# Why CLT Buildings Are on the Rise

By Jayk

Topic: Engineering

[Open the interactive Jig](https://ask.jig.com/j/YO5M0OhIv4Ui)

## Guide

### 1. Why CLT Buildings Are on the Rise

Cities are starting to build tall with big wooden slabs called cross\-laminated timber, or CLT\. Here is a real\-size sample beside a model tower\. Let us look inside a panel and find out why builders choose it now\.

Drag anywhere to turn the view\. Scroll or pinch to move closer\.

### 2. Boards glued crosswise

A CLT panel is made of solid boards laid side by side in layers\. Each layer is turned a quarter turn from the one below and glued under strong pressure\. There is always an odd number of layers, so both faces run the same way\.

Panels usually have 3 to 9 layers, each board about 2 to 5 cm thick\.

### 3. Strong along the grain

Wood is made of long fibres, like a bundle of drinking straws\. It is stiff along the fibres but bends easily across them\. Both samples here use the same wood and the same thickness\. Add the weights and compare\.

The sag is exaggerated so you can see it\. Crossed layers make a panel stiff in both directions, like a giant, thick plywood\.

### 4. Wood swells across the grain

Wood soaks up moisture from damp air and swells, mostly across the grain, hardly at all along it\. In CLT, each layer is held by its neighbours, which run the other way\. So the panel keeps its shape as the weather changes\.

Swelling is exaggerated here\. Boards are dried to about 12 percent moisture before they are glued\.

### 5. Made in a factory, set by crane

Panels are cut to size in a factory by computer\-guided machines, with openings for windows and doors already made\. Trucks bring them to the site and a crane lifts each one into place\. Building this way is fast, quiet and makes little waste\.

A panel can be up to about 3 m wide and 18 m long\. Being much lighter than concrete, it needs smaller foundations\.

### 6. What about fire?

Thick wood does not burn away all at once\. Its surface turns into charcoal, called char, which shields the wood behind it\. That inner wood stays cool and keeps holding the building up\. Slide the fire time and watch the char line creep in\.

Designers count on char growing about 0\.65 mm a minute, so about 4 cm in an hour\. Tall CLT buildings also use sprinklers and fire\-resistant board\.

### 7. Carbon locked in wood

Trees pull carbon dioxide from the air as they grow, and about half of dry wood is carbon\. A wooden building keeps that carbon stored for as long as it stands\. Making concrete and steel releases a lot of carbon dioxide, so swapping some of them for wood can help the climate\.

This only helps when the forests are replanted and well managed\.

### 8. Why now?

Building codes caught up: in 2021 the US code began allowing mass timber buildings up to 18 storeys\. Tall examples proved it works, like the 18\-storey Mjøstårnet in Norway and the 25\-storey Ascent tower in Milwaukee\. Add fast factory building and the push to cut carbon, and CLT is on the rise\.

Tall timber towers are usually hybrids, with concrete in the base or the lift cores\.