Up on the roof: a review of design, construction, and technology trends in vertical extensions

Figures & data

Table 1. Benefits of VEs recognised by existing literature.

Benefits of VE		References
Environmental benefits	Increasing urban density (while preserving green spaces and reducing urban sprawl) Avoiding or reducing demolition of existing buildings therefore reducing embodied carbon emissions Improving the performance of the base building (that beneath a VE), by using financial gain of the extension to fund a retrofit	(Ambrosini and Callegari 2021; Amer et al. 2017; Aparicio-Gonzalez, Domingo-Irigoyen, and Sánchez-Ostiz 2020; Argenziano et al. 2021; Artés, Wadel, and Marti 2017; Dind, Lufkin, and Rey 2018; Eliason 2014; Hermens, Visscher, and Kraus 2014; Jellen and Memari 2014; Pattison 2021; Soikkeli 2016; Sundling 2019)
Social benefits	Preserving cities’ historical character (as compared to demolish and rebuild) Improving safety in the city centre by increasing urban density	(Eliason 2014; Hermens, Visscher, and Kraus 2014; Jellen and Memari 2014)
Economic benefits	Increasing the income potential of the base building Financing refurbishment of the base building Reducing cost of land and new foundations (as compared to a new building) Faster construction (as compared to demolish and rebuild)	(Eliason 2014; Jellen and Memari 2014; Kussin 2016; Pattison 2021; Sundling 2019; Soikkeli 2016)

Table 2. Existing Classification of VEs.

Author(s)	Publication Type	Classification Types	VE Classifications	Study Scope
Amer, Mustafa, and Attia (2019)	Journal paper	Based on offsite construction methods of VE	(1) load bearing methods:  • direct loading  • indirect loading (2) assembly methods:  • modular assembly (3D)  • panels assembly (2D)  • component assembly (1D) (3) building materials of the  • base buildings:   masonry (81%), RC (13%), steel (6%)  • VEs: steel, timber, RC	136 VE projects in Europe, residential function, 1–3 storeys VE
Ambrosini and Callegari (2021)	Book	Based on the function of VE	historical residential buildings, social housings, factory reuse, service buildings	25 VE projects (80% in Europe), 1–3 storeys VE
Floerke et al. (2014)	Report	Based on the form of VE	roof, cube, inserted, free form, add on, gap	154 VE projects (97% in Europe)
Hermens, Visscher, and Kraus (2014)	Conference paper	Based on structural strategies of VE	spanning, building through, building on top of other buildings and infrastructure	7 VE project examples (6 in Europe)
Sundling (2019)	Journal paper	Based on structural strategies of VE	no reinforcement is required, reinforcement is required, no solution for reinforcement can be found	4 VE projects in Sweden (2 built, 2 unbuilt), 1–2 storeys VE

Figure 1. Research methodology.

Table 3. Project inclusion and exclusion criteria.

Project Inclusion Criteria Minimum 1 storey of permanent occupiable VE is built on top of the base building VE’s footprint is more than 25% of the base building’s footprint area (estimated from project drawings) There is evidence that the project has been built or is currently under construction

Project Exclusion Criteria Attic extension (i.e. an additional floor is built within an existing attic space without a VE) Façade retention (only the base building’s façade is preserved, while the structure and space is demolished and replaced with an entirely new construction) Rooftop garden or greenhouse built on the roof deck of a building Temporary/movable space and informal VE VE project proposed or in planning (16 VE projects were identified but excluded as they had not started construction yet)

Figure 2. Project locations by region.

Figure 3. Project locations by country

Figure 4. Construction periods of: a. base buildings (left); b. VEs (right). Note: There are 180 base buildings and 172 VEs, because in 7 projects, VEs were built atop 2–3 base buildings.

Figure 5. Construction time gaps between base buildings and VEs.

Figure 6. The number of storeys of base buildings and VEs.

Figure 7. Percentages of storeys added.

Figure 8. The functional classification used and its references.

Figure 9. Functions of: a. base buildings (left); b. VEs (right). Note: There are 180 base buildings and 172 VEs, because in 7 projects, VEs were built atop 2–3 base buildings.

Figure 10. Functional changes of base buildings and VEs’ functions. Note: Each node shows ‘[building function]: [no. of buildings]’. In this diagram, the number of buildings considered has been adjusted. Where multiple base buildings were located beneath a VE, these have been counted as a single building to ensure consistency in the Sankey diagram, except if they had different functions, resulting a total of 173 buildings in the graph.

Figure 11. Five forms of VEs identified along with examples of projects: (1) extruded – Adina Apartment Hotel, Melbourne; (2) setback – Deco Building, Sydney; (3) roof – Trikafabriken 9, Hammarby Sjostad; (4) rooftop village – Didden Village, Rotterdam; (5) freeform – Substation 164, Sydney [images courtesy of: (1) © Peter Clarke; (2) © Brett Boardman; (3) © Felix Gerlach for Tengbom; (4) © Rob t Hart for MVRDV; (5) © author] .

Figure 12. Project classification based on forms of VEs. Note: Combination of two forms was applied in 11 out of 172 VE projects.

Figure 13. Examples of VE projects and their facade strategies: (1) unified facade at Blue Cross Blue Shield; Chicago; (2) similar facade at Midtown Centre, Brisbane; (3) distinct facade at De Karel Doorman; Rotterdam [images courtesy of: (1) © James Steinkamp Photography for Goettsch Partners; (2) © AM Brisbane CBD Investments & DMC Projects; (3) © Ibelings van Tilburg architecten, Ossip van Duivenbode] .

Figure 14. Project classification based on facade designs of VEs. Note: Combined facade designs were applied in 3 out of 172 VE projects.

Figure 15. Project classification based on plan for VEs.

Figure 16. Three structural support strategies for VE (Julistiono, Oldfield, Cardellicchio 2023).

Figure 17. Project classification based on structural support strategies.

Figure 18. Project classifications based on: a. primary structural materials of base buildings (left); b. primary structural material of VEs (right).

Figure 19. Project classification based on occupation conditions while VE is built.

Figure 20. Summary of the most frequent trends and characteristics of VE development.

Table 4. Comparison of VE projects with three most frequent base building functions (industrial, office, residential).

Base building function (no. of projects)	Average percentage of storeys added	VE alongside refurbishment	Original function changes	VE with demolition*	Occupied
All (172)	63%	115 (67%)	86 (50%)	41 (24%)	36 (21%)
Industrial (53)**	67%	46 (87%)	49 (92%)	8 (15%)	3 (6%)
Office (48)**	54%	34 (71%)	19 (40%)	15 (31%)	9 (19%)
Residential (42)	57%	23 (55%)	10 (24%)	13 (31%)	10 (24%)

Note: *VE was built alongside partial demolition in the base building. 

**There is one project in which the VE was built atop two base buildings – industrial and office, hence this project is counted in both industrial and office base building function.

Figure 21. Construction periods of steel VEs and timber VEs.

Table 5. Comparison of VE projects with steel and timber VEs.

VE material (no. of VE projects)	Average percentage of storeys added	Structural strategy 1 (fully supported)	Structural strategy 2 (supported w/ reinforcement)	Base building occupied during VE construction
All (172)	63%	60 (35%)	69 (40%)	36 (21%)
Steel (98)	64%	36 (37%)	43 (44%)	22 (22%)
Timber (33)	52%	17 (52%)	12 (36%)	10 (30%)

Note: This table only compares steel and timber VEs (projects with VEs constructed using other materials are excluded).

Table

[Project Title]			[Code]
			Location:
			[address]
			[city, country]
			Professionals involved:
			Architect:
[project picture/s]			[architect name]
			Developer/Owner:
			[developer/owner name]
			Structural engineer:
			[structural engineer name]
			Contractor:
			[contractor name]
Base Building (BB):		Vertical Extension (VE):
Year built	[year built]	Year built	[year built]
Storeys	[no. of storeys]	Storeys	[no. of storeys]
Function	original: [original function] existing: [before extended] final: [after extended]	Function	[function of VE]
Structure	[primary structural material]	Structure	[primary structural material]
Structural strategies:		Architectural strategies:
[planned/ unplanned]		Facade: [unified/ similar/ distinct]
[supported by existing structure/ supported w/ reinforcement/ separate structure]		Form: [extruded/ setback/ roof/ rooftop cottages/ freeform]
		VE footprint ratio: [VE footprint/EB footprint]
Construction strategies:		Additional notes:
[occupied/ empty] while VE being constructed		Is the BB heritage significant? [Yes/No]
Any demolition involved? [Yes/No]		Was VE performed w/ refurbishment? [Yes/No]
		Was VE built w/ horizontal extension? [Yes/No]
Other information:
– [context story/a brief history]
– [additional information on strategies implemented to extend the base building]
References:

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Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article [and/or] its supplementary materials.