HPH205 : Specifying a house and creating tender documents – with Dan Higginbotham
Architectural technologist Dan Higginbotham from Parsons + Whittley Architects talks about producing construction drawings and a scheme of works from an architect’s design.
Interview with Dan Higginbotham
Dan has worked with Parsons + Whittley for over 15 years, as an architectural technician and now also as a director. He has recently been working on Ben and Kay’s project and previous work includes housing developments built to Passivhaus standard.
The architectural technologist details how the building is constructed
Dan’s usual role is taking the architect’s 3D conceptual model and making a full set of construction drawings that someone can actually build from. These include plans, elevations and sections. As he says, “What we try to give them is a comprehensive package of information that they’re able to sit down and accurately price.”
For a Passivhaus like Ben’s, more detail is required to ensure airtightness and avoid thermal bridges. Dan explains that there’s less risk with a house only satisfying building regs, as it would probably still pass if not sealed to such a high standard.
Being methodical helps identify all the details
Dan admits that things can get missed during this complex process. He tries to avoid this by following the same method each time but this is hard as he mostly works on one-off projects.
Additional complexity comes from the many different methods of construction, and the level of sustainability clients are trying to achieve. As Dan summarises, “Although you can re-use ideas that you’ve done before, you generally have to start the whole process again depending on how people want to construct it.”
Keeping industry knowledge current is important
New products and resources may result in faster or more sustainable construction and better dwellings overall. Dan explains, “You’re forever learning new things, new technologies, new ways to construct things, and we just somehow have to try and keep up with that process and the industry, really.”
Up-to-date knowledge might come from “build indexes”, forums, word of mouth, or working for a particular client. For example, a housing association might require modern construction methods or certain levels of sustainability. Dan has also found big social housing sites useful for finding out about cost-effective materials or shortages.
Cost, simplicity, speed and specifications all affect construction decisions
A shortage of materials could affect how you design or build things. Ben and Kay’s house will use a traditional cavity build method but this decision was threatened by a shortage of concrete blocks just before work started. Likewise, a current shortage of PIR means seeking alternative insulation materials.
Deciding on construction materials and methods is a whole team process, with input from Dan, architect John, and practice partner Chris. It helped that Ben and Kay had already selected the finishes of the building envelope. The render finish and brick plinth suggested a cavity wall as cost-effective and simple to build, albeit with more insulation than is traditional.
Items specified may change at a later point
Looking at drawings for Ben’s build shows the level of detail required. The elevation showing the joinery specifies some items that they will not necessarily end up with. For example, after various discussions, an alternative supplier may provide the windows. Dan says this is quite normal: he prefers to obtain an accurate cost from a supplier rather than leave the cost as a provisional sum.
The floor plan shows 150 mm of insulation in the cavity wall, enough for this relatively large property to get through the Passive House Planning Package. Although Dan specified high performing Knauf Earth Wool (with a thermal conductivity of 0.032 W/mK), the contractor may choose an equivalent mineral wool.
Ecological aspirations need to be balanced with budget
The section of the different floors shows lots of junctions that need to be considered, especially because it’s a Passivhaus. Dan tended towards methods and materials familiar to most builders rather than potentially expensive ecological features.
Another requirement was that height be kept to that specified on the planning drawings, so Dan avoided any ecological products where more material was needed for the same performance values. He explains the choices as detailed on the section:
- A strip foundation supports a cavity wall construction.
- The first floor is beam and block containing PIR insulation to give a thin but solid construction with the performance needed. Above is a screed with membranes and airtightness products in between.
- The floor and walls specify a very lightweight Aircrete block to get the best performance while minimising thermal bridging at the junction of the wall and floor.
- The internal walls are traditional studwork or blockwork where necessary for support.
- The second floor uses a pre-manufactured metal web floor joist (or Posi-joist) that gives a structurally sound floor with space to route the MVHR and other services without having to laboriously cut holes.
- At the top is a fairly standard, traditional truss roof, with a more complex catslide roof to the rear. To avoid complex steelwork and associated thermal bridges and/or airtightness problems, the roofing truss suppliers came up with a truss that could form the catslide as well.
Sections help contractors avoid thermal bridges and ensure airtightness
Drawing more sections is useful where there are difficult details in the building, including all the junctions. Where each part of the building is different, more drawings may be required. Ben’s Passivhaus is fairly linear, but Dan was careful to show all the construction details so nothing would be missed.
The four sections include one of the garage, one “typical” house section, one of the stairwell arrangement, and one showing the ground-to-first-floor French door, window and boarded panel arrangement.
Developing the electrical plan is iterative
Dan uses his experience to determine locations for electrical points, taking into account where the furniture will go. However, these positions might change later. As he says, “Each client will have their own input, particularly on light fittings and lighting locations.”
A flexible space, for example the spare bedroom doubling as a study, can take more time to work out, as he explains to Ben, “Once we’ve actually physically got the space on site, yourself and Kay might then go, ‘No, actually I want a few more sockets here.’” This is typical of the process: educated guesses on paper are reappraised once the rooms are built.
The team liaise with a consultant for the mechanical design
Alan Clarke is the consultant for the mechanical systems on Ben and Kay’s project: the MVHR design, the hot and cold water supply and heating the property. Dan and the design team work with Alan on decisions like budgeting for the right boiler and the site for the MVHR.
A major consideration is distributing the ductwork within the property. Most of the pipework is routed through the open webs of the intermediate floorspace with additional risers to the first floor. On the second floor landing a false ceiling provides a service void where ducts are routed to each room.
The schedule of works provides more information for the contractor
After completing the drawing package, Dan itemises every detail in the schedule of works. Reading this document alongside the construction drawings enables the contractor to produce a priced specification and to understand the methods and materials of the project.
Find out more
View Dan’s profile on LinkedIn
Visit the website of Parsons + Whittley
Download a transcript of the interview with Dan Higginbotham
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About The Author
Robin Goldberg is a teacher, currently enjoying a sabbatical during which he is overseeing building work to his house and getting ‘hands on’ where possible. The works include a loft extension, but the sabbatical is also being spent planning for his longer term goal of a self build.