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SUPER STRUCTURE
The term superstructure refers to the part of a building above its foundations. This Section will help you navigate the parts of this Guide that will help in your construction of the superstructure.
TWO SAMPLE MODELS FOR A HUT
In this section of this Guide you will find details of two structural models (with further drawings found in Appendices 1 and 2).
If you follow these models you will meet Standard 1.1 of Schedule 5 of the Building Regulations (as detailed in Which part of the Building Regulations refers to huts) and therefore will not require a Building Warrant. If you choose to use an alternative design, the Section also gives important guidance that may be followed by your appointed engineer.
Don’t forget that an alternative to the models presented herein will need to follow this Guide in all other areas that are signalled as mandatory, if exemption from Building Warrant is to be achieved.
Types of huts explains what types of hut this Guide deals with and which it does not cover. The guidance given in this Section accordingly mainly deals with light weight, timber-framed structures and does not deal with a variety of other types of possible hut structure - all of which will require you to appoint a structural engineer to help design.
The two models presented in this Guide, and detailed in below, have been fully engineered and represent the two principle types of timber framing that are likely to be used in hut design. They are:
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Bolted timber post and beam (“timber frame construction” in USA)
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Stud framed (“timber frame” in Scotland)
It should be specifically noted that this Guide does not cover (a) traditional mortise and tenon jointed post and beam structures such as associated with green oak framing in Scotland (b) stacked log systems whether natural or machined which must be fully engineered. Many cabin or hut kits of these latter types are available on the market and although they are mostly structurally robust, very few carry structural certification.
Before looking in more detail at the two structural systems below there are two general points regarding the use of timber in construction which apply to all types of structure and which must be carefully considered: that of structural timber strength and treatment, as detailed below.
WHICH TYPE OF SUPER STRUCTURE
This Section shows two sample models for the construction of a hut which could comply with the new regulations around building Type 23A.
The sample models we include here are:
These sample huts have been designed by the authors, Peter Caunt and Bernard Planterose, and have been assessed by our structural engineers to ensure soundness, stability and safety (although this cannot account for site-related constraints).
The world of hut construction is very diverse, with many creative possibilities, materials choices, challenges and opportunities. It is not possible for this document to cover all those options, and we do not attempt to do this. Instead we have focused on two basic timber hut construction methods which have been assessed and developed in relation to the new legislation on huts and building regulations. Any prospective hut builder can choose to adopt one of these construction methods, and can follow the guidance in this document to ensure that they are complying with the Scottish Government’s regulations.
SUPER STRUCTURE TYPE -
POST & BEAM ENGINEERED MODEL
Post and beam construction is one of the oldest and most globally utilised of all timber construction types. It may be chosen in hut building for a variety of reasons including:
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The physical, sculptural beauty of the frame when left exposed internally, externally or both
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The direct and readily understandable structural principles
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The pleasure and ease of using large sections of timber which may be locally sourced
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The possibility of utilising skills and craftsmanship to immediately enjoyable effect
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Its adaptation to many skill levels even to relative beginners
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Its remarkable speed
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Its particular adaptability to a small team or community approach involving frame raising for instance
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Stimulating woodland management for the ecological benefits of larger sections and longer rotations
It is particularly applicable for single-skinned (uninsulated) constructions where the frame is to be left exposed and un-compromised. It is difficult to retro-insulate a post and beam frame effectively and, if not planned from the beginning, you will lose a lot of the beauty of the frame and therefore a large part of the advantage of framing by this method.
Where a post and beam frame is to be insulated, it is most economic to utilise the insulated layer as part of the structural solution. In other words a hybrid structure is designed which will require the assistance of an experienced structural engineer to ensure soundness of construction.
5A
Structural principles - loads
Simple hut-scaled post and beam buildings are best thought of in frames (known as bents in the USA) and bays. A one bay hut has two bents, a two bay hut has three bents and so on. See drawing L11 in Appendix 1. Frames are often made on the ground and raised by either mechanical means or by communal human strength often involving ropes and winches.
The main structural principle of post and beam buildings is that the load of the roof (both live and static loads) is transmitted entirely down the posts to the ground/foundation.
The weight of the walls and any upper platform is also transmitted laterally by beams to the posts and then down through the posts to the ground. When imposed wind, snow and occupancy loads are added, we can calculate the loads imposed by each post on the ground. It is clear that the foundation must be able to take this load without subsistence or fracture. Three of the four foundation types in Types of foundations of this Guide are designed for these loads.
Structural principles - racking and sheathing
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Racking is the technical term for a wall or frame distorting under load and is dealt with by carefully designed bracing. This is usually in the form of either (a) large beams forming triangles of structure or (b) knee braces, most often at the tops of posts where they are crossed either by eaves beams (walls) or summer beams (supporting upper floor joists). It is necessary to consider racking both across the long and short axes of a building and prevent both. Racking of rafters can also take place so that roof bracing is often also required.
Racking in post and beam buildings occurs when joints fail - most often due to (a) tearing or splitting of timber around bolts (b) too few or too small bolts (c) bolts too close to edge or end of timber members splitting out.
An alternative means of bracing in post and beam is to use the walls. In this case the rules pertaining to stud frames as shown in drawing L23 Appendix 2 should be followed.
Jointing and over sizing structure
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As noted above (Superstructure), traditional mortise and tenon jointed post and beam frames (like those used in green oak framing) are not covered by this Guide and will always require a structural engineer to certify. The engineered model presented here utilises simple bolts and contemporary heavy duty, self-drilling timber screws which are now readily available on the market. It is essential to follow the rules on the edge distances for drilling holes and bolting and to take care to use the specified bolt and screw sizes. Joint failure can be catastrophic.
Whilst generally against the grain of long-standing engineering principles, over-sizing of timber dimensions is advised in small-scale post and beam building. If you do not want to use and see a lot of timber then post and beam building should not be your choice! Using a lot of timber can be justified in terms of fixing a lot of carbon and making good use of a local timber resource (where there is an adequate and sustainably managed one!). It also gives more lee-way for error by amateur self-builders.
The choice of post dimension is an early and fundamental decision in post and beam design. At hut scale it will be quite normal and well advised to use large posts. Apart from the aesthetic pleasures of large section timber, large posts accommodate multiple strong beam connections more readily.
The situation where two beams meet at a single post in the same plane is particularly pertinent. In order to achieve the minimum bolting dimensions (see Appendix 3) you will need a certain width of post.
Whilst various beam scarfing (joining) techniques and shear pin connectors can reduce this width, it is often simplest just to start with a big post, especially where you are site milling and time rather than volume of materials is of the essence.
Steel flitch plates and shoes
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The use of steel plates within joints or along whole lengths of timber is a common way of engineering post and beam timber frames but we have not considered these in this Guide due to increased costs which are not necessarily warranted at hut scale. Designed by an experienced structural engineer, steel flitch plates can however be used to almost eliminate triangulation in many structures and to confer exceptional structural integrity, larger clear spans and greater imposed loads.
Fabricated steel shoes are the preferred way of fixing the base of posts to concrete foundations. The models illustrated give an engineered design for such shoes. There are many local blacksmiths capable of making your design at low cost and they should be able to have them galvanised thus avoiding maintenance in the future.
SUPER STRUCTURE TYPE -
STUD FRAMED ENGINEERED MODEL
Stud framing is the simplest type of timber framing and, in Scotland, denotes a type of nailed (and sometimes partly screwed) framing where spacings are controlled rigorously by sheathing panel dimensions (generally 1200 x 2400 mm). This principle is applied to floors, walls and roofs.
Where the frame is to be insulated, the structure will be completely concealed within a sheathed wall or roof and, in this case, the joinery can be relatively ‘rough’ and achieved almost entirely with a nail gun. An internally exposed stud frame, by contrast, may even be of dressed timber and very carefully jointed to present a highly finished and beautiful interior.
5B
Structural principles - loads
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In complete contrast to post and beam framing, the structural principle of the stud frame is to distribute the loads of walls, roofs and upper floors rather evenly down to a solid base/foundation through the walls. Again in contrast to post and beam which erects frames onto the foundations allowing the completion of the roof first, the sequence of construction in stud framing will involve the formation of a floor platform onto which wall frame panels are erected. Multiple rafters are then located on the wall heads joined at a ridge plate or prefabricated trusses installed.
Wall frames can be ‘stick built’ (piece by piece in situ) or fabricated as complete walls or sections of walls either on- or off-site. A common way is to make them on the ground within the footprint of the building and raise them into position with one side sheathed on the ground.
An alternative to the raised timber floor platform is a ground bearing slab, which is not considered by this Guide because it is not considered a low impact foundation.
Structural principles - racking and sheathing
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A second principle of stud framing is that the sheathing stops the frame from racking. This racking strength is usually only conferred by one layer of sheathing on one side of the frame. Plywood, OSB and a variety of vapour permeable boards can all provide the necessary racking strength. In this Guide we use OSB and proprietary racking grade MDF. The studs laid edge on to the inner and outer sheathing give the rigidity across the plane. In the model with lean-to, the ‘central’ wall performs an important anti-racking function and cannot be removed without substitution of alternative structural arrangements. Such a variation will need engineering and certification.
Traditionally racking was achieved by cutting diagonal members between one pair of studs on each elevation. This method can be used in Appendix two where use of sheathing is deemed inadequate.
Voids are created which can be insulated (see Section 11 insulation). Simple gun nailing with 90 mm framing nails is appropriate but ensure that these are galvanised and intended for framing. Fixing of the sheathing to the frames would be done with 50 mm galvanised ring shank nails. Alternatively you can of course hand nail with 100 and 50 mm galvanised nails.
STRUCTURAL TIMBER STRENGTH
All structural timber MUST be strength graded when used in structural applications to meet the requirements of Building Type 23A and therefore to allow for Building Warrant exemption. This can be done by machine as marked on merchant timber or by visual grading by a qualified visual strength grader (available from some sawmills).
For the models described and endorsed by this Guide you require graded solid softwood timber which is easily procured from timber merchants. The grades detailed in Appendix 1 and 2 are C16 and 24 normally pine or spruce.
Home grown pine, spruce and fir visually graded to General Standard (GS) are designated C14 although larch may be designated C16. Higher Special Structural (SS) grades of timber of homegrown species may also be visually graded as follows: pine C22; spruce C18; Douglas fir C 18; larch C24. Home-grown Douglas fir with cross sectional area greater than 20,000 sq mm may be graded to C24 but such sections are not used in the models in this Guide.
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Hut builders attempting to use locally sourced and possibly even site-milled timber face particularly challenging decisions regarding choice of species which may in practice be determined largely by availability and cost.
Douglas fir and larch will be favoured for durability and strength but spruce and pine are usually cheaper and more readily available. Each timber has its different aesthetic and performance qualities but, as far as this Section goes, it is the strength grade to which the timber has been graded that matters, and so long as this achieves C16 or C24 then it can be used for structure.
Purchased from a merchant, structurally graded timber will generally be kiln dried (KD) but locally or site milled timber is unlikely to be.
Timber does not attain its full structural strength until dried and should not therefore be structurally graded or used in structural components until it reaches an average moisture content of less than 20% with no single reading greater than 24%.
Note that timber more than 100mm thick cannot be visually strength graded and often has to be used with a higher moisture content, eg. in the posts of post and beam framing and in log building.
STRUCTURAL TIMBER TREATMENT
(More on timber treatment including for non-structural elements can be found in Wall Cladding)
Where structure is of a durable timber such as heartwood of larch or oak and/or well ventilated it will not need further treatment. Where enclosed in an insulated wall of a non-vapour permeable type (see figure 11A) it will need to be treated. If purchased from a merchant this will present no difficulty but where site milling is taking place it presents a challenge. One solution is to use a vapour permeable wall. Although materials are more expensive for this wall/roof type they do allow use of local material where otherwise it might be very unwise.
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In certain foundation types (see Types of foundations of this Guide) timber structure is exposed to the elements and will require treatment unless built of a naturally durable species. For other species you should surface coat with a preservative followed by at least two coats of high quality paint or stain, paying special attention to saturating any end grain.
Timber that is entirely internal of whatever species does not require treatment. In many types of post and beam structures, it is only a small part of the posts that protrude into the external environment below the floor. Detailing should ensure that these areas are clear of the splash zone and separated by a membrane or by lead from any part of the foundation. Preservative should also be applied regularly to such critical exposed parts of the structure.
In the models presented here, we have been careful to separate the structural frame from the secondary roof structure. This principle is not followed in a simple, uninsulated post and beam frame where any protruding roof members should also be surface treated if not of durable timber and always detailed to ensure that none are exposed to frequent wetting.
DURABILITY OF TIMBER
Durability classes: 1 = durable; 5 = not durable. Treatability: 1 = easy; 3 = difficult. This table is taken from BS-EN 350-2.
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