Chapter 3
STRAWBALE BUILDING TECHNIQUES EXPLAINED
Loadbearing (also called Nebraska)
This is the original way of building with straw bales, pioneered by Nebraskan settlers in the USA in the late 1800s. At amazonails, we prefer this method above all others for building any type of construction, small or large, because of its simplicity, flexibility of design, enhanced thermal performance and cost-effectiveness.The potential for empowerment through working together on a shared project is one of the great advantages of this type of building, as it is such an accessible building method: pretty much anyone can do it, and it’s a lot of fun!
In the loadbearing technique the bales, which are structural blocks, are designed to take the weight of the roof; there is no need for any other structural framework. They are placed together like giant bricks, pinned to the baseplate (a continuous timber plate, usually a flat ladder structure, that sits on top of continuous-type foundations) and to each other with coppiced hazel, and a continuous rigid timber ring beam on top spreads the floor and/or roof loads across the width of the wall. For two-storey houses, the floor joists at first-floor level are attached to the ring beam before building up the straw walls again beneath the roof. The wallplate, or roofplate (a continuous perimeter plate that sits on top of the walls at each floor level and under the roof)is fastened to the bales with coppiced hazel and may be strapped down to the foundations, depending on local weather conditions and the weight of the roof. The roof is constructed on top of the roofplate, following strawbale design principles.
Windows and doors can be installed in various different ways. The simplest method is to place them inside structural box frames, which are built into the bales as the walls go up. This approach is usually used for small-scale and community buildings and self-build experiments. It may require more timber than other methods, but it keeps the actual bale building aspect to its simplest. It requires little previous knowledge of wall construction and is very accessible.
An alternative way of installing windows and doors, and one used predominantly on mainstream building sites, is to use doubledup 100mm x 50mm (4” x 2”) uprights either side of the opening, fastened securely to the baseplate and projecting through slots in the roofplate. These are housed fully into the bale either side and the window or door is fixed to them.
The loadbearing style is a simple, straightforward building method, easy for non-professionals to design following readily comprehensible basic principles. Designs from one-room to two-storey homes can be created using a simple step-by-step approach. Curves and circles are easy to achieve, though at some extra cost due to more complicated timber work. And as the straw is very forgiving, total accuracy in plumb is not necessary, but wilder variations can be brought back into shape easily! Self-builders love it because it’s fast, fun and fulfilling! Most of the larger loadbearing buildings now being built in the UK and Ireland are actually pre-compressive loadbearing designs (see page 32).
SECTION THROUGH LOADBEARING STRAW WALL
Loadbearing in a wet climate
Over the years, the main disadvantage of the loadbearing technique – namely how to keep the straw dry throughout the whole building process despite the sometimes prolonged wet weather of the UK and Ireland – has been dealt with. Ways we can address this issue are as follows.
• Prepare everything in advance (prefabrication) so the whole building goes together quickly once you start, like building with Lego. The vulnerable time of exposure to the weather, and the need to keep it covered with tarpaulins, can be reduced to a few weeks.
• For small buildings, build underneath a marquee or large tent.
• For larger buildings, increase the height of the scaffolding (needed to build the walls and roof), add a temporary roof over the top, and cover in the sides. This level of protection is required for the lime render anyway, so it can be cost-effective to protect the whole building, ensuring there will be no time lost due to the weather for any aspect of the building, not just the straw, at any stage.
• Use an impressive-compressive method (see pages 31-34) and use the floor or roof as the main weather protection.
Loadbearing for warehouses and retail units
Large open spaces can be built very quickly and effectively using mini Heston or Quad bales. At around 8’ long and 2’ high per bale you can appreciate how quickly a person with a forklift truck and telehandler machine can build a new DIY superstore. Obviously there’s a little bit more complexity to the design than this, but you get the picture!
Basically there is no real limit to design or size when using loadbearing techniques. We have built circular and curvaceous buildings, and detached and semi-detached two-storey buildings using this method. It’s all about understanding the material and having a creative approach to problemsolving.
Infill (also called post and beam or timber frame)
Next to loadbearing, this is the other ‘traditional’ way of building with straw. It was developed in the USA in the late 1970s, as their construction industry was familiar with it as a practice. In this method, the weight of the roof is carried by a timber, steel or concrete framework; the bales are simply infill insulation blocks between the posts, and a key for plaster; they are not used structurally. You might make this choice because you like the look of timber and you want to use its beauty in your design, or because you are going for something really ambitious with large openings and spans, and a loadbearing building just won’t do it. This method also has the advantage of giving peace of mind to clients and those professionals (architects, engineers and building inspectors) who aren’t quite ready to accept just what straw can do as a building material. It is often the preferred option for architects, as the structural concepts are not innovative and rely on an alreadyestablished method of building, therefore the risk associated with an experimental technique is minimised. There is no need to satisfy oneself of the capacity of the bales to take the roof weight, since the framework does this. This method requires a high level of carpentry skill and uses more timber than a loadbearing design, so has cost and environmental implications.
When designing a timber framework to have straw as an infill material it is really sensible to have somebody who understands how straw works to work with the framework designer; this means that all sorts of practical difficulties can be eliminated before you ever get on site. This type of building needs to be designed with the dimensions and properties of the bale as the first consideration, but what usually happens is that the building is designed as per standard framework techniques, and the bales have to somehow fit within that. Because of the nature of the straw, it is possible to solve most practical problems on site, but doing so can detract from the ease of build and the fun – if, for example, you’re having to make little parcels of straw, or sew packets on to the front of posts and so on just to get around the fact that the frame wasn’t designed with the straw in mind.
The main issue with this type of building is stabilising the straw ready for plastering. Traditionally, this would have been done by using external pins, fastened to the baseplate and wallplate inside and outside the wall. These long pins would be sewn together with baling twine through the body of the bale using about two stitches per bale: as you can appreciate, this is an enormous amount of work. Because this adds time and expense to the job, other methods of stabilising the straw have been devised – see ‘Compressive frame technique’, page 33.
One major drawback to the infill method is that if the posts are quite large, 200mm x 200mm (8” x 8”), fitting the bales around them can be difficult, and the ideal location for them is to protrude into the building – but if you do this, then there is not much post to push against sideways when the bales are being installed. Also, if the posts are located on the inside of the building, and the bales are essentially wrapped around the outside edge, then we have the problem again of how to stabilise the straw with no beam above it. Tricky!This type of design usually results in a lot more timber than would have been necessary for a standard timber frame building, simply because the straw itself hasn’t been taken into account properly in the design,so more timber is required as a plate below the straw and to cover the top of the straw to make a connection with the framework.
From a design point of view it is much better to use small-dimension timber buried in the centre of the bales.This makes stabilising the straw more efficient, and it also means that there’s no possibility of air leakage around posts, as there is not a flat join between straw and timber, but obviously then you don’t get the benefit of seeing the timber as part of the design.
On the plus side, all of the timber work can be prefabricated and brought to the site for assembly, which will help keep costs down, and very large open spaces can be created using specially designed roof trusses that sit on the posts. However, these advantages can also be incorporated into compressive frame techniques (see page 33.).
LIGHTWEIGHT FRAME WALL
Impressive-compressive methods
As strawbale building has become more professional, ways have been developed of speeding up and improving techniques to make them more appropriate for the modern building site.The following principles can be applied equally to loadbearing and infill methods.
Traditionally, in loadbearing buildings the roof weight would compress the bales over a period of about six weeks, but by pre-compressing the straw, settlement caused by the roof and floor loads can be controlled and encouraged to happen much more quickly. Now that straw is becoming a mainstream building material it is necessary to be able to construct loadbearing buildings fast and simply using pre-compression.Various methods of pre-compressing straw walls have been used around the world since at least 1994. The idea is force settlement of the bales and maintain the pressure by the use of tie-down straps, fastened over the top of the wall and down to the foundations. When you see this happen, it transforms a very flexible wall into a solid, strong, extremely impressive structure in a few moments! (No-nonsense builders, previously sceptical of this new-fangled building material, have become instant advocates at this point in the build and have been found showing their friends round and boasting about their building’s credentials.)Methods include rubber tubes placed on top of the walls, fastened down with chains and then filled with air (first used in Canada), industrial-strength ratchet straps, placed over the top of the walls and fastened to the foundations, and then ratcheted down tight (as used in California) or pairs of threaded rod attached to the top and base plates and screwed down (used in Australia).
You can appreciate that putting the walls under the amount of pressure required to compress the bales, particularly when using construction-grade bales of very high density, requires a lot of force. This means that whatever you fasten the ratchet straps to at foundation level needs to be very strong. For commercial house building, this is a design detail that needs to be added in at the earliest stages. The following are two ways in which it can be done.
• Lay a 25mm (1”) water pipe through the foundations so that when you are ready you can insert a metal pin, about 18mm (¾”) diameter, through it, which projects about 100mm (4”) beyond each face of the wall. Ratchet straps can then hook on either side of the wall to this pin, the pin can be removed later, and the hole it leaves then pointed up.
• Extend the width of the baseplate, normally the width of a bale, by 50mm (2”) and project it to the inside of the building, so that you have a 50mm lip of timber that can be caught by the hook of the ratchet. On the outside face of the timber baseplate fix a temporary 50mm x 50mm batten: this will perform the same function and can be removed before plastering. This method can be used only if the baseplate is securely fastened down to the foundations, or if the weight of the walls above is sufficient to prevent the baseplate from lifting under the pressure.
Pre-compressive loadbearing technique
Buildings need to be carefully designed so that the principles outlined below work in practice. These techniques will generally be used for a commercial build, but for smaller or self-build types of building you may find them unnecessary. It’s still perfectly feasible to allow the floor and roof weight to compress the walls without using pre-compression. All loadbearing designs are compressive, it’s just a matter of whether you let the building structure itself compress the bales, or whether you help it along so you can build faster. Whichever method you choose, it is best that as much as possible of the timber work of the building is prefabricated, because this speeds up the actual time of the build and therefore its vulnerability to the weather. The baseplate, first-floor ring beam and joists, roofplate and roof can all be done in advance.
Using a crane and temporary props
Prefabricated parts can either be made off-site and brought to the site later, or constructed on-site next to the building. Either method has the tremendous advantage of enabling the roof to be built at ground level, thus saving on labour, time and scaffolding costs. The roof is not completed at this stage, so as not to be too heavy, only finished up to the vapour-permeable (weatherproof) layer. A crane is used to lift the first floor, and temporary timber columns and beams are placed beneath it for support, leaving enough room to install the straw underneath.The floor will need a temporary stem wall to be built centrally along its length to create a pitch for heavy-duty tarpaulins that waterproof the floor and cover in the sides of the building to give the straw protection from driving rain. The scaffolding should be sheeted on the outside for the same reason.The straw is now installed and the columns and beams are removed by raising the floor with acro props and taking the timber columns out, leaving the floor resting on the walls, which are then compressed by the weight of the floor itself; and by ratchet straps that go through the floor.The weight of the floor itself and the walls and roof above are usually enough to stabilise the building without the need for permanent tie-downs (these would be needed at this level only if this was a single-storey building with a lightweight roof, or there were severe winds). Once the ground-floor walls are complete, the roofplate and roof are lifted up above the first floor, on to more columns and beams, and temporarily held in place while the first-floor walls are constructed. At this stage, the roof provides waterproof covering over the building and tarpaulins or sheeted scaffolding protect the sides from driving rain. Again, the walls are compressed using ratchet straps over the roofplate.
Using scaffolding
An alternative to using temporary props is to build the scaffolding up to just above ground-floor height and use scaffold trusses across from one side to the other. The floor can then be lowered on to these and waterproofed while the straw is installed. When all the straw is in place, the floor can be lifted up, the trusses removed, the floor lowered on to the straw, and the trusses moved up the scaffolding to their next position, just above the height of the first floor walls, where they will hold the roof until it is also ready to be lowered. This method was being pioneered on the North Kesteven building site in 2009.
Using props without a crane
An alternative to using a crane for lifting whole sections of the building is to construct the first floor directly on top of the temporary columns and beams, weatherproof it, then install the straw below and remove the props as described above.The first-floor walls and roof are constructed in the same way.
The timber used for the props is designed so that it can have a second use as part of the building later on. There has to be some flexibility in the building schedule to make sure that certain critical points of the build take place during dry weather, e.g. putting the roof on, as this obviously requires the straw walls to be uncovered, but these stages should not require more than one or two days to complete. We have used this method to build a two-storey semi-detached house and it worked extremely well.
Compressive frame technique
The two main drawbacks to the infill method of strawbale building, described earlier, are the difficulty of stabilising the straw enough for plastering and the extra cost associated with using a secondary frame of timber as well as the potentially structural straw. The compressive frame method addresses both these problems by making a framework building as much like a loadbearing one as possible – although for this reason it’s often more sensible just to stick with true loadbearing! The basic idea of this compressive frame method is the same as for the compressive loadbearing method: that is, using various means to hold the floors and roof out of the way while the straw is installed, dropping these elements down on to the walls and then pre-compressing the walls beneath them. In framework techniques the floors and roof are held up by posts that remain a permanent part of the building, rather than by temporary props.
The basic principle is to design a frame with the floors and roof about 100mm (4”) higher than their finished level, held out of the way by various means, and then, once the straw has been installed, to lower the floor and/ or roof on to the walls, often using their weight to stabilise the straw without the need for pre-compression straps. This gives a covered space inside the building in which to store straw, but still means the sides of the building need to be protected from rain.
There are many different ways this can be done, and you can be sure that your engineer can think of very complex ones! It should, however, be kept as simple as possible, and can be done using folding wedges of timber and mortice and tenon joints, or threaded rod of some description, metal shoes for the posts, etc. If you do end up with lots of metal in the walls (which is expensive), the straw will need to be protected from its coldness by wrapping the metal in hessian or something similar, because cold metal could cause warm moist air to condense on to it, thus causing potential damp problems.With larger frames, the straw can be precompressed between each post, tightly packing straw between the top of the last bale and the underside of the frame. If the frame is strong enough then bottle jacks can be used to compress the straw downwards from the beams of the frame, rather than using ratchet straps tied to the foundations. To do this a solid metal plate needs to be placed on top of the straw, and the jack extended between this and the top beam of the frame. Then, if you’ve designed the frame correctly, a whole bale will fit in the space you have created and you can carefully pull out the metal plates as you remove the jacks. Alternatively, you can design the frame so that you have a timber plate at compression level, which remains permanently in the wall. Again, the straw beneath can be compressed using bottle jacks or ratchet straps, and then the timber plate is fixed permanently to the posts either side, and you can carry on building above it.
Sworders Ltd, a fine-art auctioneers based in Stansted Mountfitchet, has had its new saleroom built using this method, with its Director Robert Ward Booth as the driving force and amazonails as consultants. This is pictured in the colour section of this book.
Lightweight frame technique
This method uses the structural properties of the bales to enhance those of a lightweight timber framework. The frame is so minimal that it could not stand up alone, and requires temporary bracing and props to give it stability until the straw is in place. The straw is an essential part of the structural integrity of the building, more so than the timber, and it works together with the timber to carry the load of floors and roof. Small timber posts, usually 100mm x 50mm (4” x 2”), are located at intervals and either side of window and door openings and are designed such that the timber wallplate or roofplate, at first floor and/or roof level, can be slotted down on to the posts once the straw is in place and the bracing and props are removed (illustrated opposite), allowing the bales to be pre-compressed manually using various different methods. This could be by ratchet straps, or threaded rod can be used, depending on the design. Compression of the straw is essential to achieve enough stability of the walls for structural integrity and so that they can be plastered. In two-storey buildings the first floor needs to be lowered before the first-floor walls are installed so that the posts have free movement through the wallplate. The posts do not become fully structural until the walls have been compressed and then the timber is permanently fixed to the wall and roofplates.
SLOTTED POSTS AND WALLPLATE DESIGN FOR LIGHTWEIGHT FRAME METHOD
Hybrid and other methods
There are many types of straw building that use a combination of ideas from the above techniques, or use new ideas. Being so simple, using straw allows for invention during practice. For instance, it’s possible to build well-insulated loadbearing walls to protect your house on the cold north side and combine this with a framework method on the south, allowing for lots of windows to maximise solar gain. The two-storey house that won the Grand Designs Eco-Home of the Year Award 2008 was designed like this. All it takes is a bit of design ingenuity to make the rigid parts work with the flexible parts. Other methods have been used at different times around the world too. For instance, 30 years ago in Canada, Louis Gagné pioneered a bale building method using cement mortar between the bales, called the Mortared Bale Matrix. The bales were used much more like bricks, stacked in vertical columns so the cement mortar, in effect, formed posts between each stack and the whole building was cement-rendered inside and out. This approach is rarely used now because of the knowledge of simpler, more environmentally conscious and more enjoyable methods, but it is occasionally practised, particularly in France, through the French connection between Quebec and Europe.
In Germany they suffer from having prescriptive Building Regulations (unlike ours, which are guidelines), which means they are not officially allowed to build using the loadbearing method. This is limiting in terms of being able to experiment and to use new and simpler techniques. They have to use straw as part of a timber frame method, instead of the other way round, and follow official practice for timber frame, which means placing a horizontal timber between posts at every fourth course, on top of the straw. In practice they use this to compress the straw in another version of the compressive frame technique, which works very well.
There are many types of strawbale building that use a combination of ideas from different techniques, or use new ideas. Being so simple, using straw allows for invention during practice.
Other aspects of strawbale building
Straw is a very different material from others we have become used to working with in the twentieth century, such as cement or timber. As we have seen, it is natural, breathable, flexible, non-toxic, low in embodied energy, safe and fun to work with. Once it has finished its usable life as a building material it will go back to the Earth as part of the natural cycle of nature without creating any waste, damage or pollution. So in designing with straw it is sensible to use other natural materials as well, as like goes with like. Therefore you will find roofs and floors insulated with sheepswool, hemp or recycled paper, solid floors made of limecrete and insulated with recycled glass or blown clay, and roofs covered with cedar shingles or planted with sedum. Once you start thinking about a natural system of building instead of an unnatural one you begin to see how possible it all is.
When installing other aspects of the building, such as the services, plumbing and electrics, or the roof covering, these are done in much the same way as you would expect in a conventional house, but with thought given to the way the building needs to breathe and be flexible, minimising the environmental impact of manufactured materials.
Strawbale building has become much more than simply building a house with straw walls. It is now leading the way in providing an affordable solution to the need for thermally efficient housing, with very low embodied energy, using natural materials. Therefore it is common to design foundations without using cement or deep trenches. In the same manner, instead of cement renders and plasters, traditional lime and/or natural clay renders and plasters are used. Most strawbale houses, of whatever type of construction, are plastered inside and rendered outside, so that when finished they can look very similar to a traditionalstyle cottage, very beautiful and with deep walls – it is hard to tell that they are made of straw. The walls need to be protected from the weather using either several coats of limewash, reapplied, as with all painted houses, every few years, or an active silicate paint, which is a mineral-based paint that bonds with the quartz in the render, remains vapour-permeable, and needs reapplying only every fifteen years.
Durability
Because of the simplicity of strawbale building, it is possible to build a wide range of quality structures, from a strawbale shed that might only last ten years to a strawbale house to last several generations. We can design to a standard of 200 years minimum – anything less is not sustainable, either environmentally or financially. In the UK and Ireland, the oldest buildings are fifteen years old, and some of the early ones were never intended to be more than experiments. However, there are now two-storey semi-detached loadbearing homes for social housing, classrooms and whole schools, retail spaces, centres for community groups and new extensions to existing houses, as well as hundreds of owner-built houses, offices, studios and garages, animal shelters, food and machinery storage barns, and so on, all of which are expected to have a usable life of more than 100 years. In the USA, where it all began, there are about a dozen houses built around 1900 that are still inhabited and in good condition, and thousands built since the revival began in the late 1970s.No strawbale building in the UK or Ireland has ever been refused planning permission or building regulation approval on the question of durability.
The key to durability with a strawbale house, as with any other, lies in good design and detailing, quality work, and maintenance when necessary throughout the building’s life.
• Raise the first course of bales up from the ground by at least 300mm (12”), preferably 450mm (18”); put a 450mm (18”) overhang on the roof to protect the walls from rain, and you won’t go far wrong. ‘A good hat and a good pair of boots’ – just like cob buildings!
• When using the loadbearing technique, distribute the loads as evenly as possible around the whole building. Never use point loads.
• The roof weight must be placed on the centre of the walls, not on one edge or another.
• The key to durability with a strawbale house, as with any other, lies in good design and detailing, quality work, and maintenance when necessary throughout the building’s life.
• Avoid using metal in the walls if at all possible – and if it’s not possible to avoid using it, wrap it in hessian or something similar, since it is a cold material and may encourage warm, moisture-laden air from the inside of the house to condense on it.
• Loadbearing houses are subject to settlement as the straw compresses under the weight of the floors and roof. Allowance for this must be designed in by leaving settlement gaps above doors and windows, even when using impressive-compressive methods. See Chapter 10 for more details.
