In brief
- Adding a floor or upper storey loads the existing structure, so the design depends on what is already there, not only on the new work.
- A team that has only built new single-storey walls may not raise the questions that vertical extensions experience gaps in Gabiley make critical.
- Check the existing structure first, then match the contractor's record to that specific task before signing.
Contents
Vertical extensions experience gaps in Gabiley matter because adding a storey or upper floor puts new loads on structure that is already standing. The design cannot be finished from the new drawings alone: someone has to confirm the footings, columns and wall thickness that already exist. When the team has only built new single-storey work, nobody raises that question until the concrete is ordered.
A gap of this kind comes down to whether the people pricing and running the job have done this type of work before, and therefore know which checks come first. This article sets out how the gap arises, what it does to the build, and how a client or supervising engineer can detect and close it.
What makes a vertical extension different from new build?
A vertical extension inherits a structure whose capacity was never documented for the new load, so the first task is investigation rather than construction.
On a new plot, the engineer sets the footing size, the column schedule and the wall thickness. On an extension, those already exist. The existing foundation may be a pad, a strip or something else, and its depth and width can only be confirmed by exposing it or by finding records. The engineer then checks whether it can take the added load, or whether it needs widening, underpinning or a transfer arrangement.
A contractor who has only built from the ground up may treat the existing walls as a given and start setting out the new columns. That skips the load path question. Confirm the trial pit locations and any testing with the project's structural engineer; a laboratory can quote for cores or cube tests on request.
How does the gap show up in pricing and programme?
A team without extension experience tends to price the visible new work and leave the investigation, temporary support and strengthening out of the bid.
The bid then looks complete. Missing items can include trial pits, propping of the floor or roof below during breaking, temporary weather cover while the roof is open, and strengthening of any column found to be undersized. Each of these is real work with real cost, and it appears mid-project when the frame is already open.
Detection is possible before award. Ask for a method statement that names how the existing structure will be confirmed, how the floor below will be supported while the roof is cut, and who signs off the exposed footing. Ask to walk a recent extension the bidder completed and look at how the junction between old and new concrete was treated. If the bidder has no extension to show, that is useful information, not an accusation.
How do you check the existing capacity before work starts?
Someone must record the existing structure in numbers before the extension design is finalised, and that record should name who measured what.
It is worth asking the municipal engineer's office what records, if any, it holds for the plot, such as approval drawings or borehole logs, and whether it requires anything from the owner before an extension is registered. The owner may hold an old drawing. Where no record exists, a trial pit, a measured survey of column and wall sizes, and material tests on the existing concrete give the engineer something to calculate from. Confirm the extent of investigation with the structural engineer, because a single pit at one corner may not represent the whole footprint.
Local exposure conditions affect how existing concrete and reinforcement age, and they differ from site to site. Have the structural engineer or a laboratory confirm the conditions for this plot, and let that assessment decide how much testing is needed. Keep the record as measured notes and photographs with dates, and give it to the engineer before fixing the extension layout.
What to do next
Before signing, ask each bidder for two things in writing: the name and role of the person who will confirm the existing footing and column sizes, and one extension they completed where the same check was made. Put the agreed investigation and temporary works into the scope so the price covers them, and keep records of every exposed footing and test.
Frequently asked questions
Does an extension always need the existing foundation exposed?
Not always. It depends on what the structural engineer needs to calculate the added load. Where drawings or earlier approval records exist and match the building, the engineer may accept those. Where nothing exists, a trial pit or another direct check is the usual way to get a footing size and depth. Ask the engineer to state what evidence will be accepted.
What should a client ask a contractor about extension work?
Ask which extensions they built, who confirmed the existing structure on those jobs, and how they supported the floor below while the roof was opened. Ask to walk one completed site and look at the joint between the old structure and the new. Written answers are more useful than a verbal claim, and a bidder with no extension to show should be asked how the missing experience will be covered.
Local reporting: this article is written for building work in Gabiley and Somaliland. Ground conditions, prices and rules differ between places; confirm the details for your own site.