One material is natural masonry. It can be considered the oldest building method still in common use, which is adobe or unfired clay bricks. Adobe is usually seen in drier climates. Adobe bricks are still not common in the local builders' merchants, but they are available. Claytec offers a light weight clay/straw brick and a compressed clay brick. Errol Brick also offers an unfired earth brick. All of these are non load-bearing and best suited as a skin for timber frame construction. Some 40% of new homes in Germany use Ziegel blocks ('ziegel' being German for 'brick') and this fired clay block provides a thermally efficient and easy-to-use construction system. It uses a thin mortar bed joint and a tongue-and-groove design which allows for dry vertical joints. They are said to be three to four times faster to lay than a conventional cavity wall and give a U-value of 0.23 - around 35% better than Building Regulations Part L requires - with no other insulation.
Timber is another material that can be used. It has lower embodied energy and offers the potential for better thermal performance. A single-skin, 140mm timber frame wall with 140mm natural insulation between studs and 100mm wood-fibre insulation on the outside - kind of a natural SIPs system - gives a U-value of just 0.18. That is almost half the heat loss for a Part L compliant wall, with the same overall wall thickness.
Hempcrete is a mixture of hemp shiv (the waste from hemp-fibre production) and a lime binder. Sold in the UK by Tradical Ltd under the trade name Hemcrete, the product is available as ready-made blocks for conventional construction, or more usually as mix-on-site and cast in-situ. Hemp shiv and lime are mixed in set proportions to a stiff consistency and tamped into a formwork that surrounds a timber frame. The advantages of this are that the timber frame takes the structural load of the roof, the hempcrete protects the timber meaning that no timber treatment is required, and the finished wall has high thermal performance as well as providing thermal mass.
Straw bales are a sustainable resource which creates breathable, highly insulated structures. Straw is an abundant agricultural waste product and when bales are used to build or insulate buildings, they are commonly finished with plaster. The plastered walls provide excellent thermal mass, structural strength and fire resistance. Straw can easily be used as a load bearing element in a two storey house like the above scenario, but if more is required then structural frame would need to be built from wood and the bales then function as infill.
Rammed earth - or cob has been used as a construction material for at least 10,000 years. Beautiful and versatile it is also an abundant resource that is incredibly durable, breathable and thermally efficient. Cob is simply clay, sand and straw mixed into a workable mass. In many areas of the UK, the raw ingredients for cob can be dug straight from the foundation trenches on site. This means that the bulk of construction material is free.
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Advantages of Masonry
The materials and skills required to build in masonry are widely available across the UK. It is possible to start building work in masonry instantly. It takes to build two skins of block work and render the outside for less than £40/m2. With a brick skin expect to pay up to £20/m2 more, depending on the choice of brick. Masonry cavity walling are therefore the cheapest structural system.
A masonry structure gives a house a feeling of solidity, as the density of the blocks provides a high level of acoustic mass, helping to deaden noise outside the building. Building internal partition walls from masonry, as opposed to timber stud walls covered with plasterboard, will further enhance the feeling of solidity and provide sound deadening between rooms.
The high strength of masonry walls allows the option to use suspended concrete upper floors, rather than conventional timber floor joists. This provides sound deadening between storeys, as well as making it possible to build first floor partition walls in solid masonry, rather than in timber studwork. This extends the qualities of solidity and sound deadening to upstairs rooms. Dense block work also provides a solid fixing for built-in furniture, curtain rails, pictures etc.
With the addition of steel, glue-laminated timbers, masonry is extremely versatile and is suitable for the construction of most house designs. However, ordinary dense concrete block work is a poor insulator and so in order to meet the energy requirements under the building regulations, insulation has to be added into the wall structure. It is possible to achieve extremely high levels of energy efficiency using masonry construction.
One way of improving the thermal performance of a masonry structure is to use lightweight concrete blocks - also known as aircrete. These have a proportion of air added into the mix during manufacture, creating tiny air bubbles which act as an insulant. The disadvantage is that the more air that is added into the blocks, the weaker they become. This can be a problem when it comes to fixings as special anchors are often necessary to fix heavy furniture or curtain rails.
Disadvantages of Masonry
Whilst it is possible to increase the thermal performance of masonry walls to achieve very low U- alues, there are limitations to how much insulation can be added within the cavity. The cavity cannot be built much beyond 100mm without creating structural problems. The alternatives are to use high performance partial fill insulation products, such as urethane boards, which have a relatively high cost factor, or to fully fill the cavity. This does raise concerns over damp penetration, especially in harsh weather areas where many local authorities do not allow it.
The other alternative is to add insulation on the inside face of the walls, which presents a whole different set of issues, not to mention additional labour and material costs. As a form of 'wet' construction, masonry needs time to dry out between block work 'lifts'.
As a result, masonry is likely to be slower than timber frame construction. The tall, slender walls in cavity construction may also be prone to settlement cracks. There are also fears over the cleanliness of the cavities. This being that if too much mortar is allowed to drop on cavity ties above the damp proof course level, or besides the insulation bats, bridges could be formed that might transfer damp to the inner skin.
Another disadvantage is that masonry cannot be laid when it is raining heavily or when temperatures fall below freezing. This is one reason for the popularity of timber frame construction systems over cavity systems in Scotland, where the climate can be aggressive. With timber frame much of the construction process takes place in a factory and the frame is then erected on site in days.
Advantages of Timber Frame
Timber frame systems have always been popular with self-builders because they are fast and convenient. With the main components assembled in the factory and transported to the site, the roofs and walls go up far quicker than with a conventional masonry system. A timber frame structure can be erected and water tight within a matter of days and work can begin on first fix inside whilst the roofers start on the tiling. Another advantage is that you can often deal with just one company, which will design, manufacture and erect the timber frame. Very often the company will also supply the roof structure, windows and doors.
Timber frame houses are also excellent from an insulation point of view. Nowadays the standard timber frame sections of 89mm have to be increased by extra insulation on the inside in order to conform to the U-value requirement of 0.35. However many companies are now offering 140mm as standard, a practice which some timber framers refer to as super insulation. Many self-builders also like the idea of living in a house where the main frame is constructed from a sustainable resource where the timber source is guaranteed to have come from renewable, carefully managed forests.
Disadvantages of Timber Frame
With most masonry systems the materials are readily available. With timber frame a waiting period of 12 weeks is not unusual. Beware, too, of hanging heavy objects on the inside walls. Another disadvantage is that if the labour is poor it is very easy in timber framed houses with brick outer skins to get water penetration round window and door openings.
Timber frame is also usually more expensive than brick and block, although most timber frame companies would argue that if a self-builder opts for 140mm stud walls, which gives a U-value of less than 0.30, the additional costs will generally be recovered in a few years by reduced heating costs.
Others argue that timber frame is acoustically inferior to brick and block. The fact that with timber frame is the walls are largely hollow counts against it when it comes to sound proofing. While masonry will tend to absorb airborne sound better because it is denser than timber frame, impact sound can be transmitted through all kinds of materials and there are many cases of masonry houses built with heavy concrete intermediate floors which have failed to meet the impact sound requirements of the Building Regulations. Good detailing in timber frame housing can reduce the effect of airborne sound transmission so that it matches the best a masonry house can achieve. It is also possible to add further products specifically designed to improve sound insulation, but these all add to the cost of the house. Timber frame construction using panels does not lend itself to large spans.
Whilst timber frame clad in timber is one of the cheapest finished walling options, timber frame can be a relatively expensive choice if the walls are to be clad in natural stone. A greater thickness of stone, or a backing block, will be required to increase rigidity.
Densely bound straw bales are either fitted non load-bearing within a structural post and beam system or taking loads on a ring beam fitted round what is in effect a block wall. It is a system that appeals greatly to the eco-minded because straw is an agricultural waste product, it is easy to handle and is renewable. Straw bale construction also has excellent insulation qualities and is extremely cheap. With straw bale building it is essential to have an external render to keep out the wet. The system has the disadvantage of width - it is heavy on space - and problems with water vapour at the junctions with other materials.
Advantages of Straw Bale
Straw bales are made from a waste product. Once the edible part of the grain has been harvested (such as wheat or rice), the stalks often become a disposal problem for farmers. By bailing the straw, a new life is given to the material. The farmer makes some money by selling the bales and the homebuilder gains an excellent insulation and building material. Homes insulated with straw bale can have insulation values of R-30 to R-35 or more. The thicker the bale the better the R-value. Straw bale walls are at least eighteen inches thick. This adds aesthetic value to the home as thick wall is expensive to achieve with conventional construction. The thickness of the wall helps to reflect sunlight throughout the room. Due to the thickness of straw bale walls, every window can have a window seat or shelf. This becomes both an aesthetic and useful design element.
The concept of straw bale construction is easily understood by even novice builders. With supervision by one knowledgeable straw bale trainer, first-time builders can assist in the construction process. This not only spreads the word about straw bale construction, it also means that the homebuilder can save money by using a volunteer crew to help raise the walls. Straw bales have a low-embodied energy. This means that very little energy is used to manufacture the product as sunlight was the main energy source for growing plant. The only energy needed to make a straw bale is in the bailing process and the transportation to the worksite. Other insulation materials, such as fibreglass, require a substantial amount of energy to produce. Straw bales are 100% biodegradable. Homes can last over 100 years if properly maintained. At some point, all structures will eventually be replaced. When the time comes, the straw bales can be ploughed back into the earth. Other materials can cause problems with disposal.
Straw bale walls can be carved with a knife or chainsaw. Bales can also be finished to a sharp angular edge. Also, properly constructed walls made from straw bales have proven to be more flame resistant than conventional wood-frame construction. This is because the bales are dense and tend to just smoulder when the ignition source is removed.
The insulation is the most effective in climates where heating and/or cooling of the home is essential for comfort. Aesthetically, homes can be beautiful as the natural material lends itself to multiple architectural styles.
Disadvantages of Straw Bale
Since it is not a conventional building material, the contractor may need to learn new construction techniques. If straw bale building codes are not part of the local authority codes, it may be difficult to get plans approved. Straw bale walls need to be kept dry as moisture is detrimental to not only straw, but to many building materials. Moisture entering the bales from the roof above is to be avoided at all cost. If the walls of straw bale at home are kept dry, they will last for the life of the building. Areas of extreme humidity and rain may not be appropriate for straw bale construction. Due to the thickness of the walls most of the overall square footage will be unusable due to it being within the wall space. If straw bales are not available within a few hundred miles of the construction site, the cost of shipping them, along with the potential pollution from the transportation, must be taken into account.
The specification below is for timber being used in decking:
Minimum service life: 15 years -
Decks and associated landscape structures shall be designed and built to give a minimum service life of 15 years.
2. Timber: Naturally durable or pre-treated to the correct use class
Only timber naturally resistant to decay or which can be treated by an industrial process to give long-term protection from decay shall be used.
Hardwoods: Only use species classed as durable or moderately durable.
Softwoods: Only use components treated to a standard appropriate to their use.
3. Timber grade (Strength Class): C16 minimum
The grade (strength class) of timber used for structural components such as posts, beams and joists shall be sufficient to cope with the loads placed upon it during its service life. Softwood with a strength class rating of C16 is considered the minimum standard.
For decks below 600mm in height the use of C16 timber is recommended. For structures over 600mm the use of strength graded timber is essential and required by Building Regulations.
The higher strength classes, typically C18 and C24 should be specified where smaller component sections, longer spans or commercial deck performance design considerations are required.
4. Timber moisture content at installation: 20% maximum
To minimise the effects of shrinkage eg cupping, cracking, warping etc, install timber as close as possible to its equilibrium moisture content. For outdoor wood, moisture content varies from 19% in winter to 13% in summer in the UK. For best results always install wood with a moisture content lower than 20%.
The stability of all wood used out of doors can be improved by the use of water repellent treatments.
5. Metal fixings
All metal fixings shall be made from corrosion resistant materials such as stainless steel, hot dipped galvanised or other specialist coating. Before use, verify with the manufacturer that the fixings you have chosen are suitable for use with treated timber. Aluminium fasteners should not be used in conjunction with treated wood.
Deck board screws should be 2½ to 3 times the width of the board being fixed. Ideally choose screws that are self-countersinking. Pre-drilling pilot holes will help prevent splitting. Always drill pilot holes when fixing hardwoods.
As an aid to drainage build a gentle fall of 1:100 into the deck, away from any adjacent property. Grooved deck boards are designed to assist drainage of surface water and should be laid in the direction of fall.
The Standards set out below all have a relevance to the creation of high performance timber decks designed to provide a service life in excess of 15 years.
BS EN 335-1
Hazard classes of wood and wood-based products against biological attack - Part 1: Classification of hazard classes.
BS EN 335-2
Hazard classes of wood and wood-based products against biological attack - Part 2: Guide to the application of hazard classes to solid wood.
BS EN 335-3
Durability of wood and wood-based products - Definition of hazard classes of biological attack - Part 3: Application to wood-based panels.
BS EN 350-1
Durability of wood and wood-based products - Natural durability of solid wood - Part 1: Guide to the principles of testing and classification of the natural durability of wood.
BS EN 350-2
Durability of wood and wood-based products - Natural durability of solid wood - Part 2: Guide to natural durability and treatability of selected wood species of importance in Europe.
BS EN 351-1
Durability of wood and wood-based products - Preservative-treated solid wood - Part 1: Classification of preservative penetration and retention.
BS EN 351-2
Durability of wood and wood-based products - Preservative-treated solid wood - Part 2: Guidance on sampling for the analysis of preservative-treated wood.
BS EN 460
Durability of wood and wood-based products - Natural durability of solid wood : Guide to the durability requirements for wood to be used in hazard classes.
BS EN 599-1
Durability of wood and wood-based products - Performance of wood preservatives as determined by biological tests - Part 1: Specification according to hazard class.
Preservation of timber - Recommendations
Guidance for specifiers on the treatment of timber drawing on relevant sections of BS EN Standards.
BS 5756: 1985
Specification for visual strength grading of hardwood.
BS 6105: 1981
Specification for corrosion resistant stainless steel fasteners.
BS 6399-1: 1996
Loading for buildings. Code of Practice for dead and imposed loads.
BS 7359: 1991
Nomenclature of commercial timbers, including sources of supply.
British Standards for stone
BS 8221 - 1:2000, Code of practice for cleaning and surface repair of buildings; Part 1 Cleaning of natural stones, brick and terracotta. BSI, 2000
BS EN 1467:2003 Natural stone. Rough blocks. Requirements
BS EN 1468:2003 Natural stone test methods. Rough slabs.
Masonry, cladding, sills etc
BS EN 771-6:2001 Specification for masonry units. Natural stone masonry units
BS EN 772-11:2000 Methods of test for masonry units. Determination of water absorption of aggregate concrete, manufactured stone and natural stone masonry units due to capillary action and the initial rate of water absorption of clay masonry units
BS EN 772-4:1998 - Methods of test for masonry units. Determination of real and bulk density and of total and open porosity for natural stone masonry units
BS EN 772-20:2000 - Methods of test for masonry units. Determination of flatness of faces of masonry units
BS EN 12326-1:2004 Slate and stone products for discontinuous roofing and cladding. Product specification
BS EN 12326-2:2000 Slate and stone products for discontinuous roofing and cladding. Methods of test
BS5534:2003 Code of practice for slating and tiling
BS8000 Part 6:1990 Code of practice for slating and tiling of roofs and cladding
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