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Make room without moving, it's possible. Easy and quick to implement, the wood extension offers many advantages. The Leroy Merlin Guide 2016 details the first stage of this timber construction, the foundations.
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Wood frame house extension

Enlargement wooden frame house

The concrete slab, structure carrying the extension

The extension is built on a concrete slab. In order to preserve the wood from any contact with the ground, this base is at least 20 cm above the ground.

Create a wooden extension: the foundations: wooden


If the slab rests on a crawl space, the water and electricity networks are installed before pouring the slab.
Materialize the surface of the slab on the ground with tensioned cords. Dig a hole 25 cm deep. Fill the bottom of a layer of 10 to 15 cm of gravel. Add 2 cm of sand. Install boards plastered with oil all around the future slab. Calculate the concrete dose by multiplying the surface of the slab to sink by its thickness. For an armed slab, opt for a concrete dosed at 350 kg / m3. Check the slope.
Set up the sealing film. Lay the strips so that the edges of the slab are completely covered and cover them with a minimum of 20 cm. Install splice joints to divide the slab. These PVC expansion joints also facilitate concrete equalization.
Ask them lattice panels 3 cm from the ground on the whole surface (one mesh superimposed on each end) keeping 3 cm on each side. Attach the panels to each other with annealed wire.
Pull the concrete in each section. Use an aluminum mason ruler to level the slab, guiding it over the splice joints. With a trowel, smooth the surface to measure.
Leave a day later. The patching takes place one month later.

Separate the extension of the house

Create a wooden extension: the foundations: foundations


If the extension is an extension of the house, ensure that the two frames are independent. Thus, the slab and the foundations of the extension are contiguous to the structure but do not rest on it.
Allow a space of 2 cm between the constructions thanks to an expansion joint. This separation avoids structural imbalances and creates a lasting whole.
Install expansion joints before pouring the concrete slab to limit settling and cracking.
Inside the house, for a slab less than 2 m wide, have an expansion joint every 6m2 and a joint every 15 m2 for a width of more than 2 m.

Protect the extension of moisture

To avoid humidity and thus preserve the structure, it prevents the wet lifts of the soil. The sealing in details.

Create a wooden extension: the foundations: create


Extension on crawl space
Lay a strip of arase over the entire width of the walls. Set up before pouring the concrete slab, it allows to cut capillarity. For example, use an anti-salt strip based on waterproof mortar and a bitumen strip.
Extension on solid ground
This construction in direct contact with the ground requires greater insulation. To seal the slab, install a polyane film and thermal insulation. Install a drain and prepare the base of the walls.
The slab on hedgehog is a good option. Composed of a compacted layer of gravel, the hedgehog ensures the stability and waterproofness of the slab.
To limit the wet lifts on the walls, take care of the preparation of the foot of the walls. Coat the substructure with waterproofing and cover with a protective membrane ensuring the circulation of air. To prevent infiltration from the ground, install a profile at the top of the membrane.
The drain and the draining trench are covered with a geotextile film. Then fill the trench with a layer of gravel then a layer of gravel and sand.

Prevent termite risks

Termites and risks in France,

Termites are real dangers for the home. To prevent their appearance, it is essential to act before construction. Ensure the watertightness of the basement, point of access for these pests. Install an antitermite film under slab or treat the soil under extension with an insecticide. Also use an insecticide for all wood: coat the frame and all woodwork.


Video Instruction: How to Dig the Foundations: How to Build an Extension (2)