When it comes to drawing up the structural designs for a house extension, the Structural Engineer will sometimes make assumptions about the building before crunching the numbers. Doing so carries risks and where possible it is preferable to carry out opening up works to the property as a means to baseline what is currently there, minimise guesswork and better plan for what needs to be done to open up the new space safely.
Opening up works are usually requested by the Engineer (and occasionally the Architect) when structural investigations are required. Opening up works involve stripping away precise surface areas to reveal a specific section of the underlying structure that the Engineer need to physically eyeball prior to producing a suitable structural design for the extension.
The need for opening up works for an extension can be triggered for a couple of reasons. The Engineer may need confirmation that part of a structure is as they believe it to be, or he/she has spotted something around the building (such as cracks) that gives cause for concern, and so further investigation of the issue is deemed necessary.
There are four types of opening up works common to extensions.
Trial pits. Trial pits are a type of opening up work. They are holes (usually 1㎡) excavated by hand, abutting the outside wall of a building. They are dug to check the depth and type of foundations that the building sits upon.
Lifting floorboards. This is done to check one of many things, such as the continuity of suspected load-bearing walls, condition of dwarf walls joist integrity, water table saturation and more. Care should be taken to disturb only the smallest area of flooring as necessary.
Boarded attics. Attic spaces are sometimes boarded over. This boarding may need to be removed to assess the extent of any voids behind or on the underside of the boarding, or to check joist direction and sizing. The advent of keyhole cameras allows for disruption to be kept to a minimum.
Exposing walls and ceilings. Plastered sections of wall and/or ceilings may need to be removed, typically to check for the presence of supporting members like reinforced steel girders. Furthermore, to check for the existence of spreader plates supporting said girders, integrity of the supporting wall and the nature and quality of any connections.
When carrying out opening up works for an extension, it is best practice that the instructing Engineer is present along with the tradesmen, so they may work under his/her instruction. It allows the Engineer to inspect what they need first-hand and not have to rely on photos and importantly, keeps the Engineer firmly on point and accountable for the work they produce. Size of openings should be kept as lean as practicable and any making-good should ideally be completed on the same day, straight after the inspection.
Carrying out opening up work for an extension can be a headache, but the benefits of doing so outweigh the inconvenience, because they allow the Structural Engineer to accurately baseline the make-up of the building before producing an optimally designed extension structure.
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