Appendix 5: Case Studies of Generic Residential Unit Types Ireland

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1 Appendix 5: Case Studies of Generic Residential Unit Types Ireland A5.1 Three notional generic house residential designs are included in this report to illustrate particular issues specific to timber frame construction and which, in the course of the Consortium s research, have been found to require either additional guidance, regulation or consideration. Some of these issues will be common to more than one of the house types. The case study units will be referred to as House Type 1, House Type 2 and Apartment Block Type 3. The designs are generic and not intended to demonstrate specific architectural features or style but to clearly illustrate timber construction, and issues of quality, reliability, compliance, cost, marketability and sustainability as it relates to various prevalent types of residential construction. The terraced house type 1 and the semi-detached house type 2 were chosen on the basis of being representative of a large percentage of housing types currently being constructed in Ireland. The 4 storey apartment building, type 3, is modelled to illustrate timber frame construction issues as they apply to apartment buildings. The inclusion or exclusion of a house type is not intended to warrant or exclude other types of residential units: House Type 1 2 storey terraced house unit Selected Issues (Internal) Fire Separation Fire resisting construction is required in party walls and in other, load-bearing walls and floor separations. The performance of the construction is dependent upon proper detailing and construction. Extensive guidance is given in the HomeBond manual as to the preferred detail. Fireplaces and Flues in Party Walls Fireplaces located in timber frame party walls when constructed of masonry on site require particular attention to detail during design and construction of timber frame buildings. Errors in setting out can affect the required fit of the accurately fabricated timber frame panels. Services in External Walls Services in timber framed external wall construction currently run within the insulation void and protrude through the vapour barrier beneath the plasterboard. However, they can be installed in a service zone so that the vapour resistance and thermal insulation are not compromised in the wall. This adds cost to the construction and reduces the available floor space but eliminates the possibility of damage to the vapour control layer and insulation. Fig.A5.1 Floor Plans of House Type 1 Appendices A 5.1

2 Stair Opes Manufacturers may span the long dimension of the house to make better use of prefabricated floor panels. Internal load-bearing timber frame partitions must be designed within span limitations. Internal Partitions For small area houses and bungalows, internal wall partitions are generally non-load-bearing and can be easily moved and manipulated in the future for different needs. Internal partitions with timber studs at 600 mm centres form internal walls which can help to break up the floor or roof spans above. Services in Party Walls Services are not permitted in timber frame party wall construction in Scotland or Ireland. Designers can either avoid the party wall or detail a service zone, which is in addition to the party wall. This will have cost implications but maintains fire integrity of the party wall where site skills are questionable or atypical structures are being built. Structural Spans As with stair opes, the manufacturers optimise use of materials and attempt to provide for the largest transportable panel sizes. This results in floor framing timbers that may span the long dimension of the house with internal load-bearing partitions, or steel being used to stay within span limits for timber floor joists. Domestic suspended timber floors generally use standard 225x44 joists at either 400 or 300 centres to span approximately 5.5 m. TF manufacturers tend to panelise these joists into floor cassettes for ease of transportation and site erection. Fig. A5.2 Masonry fireplace and chimney construction in a timber frame building. Services run through the joists at specified locations throughout the spans. Engineered joists can achieve lateral spans of up to 7.5 m. The OSB web can easily be penetrated to allow services through. Excessive spans are usually broken up by the use of internal loadbearing partitions at wall locations by the use of engineered gluelaminated timber beams of modest steel sections packed with timber. Selected Issues (External) Ventilation of the Cavity (Bottom of Wall) Ventilation opes at the base of the cavity wall are required to allow vapour to escape out of the wall system. Omission of ventilation may cause the moisture content of the timber frame to rise above the decay threshold (nominally 20% moisture content) resulting in an increased risk of fungal decay occurring. Masonry Cladding Materials Cladding materials vary. Brick and block and render are most frequently utilised as they have a history of durability. Fig.A5.3 Front elevation of House Type 1 Appendices A 5.2

3 Changes of Materials Special care must be taken in detailing the transitions between dissimilar materials and changes from one external cladding to another. Render Render finishing to timber frame buildings can be applied over blockwork outer leaf. The HomeBond manual recommends limiting areas of render finish applied over metal lath and timber battens as past experience has shown these areas to fail over time. Other synthetic cladding systems are being used in other countries; however, their use in Ireland must be demonstrated by way of Agrément certification. See Appendix 3 (International and National Case Studies) for failures in cladding that affected timber frame buildings in other countries. Ventilation of the Cavity (Top of Wall) Ventilation is always required at the top of a timber frame external wall. Window and Door Opes Special care is required at window and door opes and these are likely locations for imperfect installation and possible water intrusion. Many documented cladding failures have taken place around doors and windows. A.5.3 House Type 2 2 storey semi detached unit with attic extension A three bed semi-detached house. The design raises questions about the potential future adaptation of loft spaces into additional rooms. Currently in Irish regulation, certification and insurance schemes, it is difficult to gain approval for a detail for a chimney to be in a party wall of timber frame construction. The junction between the two construction methods (a masonry chimney within a timber frame wall) is critical. There are implications about the onsite application where wet trades and dry are both required, how the detailing of such a junction might be addressed and how the issue has been approached in other countries. Selected Issues (Internal) Fire Separation Fire resisting construction is required in party walls. The performance of the construction is dependent upon proper detailing and construction. Fireplaces and Flues in Party Walls Same as in house type 1. Services in External Walls Stair Opes Stair opes in this type of house pose no particular problems, manufacturer s however, may span the long dimension of the Fig.A5.4 Floor plans of house type 2 Appendices A 5.3

4 house to make better use of prefabricated floor panels and use internal bearing partitions stay within span limitations. Internal Partitions Internal partitions can be used as load bearing elements. Services in Timber Frame Walls Structural Spans Compartmentation of Garage Garages, formed as annexes to houses, tend to be converted to additional living space at some stage in the life of a house. There is no additional difficulty associated with TF construction. The adjoining wall to the house is generally a cavity external wall and whilst the removal of the panels may be an easier process, the same issues relating to the forming of an ope apply to TF; namely, temporary propping of the supported floor and panel and the necessity to support the external cladding above the single storey height. Dormers and Attic Conversions More often than not, TF houses employ prefabricated timber trusses as a roof structure. Pitches are normally in the region of 30 to 35 degrees which in most cases leads to problems with headroom in most domestic scale situations. The ability of prefabricated trusses to span large distances with light framing members makes their removal or alteration extremely difficult given their lack of structural redundance. The best solution for future conversion is to design the attic space initially for future use by employing a traditional cut-roof or dormer type arrangement and possibly use the space as storage. Party Wall (Separating Wall) Construction Technical Guidance Documents require non-combustible party wall construction over two storeys. The timber frame manufacturers argue that this is too restrictive relative to limits in other jurisdictions. See TFCH recommendation R4.3. in Chapter 4 of this report. Selected Issues (external) Ventilation of the Cavity (Bottom of Wall) Bricks or Masonry Cladding Materials Cladding materials vary. Brick and block and render are most frequently utilised as they are perceived to be durable. Changes of Materials Render or Timber Cladding Appendices A 5.4

5 Ventilation of the Cavity (Top of Wall) Window and Door Opes Fig storey timber frame semidetached house with attic conversion. External Wall Sections Separating Wall Construction A 3 storey timber frame party wall in timber frame construction is not permitted by the Technical Guidance Documents. A.5.4 Apartment Block Type 3 4 storey apartment block A 4 storey apartment block consisting of eleven apartments - eight two beds and three singles - served by a single stair and a lift. In this study issues such as compartmentation for fire and sound attenuation are examined, particularly from floor to floor. Mechanical and electrical fit-out in compartment walls is flagged as well as construction of stair and lifts shafts. Selected Issues (internal) Fire Separations and Compartmentation Fire resisting construction is required in compartment walls. The performance of the construction is dependent upon proper detailing and construction. Services in External Walls Services in timber frame external wall construction can be installed in a service zone. This adds cost to the construction but eliminates the possibility of damage to the vapour control layer. Internal Partitions Internal partitions are sometimes used as load-bearing elements particularly in mid-rise developments in areas where high wind loading is expected. Internal partitions forming compartment walls within the apartment block will generally be load-bearing and depending on loading, studs will be either increased in size or spaced at 400 or 300 mm centres. Services in Compartment Walls, Floor and Ducts Services are not permitted in timber frame compartment walls. As with external walls, an additional service void should be designed into the layout where required. Structural Spans Same as house type 1; however, the height of the building will require higher levels of engineering and expertise in timber frame construction. Timber Frame Balconies There are separate issues for exposed balconies and covered ones. Exposed balconies will need to use naturally durable species. Cantilevered structures will have to interact with the house structure whereas self-supporting balconies will need to be designed to compensate for differential settlement. Covered Fig Storey timber frame Apartment Building Appendices A 5.5

6 balconies will require waterproofing details and will need to interact with the house structure. Lift Shaft Construction Should lift cores be installed, they should be constructed using load-bearing masonry, reinforced concrete or adequately protected steel framing with infill panels. The TFHC found no instances where a timber frame lift shaft was constructed in a timber frame building. The use of concrete or masonry lift shaft construction for will require threshold details capable of overcoming differential settlement and lifts will be required to auto compensate for this settlement. Stair Enclosure Construction Stair enclosures are required to be 1-hour fire resistive construction. A timber frame fire resistive construction wall construction would be allowed under current regulations. Construction of Stairs Timber stairs would not be allowed under current regulations which restrict the use of combustible materials in a building of this type. See recommendations in chapter 4 of this report regarding relaxation of this restriction. Selected Issues - External Ventilation of Cavity Cladding Changes in Materials Render of Timber Cladding Differential Movement Timber as a material will shrink over time as its moisture content is reduced by drying. This results in vertical movement of the frame. Designers usually allow for as 8mm per floor. In multi storey construction the cumulative effect must be allowed for and designed into the construction of all of the wall and floor elements. Where movement is not properly allowed for, failures in the weathering of the building may occur. Balconies Balconies must be designed with particular care. Water intrusion at locations where internal floor joists extend through the external wall construction may cause decay of concealed timbers and should be avoided. Appendices A 5.6