Roof extensions Means

In tight sites the capacity to extend a building laterally can be severely limited. Mid-terraced properties for example can only allow an extension of a suitable size at the rear. Even so there may be little if any room

Schematic vertical cross-section (not to scale)

Schematic vertical cross-section (not to scale)

Notes

A. Traditional method of forming a light-well with skylight.

B. Modern method of forming a light-well using a reflective duct (e.g. Sun Pipe).

1. New opening formed in existing roof. A new rafter is added to the existing on either side of skylight to strengthen the roof structure if more than one rafter is trimmed.

2. Duct consisting of framing and plasterboard/plywood to form vertical or slanted light-well.

3. Security grille.

4. Hemispherical glass can come complete with flashing.

Notes

A. Traditional method of forming a light-well with skylight.

B. Modern method of forming a light-well using a reflective duct (e.g. Sun Pipe).

1. New opening formed in existing roof. A new rafter is added to the existing on either side of skylight to strengthen the roof structure if more than one rafter is trimmed.

2. Duct consisting of framing and plasterboard/plywood to form vertical or slanted light-well.

3. Security grille.

4. Hemispherical glass can come complete with flashing.

Figure 6.9 Methods of forming a light-well to extend the property at the back. Planning controls as well as site constraints often preclude such extensions, especially in inner-urban areas. Moreover, a flat-roofed building in such circumstances cannot offer any loft conversion capabilities unless it is being over-roofed (see next section on Over-roofing). The substructure conditions or the proximity of tunnels or other encumbrances such as major services or utility lines may not make a downward vertical extension possible.

The only means of increasing the building's capacity in such circumstances is to add another one storey or more. This may include the provision of a pitched roof on the extension. However, as over-roofing a flat roof is a form of improvement that does not necessarily involve any increase in habitable space, it is included under the chapter dealing with refurbishment buildings.

In refurbishing framed commercial buildings, adding another storey is usually best achieved by using a steel structure. This approach can be adopted for steel and reinforced concrete-framed constructions. In the case of a reinforced concrete- or steel-framed building, for example, the new storey could comprise a steel portal frame supporting a curved roof. The portal frame could set back from the existing parapet to minimize disruption to this part of the building as well as provide an access path around the top storey.

The constructional requirements for adding a new pitched roof to a flat-roofed building are shown in Figure 6.10. The drawing illustrates how a steel portal frame can be used to form an extra storey with a new curved roof on a reinforced concrete-framed building. The new storey is set back from the existing parapet to provide a distinct break in the design as well as to give an access route for maintenance. Alternatively, a similar style of roof could form part of an over-cladding scheme to a building (see Chapter 9). There are essentially three designs for a roof extension. In order of likely preference these are:

1. an additional storey incorporated within a new pitched roof (see next section on Over-roofing);

2. an additional storey (or storeys) with pitched roof;

3. an additional storey (or storeys) with flat roof.

Structural implications

A building that collapsed in Middlesex, England, during its adaptation in the mid-1990s highlighted the risks to health and safety associated with this type of work. The case in question was a three-storey commercial building that was halfway through a refurbishment contract in 1995. During the early afternoon of 1 August of that year a substantial part of the structure suddenly collapsed, killing four construction workers.

The following extract from the official report into the tragedy (HSE, 1999b) neatly summarizes the background to and causes of this tragedy:

... The building was constructed in 1969/70, initially as a single-storey structure. It was extended upwards in 1970. In 1995 it was being refurbished by building contractors advised by a structural engineer and an architect.

As refurbishment progressed, three men were engaged in removing panel walls between the key structural supporting columns. They discovered that a column at first-floor level was supported on a single lightweight concrete block. Within minutes of this discovery the building collapsed, killing the three men and the site agent who had been called to investigate.

Subsequent investigation by inspectors of the Health and Safety Executive (HSE) and staff at the Building Research Establishment (BRE) revealed serious defects in the original construction of the vertical extension of the building. Initially, the building had a flat roof with a low parapet wall round the edge. When the building was extended top three storeys, the lightweight concrete blocks forming the bottom course of the parapet wall were left in position and used to support the load-bearing column at first-floor level.

The collapse was caused by the failure of one or more of the lightweight concrete blocks supporting the brick columns at first-floor level. This led to consequent rapid and catastrophic collapse of two-thirds of the building. The extent of the collapse was much greater than might have been expected because of the lack of con-tinuity(ties) between key structural elements.

Examination of four brick columns still standing at first-floor level showed no externally visible signs of the lightweight concrete blocks in the columns. They had effectively been hidden by the facing brickwork, internal plaster, and the inclusion of infill brickwork .

Two recommendations (82a and 82b) were made in the above report. They are worth stating because of their relevance to many types of adaptation work:

82(a) Clients, planning supervisors, contractors and their advisers when renovating, refurbishing, extending or demolishing a building, particularly if it was built before the Building (5th Amendment) Regulations 1970 took effect, should address the possibility that it may not be robust, and that damage to a key structural element could lead to disproportionate collapse. Where this is the case the risk assessment should include an evaluation of the risks of such collapse. For instance, if heavy plant is to be used near to key structural elements it may be necessary to provide barriers to prevent contact with the building. Planning systems would be needed with crane operations and propping of the building could be appropriate.

82(b) The defects discovered in the brick columns in this building reflect either gross incompetence or total irresponsibility. Everyone in the construction industry needs to be vigilant against such blatant malpractice. DETR Building Regulations Division will issue advice shortly to local authorities and others on the inspection and appraisal of open plan precast concrete and masonry structures built before 1970.

Structural implications are, therefore, inevitable, regardless of which of the three options listed above is selected. To avoid problems such as overstressing, distortion or even disproportionate collapse, consideration must be give strengthening the main structural elements affected by the additional storey (see Chapter 7). It is possible, of course, that the existing building was designed and built to accommodate an additional storey. In such a case, the structure may require little or no strengthening, but this would also be dependent on the building's overall condition and on the extent of internal alterations associated with the adaptation work such as forming new openings in the old roof to accommodate the new access stair.

Thus, the issues that need to be taken into account when adding another storey to a flat- or pitched-

roofed building are as follows:

• In the case of a building with a pitched roof, the removal of the old roof structure may be needed - to avoid risk of bearing new structure on defective substrate.

• In the case of a building with a flat roof, the removal of the entire existing parapet wall may be advisable - to avoid risk of bearing new structure on defective substrate.

• The joint or connection of any new load-bearing element to the existing substrate.

• Matching or contrasting wall finish of the new top storey.

• Detailing the junction between the new/modified existing wallhead and new extension - to ensure weather-tight construction.

• Sealing and masking the junction between the old and new parts of the building - to maintain structural integrity and resistance to rainwater penetration.

• Forming an opening in roof deck for well of new stair - to provide improved access to the roof space.

• Anchoring the new roof to the existing wallhead - to minimize the risk of wind uplift, particularly if the new roof has a wide overhanging eaves.

• Possible underpinning of the building along the length of the roof extension or inserting of new columns to support the additional loadings (see Figure 7.18) for strengthening the structure to take the increased loading. However, care must be taken to avoid creating a hard-spot in the substructure that could encourage differential settlement. Figure 6.10 illustrates the substructure problems associated with adding a roof extension to part of a building. As highlighted earlier, there is a risk of differential settlement occurring in such circumstances.

Over-roofing Preamble

Over-roofing usually forms part of a modernization scheme rather than a vertical extension to a building. In the former there is usually no attempt to increase the property's usable space. In this sense the over-roofing is still an extension but only of the building's height, rather than its occupational capacity.

In the 1950s and 1960s with the third phase of non-traditional construction, flat roofs were the norm in many new housing developments. Flat roofs were very popular for the simple reason that they are much cheaper and simpler to install than their pitched counterparts.

As indicated in any elementary construction textbook, flat roofs have a number of other advantages over pitched roofs. They offer easy access for maintenance and can easily accommodate complex plan shapes.

In the past flat roofs have, of course, had a poor reputation in terms of weather-tightness and thermal efficiency. Early versions of such roofs usually contained only minimal insulation (often not more than 25 mm thick) under a traditional three-layer felt system. As a result they were prone to excessive heat loss, ponding and leaking, all of which resulted in their having high maintenance costs. Moreover, they were not very durable as their service life was usually between 15 and 20 years.

Modern flat-roof designs have done much to rectify these deficiencies using high-performance single-ply membranes on thicker insulation in a conventional 'warm-deck' or 'inverted warm-deck' system laid to better falls (i.e. 1:40). Such systems have a life expectancy of at least 30 years.

Another difficulty with re-roofing a flat roof, however, is the knock-on effect of increasing the thickness of the insulation. This involves the need to raise the entire parapet, which is a very expensive option and creates a potential mismatch with the finishes at the wallhead.

As a result of these problems with flat roofs many designers and clients consider over-roofing rather than re-roofing as an option for dealing with their flat-roofed buildings (see Hillier et al., 1998; see also Figure 6.11). Over-roofing frequently forms part of a comprehensive modernization scheme that includes over-cladding (see Lawson et al., 1998). This form of roof improvement is not restricted to flat roofs or single buildings. As shown in Figure 6.12 it can be used to integrate a number of small roofs or different shaped roofs into one composite roof. It can also be used to cover over an open courtyard. The design of the new over-roofing system can adopt a straight- or curved-pitch profile for the slope (see below).

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Office

Office

Office

Part plan of west wing

Part plan of west wing

Office

Part plan of west wing

Service yard

Service yard

Part plan of west wing

Service yard

Notes

1. Existing building.

2. Refurbished building with new storey and barrel-vaulted roof.

Figure 6.11 A vertical extension involving a steel portal frame on a flat-roofed building (from drawings of refurbishment of Post Office building in Chesterfield)

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