TIMBER FRAMING

 

What is Timber Framing

Timber Framing also called post and beam, is as the name suggests a method of timber construction that relies on the timber frame as the primary structural support for the building. Timber Framing is a centuries old construction method of creating timber framed structures jointed together with wooden pegged mortises and tenon joints.
People throughout the world have been living in timber framed structures for thousands of years. This has been for many reaons, the ready supply of wood for timber frames, the ease of construction, the durability of the material and of course its aesthetic appeal.

Benefits of Timber Framing

• Fast, efficient and cost effective
• Flexible to add further extensions
• Environmentally friendly
• Hand crafted
• Unique character
• Buildings with an organic, natural, earthly charm

Types of timber frames

There are a wide variety of types of timber frame truss that can be used. The majority of these can be divided into "closed" or "open" trusses.
A closed truss has the form of a triangle, the horizontal member of the triangle is called the tie-beam which is jointed in to two angled members called the principal rafters.
With an "open" truss the horizontal member is interrupted with other members such as crucks or sling-braces.
The principle purpose of the trusses is to deflect the loads on the roof to the wall post who then direct the load to the foundations. The horizontal member, the tie-beams main purpose is to prevent the walls from being pushed apart by the load of the roof.

Closed trusses

The closed trusses can be sub divided into two types the queenposts truss and the kingposts truss.

Queenposts truss

The queenposts truss has the shape of a triangle with two curved members running from the principle rafters to the tie-beam.

Timber frame with queenposts
Frame with queenposts under construction


Finished frame with exposed queenposts

Kingpost truss

The kingpost truss has a triangular shape with a vertical member called the kigpost in the centre running from the tiebeam to the ridge. This type of truss is used to make frames with larger spans. The kingpost prevents the tiebeam from sagging.

Timber frame structure showing kingposts
Combination of king post and queenpost truss

Open trusses

The open trusses can be sub divided in to several types, arch brace truss, interrupted tie-beam truss, slingbraces truss and crucks.

Arch braces truss

Arch braces trusses are made up of two principal rafters connected with a collar. Underneath the collar there are two curved full braces jointed into the collar and rafters.
With this type of truss you can create a curved ceilings as are often seen in churches.
Timber framed roof with arch brace trusses

Interrupted tie beams truss

The interrupted tie beam trusses are used when there is not enough height between floorbeam and tie beam. This can be the case if the building height is restricted by planning regulation for instance, and it is not permitted to build two full storeys but there is room for one and a half storeys.
To retain the out worth truss of the principal rafters you can omit partly the middle section of the tie beam, so you are left with two short tie beams where the principal rafters are joint in.
The interrupted ties are connected to upright post, slingbraces or crucks. (See drawings of the interrupted tie beam, slingbraces truss and crucks). This will stop the outward trust of the principal rafters and walls.

Slingbraces truss

The slingbraces is a truss that has two angled curved members that run from the horizontal wall post up to the collar. As with the interrupted tie beam truss this truss also has two interrupted tie that are jointed to the slingbraces to stop the rafters and walls from spreading.
Timber frame showing slingbraces Timber framed building with slingbrace trusses


Example sling brace truss


Sling brace truss in house construction

Hammer-beam truss

The hammer-beam truss is an open truss that is constructed by omitting the middle part of the tiebeam. This type of frame resembles triangles that are stacked on top of each other.
Scematic shows the principle of a hammer beam truss
By omitting the middle section of the tie beam you create a horizontal thrust that needs to be absorbed by either buttresses on the outside of a masonry wall or a steel rod connecting the two pieces of tie beam.
Hammer-beam trusses where used in prestigious buildings like great halls and barns so as to create great spaces.

Timber frame with hammer beam trusses

Cruck frames

Crucks are made from curved trees that usually are cut in half to from two identical cruck blades. With full crucks they run from ground level up to the apex of the building. Then there is the upper cruck where the cruck blades run from floorbeam level up to the apex. The last type is the jointed cruck here the two members are jointed to form a cruck. Here a post runs up to topplate level where it flares out into the roof angle. There it is jointed with a straight principle rafter.
Full cruck timber frame
Upper cruck timber frame
Jointed cruck timber frame

Cruck timber frame

Cruck timber frame

Woods Used In Timber Framing

There are numerous types of wood that you can use for timber frames. The most common ones used are oak, Douglas Fir, larch, sweet chestnut, pine or modern day Glulam.

Oak

Oak is one of the main timbers used for timber framing. It has been a building material for more then 9000 years. Trough it’s fast occurrence in Europe it was the building material of choice for many years. Because of it strength, durability, beauty and easiness to work it was used for many purposes, dwellings, ships building, wheels, household utensils, roof covering, charcoal etc.
The wood structure and quality depends on the growing conditions. For instance oak from Slovenia grows slow and uniform, is straight of grain and has an even colour and is easy to work. Polish oak is tougher and harder. It also contains a lot of tanning acid so when it comes in contact with steel it will corrode quickly. Oak tree between 40 and 140 years old are mostly used for timber construction. Wikipedia article on Oak.

Douglas-Fir, Larch, Pine

Douglas Fir is an impressive pine tree. It originated in North America, and is now widely planted throughout Europe including areas of Scotland.
The wood has very good structural qualities and has a particularly high weight/strength ratio. It is easy to cut providing the cutting tools are sharp otherwise it as a tendency to splinter on the cutting edges. Wikipedia article on Douglass Fir.
Larch can also be used for timber framing and is valued for its tough, waterproof and durable qualities. It is also easy to process, but is highly resinous. Wikipedia article on Larch.
Many sorts of Pine can be used in Timber Framing. Pine has almost the same quality as Douglas-Fir, it is easy to process and cheap. Wikipedia article on Pine.

Sweet Chestnut

Chestnut is a wood that is used for timber framing for centuries. It has not the same structural strength as oak but it is not far from it. Quatersaw wood has almost the same texture as oak except for the rays. It has a lot of tanning acid in the wood. Wikipedia article on Sweet Chestnut.

Glulam/laminated wood

Modern timber frame structures can also be made from glue laminated wood which gives a timber frame a more contemporary style. Gluelam or laminated beams are specially sorted woods, where the bad bits are cut out, then finger jointed and glued together in any dimension you want. The quality and strength is superior to normal wood so you can used it in bigger spans, thinner dimensions etc. So it can save money. It is also available in oak. In some structural construction it can replace steel and concrete and be a cheaper alternative. Wikipedia article on Glulam.

Daisywheel and compass layout technique for timber frames

Before the age of computers and drawing software carpenters and stonemasons used the dividers or compass to construct the necessary angles and layout of buildings even as big as cathedral roofs. Be side the compass on used straight edge, wooden batten with a perfect straight edge, and scratch awl, a steel spike with a point to scratch marks or lines on the wood.

How to construct a daisywheel

A daisywheel is constructed by drawing a circle with the compass. By keeping the same radius you can mark six other points on the circle. These are the centre points for six other circles that will form the daisywheel.
These daisywheels are often found on timber frames or cut stones.
By connecting the four points you end up with a rectangle, see drawing below.
In this rectangle the short side is equal to the radius of the circle used to construct the daisywheel. The diagonal of the rectangle is the same length as the circle's diameter. The exterior ratio is approximately 1: 1, 75. The ratio that is more of interest is the ratio between the rectangle's short side radius and the diagonal (diameter) which is 1:2.
The six points on the circle can also be used to generated angles. Connect the six points and this will give you six angles equal to 120 degrees. The corners of the rectangle are 90 degrees; the diagonal of the rectangle will give two equal triangles which consist of 30,060 and 90 degree angles.
When the six points on the circumference of the circle are connected with the centre of the circle this will give six 60 degree angles. These angles can be divided in 30 degree and 15 degree angles.
To use this geometrical layout method in practice this garden shed was build in Cressing Temple, Essex. The design was based on the daisywheel, the front, side and back wall.
All the timber where hewn on site, then the shed was laid out with the daisy wheel method. In principal all you need is the six end points of the pebbles which you mark out on a floor and use these points to lay the wood in position and then mark out he joints.