Technical Information

Return to Technical main page


CLT Frequently asked questions

Below is a summary of the frequently asked questions with a brief answer. For a more in-depth understanding please look for content within our Technical section or contact us via email or phone.

CLT Frequently asked questions

Q. What is CLT?

A. Cross Laminated Timber (CLT) is made of strips of spruce, pine or douglas fir (called lamellae) laid orthogonally and glued under pressure​.

It is made in dedicated factories in standard panels which typically measure 13 metres by 3 metres.​
The glues used in the production of CLT by and  large are zero VOC. If you wish to order CLT, Lamella MMC will check the glue with the manufacturer for you before processing the order.

Q. What is CLT’s carbon Performance vs Conventional building materials?

A. “One ton of CO2 is emitted into the atmosphere for every cubic meter of concrete
created. In contrast, CLT contains “sequestered carbon,” or carbon naturally stored in
wood during tree growth.

Thus, despite all the energy used in the extraction and manufacturing processes,
emissions from wood construction will never match the amount of carbon that is kept
“sequestered” in the CLT.” (source: Jose Tomas Franco-Arch Daily)

There is a lot of publicity regarding the imminent development of Sustainable Concrete
and Green Steel. Both require the application of new(ish) technologies
namely renewable electricity, green hydrogen and carbon capture.

Carbon capture and the generation of green hydrogen are costly unproven technologies, each
requiring a great deal of energy input. Green Steel and Sustainable Concrete will
undoubtedly be more expensive than the equivalent current materials, but we will
never be able to reduce their carbon performance below zero.

By contrast CLT is made of a naturally carbon negative material, wood, which
sequesters carbon as it grows and draws its energy directly from the sun, as it has
done since the sun first shone on the Earth, always reliably, and at no financial cost.

Q. How strong is CLT ?

A. CLT delivers at minimum the same structural strength as reinforced concrete, but it’s a
material with a high degree of flexibility that has to undergo great deformations to
break and collapse – unlike concrete (source Jose Tomas Franco).

CLT is strong in compression, tension and shear and can be used for walls, floors,
ceilings, roofs and lift shafts. It is a valuable, strong structural material that can be
used to span up to 12 metres and has been proposed for buildings of up to 40

Q. How much does CLT weigh?

A. CLT is a lightweight building material. It weighs just under half a tonne per cubic
metre. This compares to concrete which weighs 2.7 tonnes per cubic metre, more
than 5 times as much.

Consequently, buildings made of CLT require much lighter foundations and less
expensive ground works. CLT’s lightness also makes it ideal for the upwards
extension of existing buildings (e.g. low rise blocks of flats and offices with the benefit
of permitted development).

Q. What are the Insulation properties of CLT?

A. CLT made of spruce has an insulation performance similar to airblocks, aircrete
blockwork and far superior to concrete blockwork or brickwork. In keeping with local
legislation additional insulation is normally required for external cladding and internal
finishing where the exposed timber finish is not authorized.

One commentator said, “Physically, to achieve the same degree of insulation that a
100 mm thick CLT wall would afford, we would need to build a concrete wall 1.80 m
thick (1/18 ratio)”.

Due to the tight tolerances in its manufacture, CLT buildings are draught free and
lend themselves to being engineered to passiv haus, or better, standards.

Q. What is the Cost and Construction Risk of CLT?

A. Very often costs are spoken about very narrowly as if they were just the cost that the
owner has to pay the contractor to build his building, assuming nothing goes wrong.
This discussion often takes no account of finance costs, health and safety costs, the
benefits of fewer deliveries to site, the cost of unforeseen delays, the opportunities
afforded by quicker builds etc.

This is a complex subject and needs to be considered together with Construction
Risk as abstract risks can very easily become firm costs- for example, exposure to
bad weather can easily lead to delays with obvious cost implications.
The economics of the construction industry are changing fast. Material and skills
shortages are leading to significant cost inflation in conventional materials in the
short term.

In the medium-term carbon pricing and/or the application of expensive technologies
such as carbon capture are likely to increase the costs of carbon emitting materials
such as concrete, cement, steel, bricks and concrete blocks, while not affecting the
costs of carbon negative CLT.

We have developed a paper on the cost and construction risks of building in CLT vs
Conventional materials. Contact Lamella MMC for info.

Q. How quickly can you build in CLT?

A. Building in CLT is much quicker than in traditional building materials. In most cases
you can reckon on a 25% to 30% reduction in overall project times, and a 50% or
more reduction in superstructure construction.
The trade off is that the planning and design needs to be more rigorous and is more
time consuming. But this can be mitigated by building the same design in volume so
that the design can be done once for multiple builds.
The results include reduced funding costs, reduced waiting lists, hitting central
government targets and achieving social and political objectives in a term of office.

Q. What types of Buildings can CLT be used for?

A. CLT can be used for the building of all property types. From single residential homes
to skyscrapers. From hospitals to schools, sports facilities to office buildings. Not
forgetting retail and industrial buildings. The only limitations are your imagination and
local legislation.
If we consider Mental Health, Education, physical health and wellbeing to be
paramount values we would argue that every building should be built of natural
products such as CLT.

Lamella CLT Buildings

Q. How long do timber buildings last?

A. “The magnificent hammer-beam roof of Westminster Hall is the largest medieval
timber roof in Northern Europe. Measuring 20.7 by 73.2 metres (68 by 240 feet), the
roof was commissioned in 1393 by Richard II, and is a masterpiece of design”

It was prefabricated in Farnham and delivered by barge and wagon to Westminster,
where it was erected on site. It was in fact an early example of MMC (Medieval
Methods of Construction!).
It is still giving good service today, over 700 years later.

This is by no means the oldest wooden structure in the world. Horyuji, an ancient
Japanese temple, was built in 607 CE. It is the world’s oldest surviving wooden

Horyuji was constructed from Japanese cypress that were roughly 2,000 years old. It
has been 1,500 years since the cypress were cut down, and the wood still stands
There is no doubt that timber is a strong, long-lived, high performance construction

Q. What is the fire performance of CLT?

A. Fire is a contentious subject and is one of the first issues raised when timber
construction is discussed. CLT buildings must of course meet the same fire
regulations as all other types of construction and therefore pose no greater risk to
their inhabitants. In fact, in many respects CLT performs better in a fire than steel or
concrete. Timber stud partitions and timber joist floors have been used for many
years as fire separating elements and their performance has been proven time and
time again in test and real fire situations.

CLT burns slowly and the char resists heat penetration. When large timber members
are subjected to fire, the uncharred inner portion maintains its strength, giving the
structure a higher survival factor. The rate of progress of pyrolysis is governed by the
low thermal conductivity of the timber and the lower conductivity of the charred layer,
which also hinders the access of oxygen to the timber surface.

The total insulating effect of the char and timber is such that temperatures only a
short distance in from the char line will not rise sufficiently to impair the strength of
the wood. In the same timeframe steel will melt, and concrete will crumble.
The Structural Timber Association and Stora Enso are carrying out fire tests of
encapsulated CLT, the results of which we expect to know at the end of 2021.

In practice the answer for taller residential accommodation in the future may be to
build the outer walls either of encapsulated CLT or lightweight steel with the inner
walls in CLT (the hybrid solution). Lamella MMC can offer independent advice on the
appropriate approach.
In all cases buildings need to be designed in line with legislation and with fire risks in
mind. Lamella MMC have partnered with MLM Sweco to offer the best advice.

Q. What happens at the End of Life of a CLT building?

A. CLT is unique in its versatility. Not only do well built and maintained timber buildings
stand for hundreds of years. CLT can be dismantled and repurposed, rebuilt and moved, not simply used as landfill or hardcore. CLT structures can be erected with
fittings that will allow for simple future disassembly and reassembly. But as CLT
buildings in the UK are relatively new we have at least 50 years to imagine how we
will re-purpose our CLT.
Reuse / Recycle.

Q. What are the Forestry standards of suppliers to Lamella MMC?

A. Lamella MMC only orders CLT that has been produced from timber harvested from
forests certified by either the FSC (Forest Stewardship Council) or PEFC
(Programme for the Endorsement of Forest Certification).

Q. Where is CLT manufactured?

A. In the northern hemisphere, CLT is manufactured predominantly in continental
Europe and North America, generally close to forests and sawmills.
However CLT can be cut and stocked anywhere there is a demand. Cutting
represents about 40% of the value of the finished panels.
There is high and fast-growing demand in the UK and a very real opportunity for
Brighton to become a UK cutting and distribution centre.

Q. What are the Wellbeing and Learning advantages of CLT buildings?

A. As a natural material, wood is thought to provide a connection to nature and
therefore improve physical and mental wellbeing. Internationally, studies have
demonstrated this relationship in offices, schools and hospitals.
Office design: productivity can be increased by 8% and rates of well-being increased
by 13%
Education spaces: increased rates of learning, improved test results, concentration
levels and attendance, reduced impacts of ADHD
Healthcare spaces: post-operative rates of recovery reduced by 8.5%, reduced pain
medication by 22%
Retail: the presence of vegetation & landscaping has been found to increase average
rental rates on retail spaces with customers indicating they were willing to pay 8-12%
more for goods and services.
Homes: 7-8 % less crime attributed to areas with access to nature and can command
an increase of 4-5% in property price.

Q. Is CLT used in other countries?

A. As part of the French governments push for sustainable urban development all new
public buildings financed by the state must contain at least 50% wood.

The World’s Tallest Timber Buildings

CLT is widely used globally but predominantly in North America, Scandinavia and
Europe. Currently projects are also underway in two of the world’s most extreme
climates, Singapore and Dubai.

Back to Home Page