Architectural design of oak framed houses
When considering the architectural design of oak timber framed houses, more than in any other building type, it is essential to think through not just the structural functionality of the oak frame, but also how it looks in the resulting space. The oak frame will not be covered up, so if anything incongruous is incorporated into the design, it will remain on display. The best architecturally designed oak frames display a comfortable marriage of form and functionality where the structure is celebrated and expressed. At one end of the spectrum, a poorly considered oak timber frame might work well structurally but have within it room layouts or window and door apertures that are juxtaposed with the rhythm of the frame, or worse still, oak posts that are located where a door should be or braces across the middle of a window. At the other end of the spectrum, a well-designed oak frame will simply complement the space it creates and therefore will make the architectural design sing.
Basic design rules
There is no way of getting round it: the architectural design of a successful oak frame house requires a different approach to a house where for example the masonry is the only structure. If you want an oak framed house rather than a house with some feature oak in it, the following basic rules are well worth keeping front of mind both before commencing, and during the design.
One: Understand oak as a building material
As with any building material oak has its limitations. To an inexperienced oak timber frame designer, this first rule is probably the most foreign and possibly surprising.
The old adage of form follows function, while not entirely contradicted in this case, does need to be treated with care. For an oak timber frame house, form must walk hand in hand with function and therefore the oak frame should be a prime consideration from the outset.
Starting from a floor plan and then attempting to place the structural oak to provide a setting for the spaces created by internal walls will at best create an inefficient and overly expensive oak frame that will, in all probability, look contrived. The evidence of this is all around us, and can most readily be understood by our response to seeing Victorian or modern ‘plant on’ oak trim.
Conversely to concentrate only on designing an efficient oak frame with disregard for the architectural design of the rest of the house might result in an impressive and beautiful oak frame home that is simply inappropriate for the client.
Neither of these extremities of architectural design are particularly clever; rather the best designs comfortably balance the two approaches to achieve the best of both worlds. This best approach can be summarised as delivering the functional requirements within an efficient and aesthetically pleasing oak frame. Thus, once the brief is developed and the room formats decided on, the oak frame design should be bought into play to inform how those rooms are best incorporated in the building shell.
The oak frame is a structural support; a skeleton around which the ‘skin’ of the building hangs, and within which any non-structural internal partitions can be placed. The oak frame is not just a material, it is both the structure and the heart of the building. Designing with this in mind ensures that the oak frame’s structural purpose is honestly expressed, and that there is a natural rhythm that sits comfortably throughout the space. Inherently architectural designers know this, but when it comes to working with the oak frame and making sure that form and function take equal precedence can be difficult. Perhaps this is because there is a resistance to the perception of being dictated to by a construction material, but this is really missing the point that an oak frame house is an oak frame house, not a house with some oak frame in it. It is always worth persisting to ensure that this balance is achieved.
Two: Design only what is appropriate
One of the delights in an exposed oak timber frame is that it always looks good. The frame as we’ve said many times is one of, if not the major feature – and that feature speaks for itself.
Oak timber frames already make a strong statement, and that statement must be carefully managed by the architectural designer to ensure that the oak frame is exactly what is required and no more. One can have too much of a good thing.
Many historical oak timber frame buildings are full of timber – close studding, floor joists, acres of rafters and trimmers everywhere. The only real reason to incorporate these into an oak framed house is because this is what the client wants; otherwise they simply aren’t necessary. Additionally, having too much timber in the house can be oppressive, especially if it’s overhead and too close to the occupant.
Another factor that should be taken into consideration is the potential oversizing of oak frame members. Often this happens because of an assumption of engineering requirements, which is often incorrect. Rather than build in safety factors for the engineer to approve, it is better to design as aesthetically appropriate, and leave it to the engineer to do the engineering!
Three: Use the frame as the main structure
Allied to the above two rules is the third. Because the oak frame is structural, to not treat it as such would be missing a trick. If the oak frame is designed to be structurally efficient, then the rest of the building system and the internal walls need not be designed to take the loads that the frame is already dealing with. In simple economic terms, this makes good sense.
Oak frame layout techniques
Bearing in mind the golden rules for the architectural design of oak timber framed houses, we go on to look at how a frame should be organised.
Using a Grid
The concept of setting out a building on a grid is well established, and in oak frame design it is of paramount importance.
In a grid layout, the longitudinal divisions are the bays, while the oak cross frames traverse the grid.
The simplest expression of an oak frame, and the basic historic form is a rectangular grid. The house is divided up into a series of bays with oak cross frame delineating each bay, and the longitudinal members spanning each bay defining the bay width. This is a typical and efficient layout for buildings such as an oak framed barn where there is no requirement for internal division but simply to provide cost effective space.
An oak cross frame is designed to traverse the span of the house, and the usual form of a building is to have it just one cross frame deep. In this case, the depth of the oak frame house is limited by the practical limits of a cross frame assembly, and the oak tie beam in particular. Deeper houses can be achieved through the use of untied trusses and spans up to 20m are achievable.
The primary problem with increasing the size of an oak truss is that other associated members, such as oak floor beams, also probably need to be increased. The other issue is that when the span is increased on a like-for-like roof pitch, the ridge height is raised and the roof area becomes proportionately large. Planning constraints are quite likely to have a significant impact here. Practically then, these larger spans are really only appropriate to very large buildings, areas with oak vaulted ceilings, and when dramatic impact is sought. Otherwise, it is wise to assume a maximum span of 8m, and be aware that while it is eminently possible to exceed this, engineering advice should be sought before assuming that larger spans will be possible.
Apart from increasing the truss span to keep one clear open span, there are other techniques available to deepen the house:
Use a single oak truss, but support the tie and other members with internal oak posts, thereby losing the clear span. The internal oak posts could be at the middle of each truss, but not necessarily so, and could be at appropriate locations specific to the room layout e.g. in line with internal walls.
To have more than one truss in a line, thus creating a multiple pitched roof with either a section of flat roof between the two, or a gutter between. The house would then have a series of internal oak posts down the middle of it. The advantage of such an arrangement is that the roof line can be reduced externally.
To use an aisled cross frame to one or both sides of the main span. This lends itself to a series of smaller rooms to the side of the central area.
Most early oak framed houses however were not based on a uniform grid where all bay sizes were the same. The oak cross frame spans needed to be consistent, but the placing of them and therefore the width of the bays would vary according to the internal use of the space. Thus while the discipline required for the efficient construction of the frame placed constraints, within these constraints, the function would indeed dictate the form.
As is the case for cross frame spans, bay sizes are also commonly constrained by the oak available to make the structure work. Beyond a certain point, the bays become too large, or the section size of that timber increases to an extent that the timber becomes difficult to find or incongruous to the overall appearance of the oak frame. While increasing the section size of these spanning members (top plates and purlins) is an option, it is important to note that it is more than likely that the size of other adjoining oak timbers also need to be increased. Larger oak purlins to one enlarged bay may require larger oak principal rafters either side of that bay as well as larger posts to hold them up, then making the adjacent cross frames and purlins look undersized, and so on.
When constructing the initial grid, placing cross frames closer together than 3 metres is structurally unnecessary and increases cost as well as making the house look very busy. If a particularly oak timber rich environment is desired, it would almost certainly be better to incorporate additional oak posts mid bay or studs to achieve the desired effect. Increasing the bays over 4.5 metres is possible, but chances are the section sizes and knock on effects will become a consideration. As a result, it is recommended that typical bay widths to aim for are ideally between 3.0 and 4.5 metres, with a target width of 4 metres.
It should be apparent from these considerations of limitations to building depth that an oak framed house on a large scale would inevitably tend towards being very long as ever more cross frames are added to provide the internal volume. The typical solution to this is to add wings on to the basic house making the plan L, T or possibly H shaped. This works very well for a large structural oak frame, making the internal space more interesting since spans can be varied and the flow of movement less linear, but also makes the external elevations more interesting.
Just because working to a grid is essential, this does not mean that a grid must be linear.
In such cases, plans can incorporate grids with kinks in, or wings that are positioned other than perpendicular to the main area. Provided that the change of direction does not coincide with changes to spans and difficult geometry to work round during the build, this does not make for a particularly disproportionate increase in costs.
A plan that seems to have become increasingly popular, and has been executed to great effect is that of a curved layout. Again, keeping the oak cross frames the same span means that this need not be a particularly difficult method of building, and is therefore well worth considering. Curved oak frame houses have great visual impact, and can either be very modern or still feel surprisingly traditional depending on their detailing. Their layout, apart from being used to focus the views on that amazing feature of the plot, are also well placed to deal with solar gain and light control since different parts of the oak framed house can be in shade when others are in the light, and that relationship can change during the day to provide a dynamic use of the internal spaces.