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Built in the sixties, this masonry house deserved a renovation and better insulation. The idea of ​​a wooden extension naturally imposed itself.

DIY: wood frame extension, all stages of construction

Practical advice

Level : professional
Cost: 400 € HT / m2 (supplied, laid, out interior finishes)
Time: 1 month (to two companions)
Equipment: circular saw, carpenter's hammer, pneumatic nailer, electric planer, ripper, handsaw, impact driver
Rain shield and vapor barrier
These screens protect the frame and isolate it from external (weather) and indoor (vapor) moisture. The rain screen is a microporous plastic film that lets air through but not water. It settles on the outer walls. The space between the cladding and the rainscreen (called the air gap) must be ventilated to remove moisture or condensation that may accumulate.
The vapor barrier (or vapor barrier) is placed between the insulation and the interior facing. It minimizes the penetration of water vapor into the insulation. Evacuated to the outside, it crosses the rainscreen before being eliminated by the ventilated air space. According to DTUs (Unified Technical Documents), the water vapor permeance of the rainscreen must be at least one hundred times greater than that of the vapor barrier. Only products benefiting from a technical opinion from CSTB * give the assurance of this effectiveness (Doerken, Onduline, Siplast, Soprema...).
Scientific and technical center of the building. cstb.fr

The expansion

The coating covering the exterior walls of the existing building required serious refreshment. The 5.5% VAT and the tax credit helping, the owners decided to improve the interior comfort by insulating the walls in accordance with the 2005 thermal regulations (energy saving...). They also programmed an extension of 35 m2 which logically fits in a setback of construction. On the advice of an architect, they opted for a wood frame extension that will house a bedroom and a bathroom.

The structure and frame of this enlargement have been entrusted to the men of art because of their technicality. The owners took care of the interior finishes (laying plasterboard).

A semi-industrial solution

In the booming wood construction market, the companies offer semi-industrial solutions ("Mobissimmo" systems from Point P or "Batimob" from Réseau Pro and Panofrance). Their design offices design and dimension the elements of the structure according to the plans established by the architect.


Like concrete and brick blocks, standardization of elements (floors, walls, frames, partitions) simplifies supplies and reduces assembly time on site. In the case of a panel frame construction, four sections were selected by the design offices. Their dimensions: 45 x 220 mm for the floor joisting; 45 x 120 mm for the frame of the walls (smooth or sand plates, uprights, sleepers); 45 x 95 mm for interior partitions; 45 x 22 mm for fastening rain screens and cladding boards.


The species are mainly spruce fir (class 2 * minimum), northern pine or Douglas pine. Composite woods (glulam, lamibois, "I" beams) are used for some structural parts whose mechanical strength must be greater (girders, floor joists, lintels, framework, etc.).

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Foundations on piles

After a geological study, the clay soil on which the house is built does not have sufficient lift. In addition, there are risks of sliding due to its slope. Deep foundations are therefore envisaged.

Tubular metal piles are driven 7 m deep to reach the bearing soil (their diameter has been determined by the design office). Upgraded to catch the slope of the land, they give the building a solid foundation. Equipped with a steel plate, they will support two glulam beams positioned in the length of the future extension.

A wooden floor on crawl space

This structure on piles (three piles for each beam) creates a crawl space that avoids rising damp but does not solve the problem of termites. Prior to the construction of the floor, a precautionary treatment termicide (certified CTB-P + by the FCBA **) is sprayed on the ground and around the house, at the rate of three liters per square meter.


Fixed on the flanks of the main beams, metal shoes support joists. These joists are "I" composite beams with a total height of 220 mm. To improve their strength, they comprise a central core of corrugated galvanized steel, inserted between two solid pine chords (Nailweb).


The fixing of horizontal panels (OSB *** 18 mm thick) on the high and low chords creates a series of watertight boxes in which an insulating material takes place (mineral wool panels or insulated bulk insulation).

A panel structure

On this floor, the walls are high. The framework of the constructive panels is assembled flat on the ground. Whole wall sections are built this way. Then simply straighten them manually or with hoists.


Nailed to the outside of the walls, OSB Class 3 * panels (9 mm thick) provide bracing ("Isoroy's Lameply OSB Class 3"). They participate in the rigidity of the frame and compensate the "weak" sections of the woods. This bracing also serves as formwork when the loose insulation will be blown.


As the cladding is not enough to seal the water and dust, a rain screen is summarily stapled on the bracing panels. It is then held by cleats of 45 x 22 mm on which the cladding will be fixed.

A "heated" cladding

The cladding boards are laid vertically, which imposes a framework of cleats nailed horizontally and spaced 40 cm. To facilitate the ventilation of the walls, an air space is arranged between rainscreen and clapboard. In addition, the lines of the cleats are interrupted to increase the circulation of the air.


The maritime pine clapboard is "heat-treated". A non-chemical component process, called "high temperature" or thermally modified wood (BMT) that can treat local woods.


Heated between 200 and 280° C in an oxygen-free atmosphere (to avoid burning wood), the main organic components are neutralized: they become "inedible" by xylophagous insects. This type of process gives the maritime pine job characteristics equivalent to exotic species and autoclaved pine (class 3 or 4 *).


* For job classes and resistance to humidity (ex-risk classes), consult the website bois-construction.org (tabs "Documenting the wood material" then "Wood in construction" ).
** The Afocel (Forest Cellulose Association) and the CTBA (Wood and Furniture Technical Center) merged in 2007 to give birth to the FCBA (Forest Cellulose Technology Institute, wood-construction, furniture).
*** Oriented strand board or panel of oriented slats (panel derived from wood).

Provide a sanitary space

Provide a sanitary space

Metal piles are driven into the load-bearing ground to create solid foundations. Plates allow to wedge the two main beams. Hoofs were previously fixed, they support the joists.

Equalize and fix the main beams

Equalize and fix the main beams

The main girders are ground with a ripper to make up the differences in the level of the piles and obtain a rigorous flatness before their final installation. They are then screwed on the decks.

Fix the joists to form the floor

Fix the joists to form the floor

"I" joists are composite beams 220 mm high. They are inserted and screwed into the metal shoes fixed every 50 cm on the flanks of the main beams.

Laying the derived panels

Laying the derived panels

Wood-based panels (OSB Class 4) are stapled between the joists on their lower chords. They act as spacers and constitute the lower part of the formwork of the insulation.

Insert mastic between the panels

Insert mastic between the panels

Junctions between panels (pipe passages, etc.) are made using sealant or polyurethane foam to prevent thermal bridging and to make the structures airtight.

Assemble and secure the panels on the top of the joists

Assemble and secure the panels on the top of the joists

Tongue and groove panels (OSB Class 3) are nailed or stapled to the top chords of the joists. Installed with staggered joints, tightly assembled and glued preferably, they must rest on at least three supports.

Mount the structure on the ground

Mount the structure on the ground

The wood structure is mounted on the ground. Around the joinery tables, the amounts are doubled to reinforce the resistance. Squareness is checked by measuring the diagonals.

Fix the bracing panel

Fix the bracing panel

The location of the uprights is marked with the line. Then the OSB bracing board (cut to dimensions plus 10 cm at the bottom) is stapled or nailed to the structure.

Lay the rain screen

Lay the rain screen

The rain shield is stretched and stapled onto the bracing panels assembled and bolted together. A lateral overflow of 20 cm is provided for the overlaps.

Fix and consolidate the structure

Fix and consolidate the structure

The straightened structure is held in place by bastaings. Once its alignment and plumb with the spirit level verified, it is screwed (or nailed) into the main beam.

Screw structure panels

Screw structure panels

The structural panels are assembled together by bolting or screwing. Fasteners and bolts must be made of stainless steel or hot dip galvanized.

Install the sliding bay lintel

Install the sliding bay lintel

The lintel of the sliding bay is made of laminated wood. Its section is calculated according to the range and the weight to bear. He is posed and screwed to the top of the walls.

Seal the frame to the wall

Seal the frame to the wall

Composed of industrial half-trusses, the frame is placed on a wall beam. It is pegged and bolted into the existing facade.

Isolate and apply bandages

Isolate and apply bandages

Planks (spacers) are screwed on the entruites of the farmhouses to maintain their spacing. Others are fixed diagonally under the rafters, they ensure the bracing.

Lay the vapor barrier

Lay the vapor barrier

The vapor barrier is stretched and stapled on the interior frame. Around, a bead of mastic-glue ensures airtightness. Attached over, the cleats are the framework of the siding.

Inject the wadding

Inject the wadding

Cellulose wadding is injected under pressure into the hollow structure of the walls, the floor and later into the attic (when the ceiling is mounted).

Lay the cladding supports

Lay the cladding supports

Cladding supports, cleats of 45 x 22 mm are nailed through the rain guard in horizontal lines spaced 40 cm. Spaces between brackets will allow the free flow of air.

Lay the cladding boards

Lay the cladding boards

The cladding boards are stapled (or nailed) to the right of each cleat. A batten attached at the bottom of the cladding serves as a level guide. It will be removed at the end of the pose.

Conceal the pinion

Conceal the pinion

The high cut of the gable cladding is concealed by the facing of the advance of the roof (carried out later). An anti-rodant grill is placed on the bottom of the cladding.

A wood frame extension ready to install

FAQ - 💬

❓ How long does it take to build a timber frame extension?

👉 How long does it take to build a timber frame extension? There are a number of variables involved in the timescale of a timber frame extension built. However, on average, you can expect a small extension of around 15m2 to take four to six weeks, while one of 30m2 is likely to take at least six to eight weeks.

❓ Can I build a wooden extension?

👉 A timber frame extension is an impactful way to increase your home's space and add value to your property. This type of extension is often quicker and easier to build than blockwork projects and is a great option if you're building on a budget.

❓ How long does it take to build a timber frame?

👉 If everything goes to plan, ideally your timber frame self-build could be completed between 8-12 months. However, bear in mind that the precise design for a timber frame must be finalised in advance of manufacture to ensure efficient delivery.

❓ What do you need to build an extension?

👉 You can choose a build route that suits you, your budget and the type of extension you want to build.

  1. Hire an architect. ...
  2. Employ a Design and Build company. ...
  3. Project manage the scheme yourself. ...
  4. Split the project management. ...
  5. Internal joinery. ...
  6. Flooring. ...
  7. Decoration. ...
  8. Tiling.

❓ Do I need foundations for a timber framed extension?

👉 When adding timber decoration or supporting structures to an existing building, the current foundations will usually suffice. Although hardwood timber such as Oak is a heavy material, a foundation designed to support concrete or brick should be strong enough.

❓ Is a timber frame extension cheaper than brick?

👉 A timber frame extension often costs less than bricks and mortar because there is less skilled labour involved.

❓ Is a wood extension cheaper than brick?

👉 A timber frame extension often costs less than bricks and mortar because there is less skilled labour involved.

❓ How long will a timber frame extension last?

👉 Usually, structural-grade construction timber has a guaranteed lifespan of around 40 to 50 years. In comparison, masonry products can last up to 150 years.

❓ Can you get a mortgage on timber frame house?

👉 Can you get a mortgage for a timber framed house? It is certainly possible, but as many lenders consider them non-standard construction properties, your choice of lenders will be more limited. There's a number of factors that can influence a lender's decision, such as the build quality of your chosen home.

❓ What is the cheapest way to build an extension?

👉 What is the cheapest way to build an extension?

  1. Loft conversion – average value add: up to 20%
  2. Single-storey extension – average value add: 5-8%
  3. Double-storey extension – average value add: up to 12%
  4. Conservatory – average value add: 5-7%
  5. Garage conversion – average value add: 10-20%

❓ What size extension can I build without planning permission?

👉 Detached House Extension Rules 2022 Under permitted development for a detached house, you can extend up to 4m under permitted development, and up to 8m under the larger home extensions scheme / prior approval.

👉 Adding a timber frame extension is a time and energy efficient way to add space and value to a home, but it can be hard to decide on the right style. Take a look at the amazing homes below, all of which have been transformed by extending in timber frame, for some innovative ideas.

👉 This large two-storey wrap-around timber frame extension was designed by PAD Studio and has been clad in timber and render. The large expanses of glazing help to maximise the garden views. (Image credit: Nigel Rigden) Get three detailed quotes and go over them with your designer to ensure you are comparing on a like for like basis

👉 This single-storey timber frame extension has been clad delicately in timber to compliment the brick and flint of the original period house. Taking advantage of the large garden means the new structure enjoys stunning views. Assistant Editor Amy began working for Homebuilding & Renovating in 2018.

👉 This might be oak flooring that runs throughout, introducing timber doors, exposing previously painted woodwork, or even choosing a few pieces of furniture in the same woody tones. What is the timber frame extension build process like?


Video Instruction: timber roof on a single storey extension