• Multi-storey timber frame developments usually use the platform frame system.
• Structural codes are adapting to take account of the popularity of multi-storey timber frame.
• The biggest challenge for multi-storey timber frame is to break into commercial build.

NHBC statistics show that flats and maisonettes have increased from 17% to 23% of timber frame starts in the last year.

There are many excellent reasons for this popularity: the structures are lightweight, so lend themselves to prefabrication and only need relatively shallow foundations, and the erection process is quick and dry. The panels are easy to insulate and services can be routed through the voids. And, of course, timber has the best environmental credentials of all construction materials by some distance.

In the UK, timber multi-storey residential developments are built using the system known as platform frame. The turning point for this type of construction can be clearly fixed as the multi-storey research project TF2000 conducted at BRE’s large-scale test facility in Duxford.

Overall, the performance of the building gave the regulatory authorities and the building industry alike the assurance that multi-storey platform frame was not merely viable, but hugely beneficial.

Code changes

The structural codes are adapting to take account of the popularity of multi-storey timber frame. A major amendment to BS 5268 part 6.1 was issued in November 2007, taking the height of timber frame walls covered by the code from four to seven storeys. The main part of BS 5268 part 2, was also amended in December 2007 to include more detailed guidance on designing to resist accidental damage and disproportionate collapse.

Of much greater importance in the near future is the withdrawal date of the British Standards, currently set at March 2010. By this date, national standards will be replaced by Eurocodes. This change represents a great opportunity for UK timber frame companies, because they should be able to transfer their considerable experience in multi-storey timber frame to markets in the rest of Europe without the barriers of national standards. However, they will also have to learn to live without some of the familiar guidance given in the British Standards. TRADA is working hard to support members and the wider timber industry in the transition to Eurocode 5.

Build speed advantage

The build speed advantage of timber frame erection is realised when multiple building phases run concurrently. For instance, when timber frame erectors sign off each storey as they complete it, they can be fixing the wall for the second storey while the windows are being installed in the first storey and the first fix electrics are being fitted in the ground floor.

As soon as the roof is on and the building is dry, the internal linings can be installed, followed by the second fix services. The dry linings are a large part of the weight of the building and cause significant settlement. It is best to leave any external brick cladding until the linings are in place so that some differential movement is eliminated. Provided it is properly installed, the breather membrane has enough weather resistance for the build period, so it is not necessary to start the brick cladding too early.

Optimising the order of work and quality of workmanship are key challenges in the immediate future as the current economic downturn means that it is not possible for developers to rely on a buoyant market. Timber frame buildings must be erected quickly and be right first time.

Closed panels

In the medium term, increased use of off-site construction techniques will mean that wall elements and floor cassettes will arrive on site with both sheathing and linings installed, providing greater resistance to construction site fires. These units will need careful handling on site so that surfaces that are on the inside of the finished structure are not damaged by exposure to the weather.

Other timber construction types will come to challenge the dominance of platform frame. This is already happening, as Waugh Thistleton’s nine-storey tower in Murray Grove, Hackney nears completion. It is the tallest modern residential building in Europe made entirely of timber, but instead of platform frame construction, the tower uses cross-laminated wood panels from Austrian company, KLH. These panels carry the loads as well as forming the shell of the building.

Commercial sector challenge

But perhaps the greatest challenge for multi-storey timber frame is to break out of the residential sector and to be considered by architects and engineers on the same footing as other materials. The timber industry must work hard to make timber as easy to specify as steel or concrete for the higher loads and increased occupancy levels of commercial offices and hotels.

And the industry must provide easy routes to specification for the elements that make up medium-rise commercial buildings. If this can be achieved, then all the advantages that have made multi-storey timber buildings so popular in the residential sector will see to it that the market share of timber in construction will continue to rise.