There are four principal types of oak frame in the UK: cruck frame, box frame, post and truss frame and aisled frame. All of these can be reproduced today, but most new homes are built using post and truss (also known as post and beam) framing, as it is the most cost-effective and flexible.
Oak is not nearly as expensive to build with as people think, though a full oak frame home, while a beautiful and ecological construction system, is not cheap. Where budget is limited, there are many options for building with oak without using a full frame. Oak can be combined with other construction systems to create oak features in individual rooms or wings of a building, the first floor, or just a section of roof.
The earliest and simplest oak frame homes are based around a series of simple A-frames made of substantial curved timbers created by splitting a large tree trunk into two matching posts (cruck blades), joined by a collar or tie beam. Several A-frames can be lined up to form two or more bays, linked by purlins to carry the rafters. As the cruck blades take the load of the roof, the rest of the walls and floors are made of lighter sections of timber.
In Wales and the West of England, where there was less pressure on the supply of large oak trees, this remained the predominant system until the 16th century. However, few houses are built this way today, perhaps because of the size of tree required.
Rather than A-frames, the walls provide the strength of the building. Goalpostlike frames of posts and beams are connected at bay intervals by cross tie beams, forming rectangular ‘boxes’ that can be assembled in a series of bays. With box framing it is also possible to stack two or more storeys on top of each other, making it a very flexible building system compared to cruck framing, which is limited by the size of oak available to form the cruck blades.
The roof is built from a series of pairs of principal rafters supported by the external walls, with collar beams and/or tie beams spanning from rafter to rafter to stop the roof from spreading. The rafters are braced against lateral movement (racking) by the introduction of roof bracing, with posts (king posts or crown posts) between the collar and tie beam, and possibly a through purlin running lengthways, helping to brace the collars.
Post and Truss Frames
This is the construction system used in the majority of the surviving oak frame homes and in most new oak frame homes. A series of cross frames, made up of goalpost-shaped wall frames together with an A-shaped roof truss (principal rafters, plus collar or tie beam and bracing) are assembled together using tie beams to form a series of bays. The common rafters are supported by oak purlins, spanning from truss to truss, and bedded down onto the gable-end walls.
Oak frames of any form can be enlarged or extended at ground floor level with the addition of an aisle down one or both sides of the frame. This is created by extending the principal rafters down to first floor level, supported on wall posts. This is a common feature on oak barns, churches and farm buildings, but can also be a useful way of adding space to a home.