, about 60 cm (24 in) apart. This is the moment when you find out whether you worked accurately enough: the frame must overhang the foundations everywhere. We then screwed the joints together with wood screws and gave the whole frame a final coat.

Fixing the frame

To stop the frame from shifting during the rest of the build, fix it to the foundations at several points with metal angle brackets. Fit them on the inside: there they are protected from the weather and hidden once the cabin is finished.

With the base in place, we could start preparing the logs for the walls. If you are building a kit, you can now assemble it on the frame – which, of course, must match the dimensions of your cabin.

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