Before work began on Ian's extension project

With our Building Regulations application for our new extension ready to roll, with the exception of the structural calculations, it was time to find a structural engineer to solve the piece of the puzzle.

We couldn’t afford to delay things because getting builders’ quotes depended on having all the key structural components confirmed. Plus, it doesn’t look enormously professional if you submit a ‘full plans’ application to building control with vital information missing.

A couple of days later, following up a recommendation, I found myself sitting in the plush surroundings of a highly regarded engineering consultancy for a preliminary discussion.

But, inevitably, when their quote arrived, it turned out to be heftier than budgeted. It was back to square one.

A quick online search of the surveying website produced a list of structural engineers located within a 15-mile radius. Local firm Cox Clifford Partnership quoted a reasonable sum (£510 + VAT) to provide all necessary calculations for the roof structures, beams, lintels and floor joists, together with advice on foundations. Any necessary site visits would incur an additional charge.

Structural Calculations

Having emailed over a full set of PDF drawings and photos, I picked up the phone to Anne Wiseman at Cox Clifford Partnership. Anne helped clarify a number of issues that had been bothering me.

For starters, the sub-strata below the ground surface in this part of Buckinghamshire comprises layers of thick clay, which, being notoriously prone to seasonal shrinking and swelling, can play havoc with shallow foundations.

I wondered whether we’d be required to excavate the trenches to uneconomic depths to reach down to solid ‘bedrock’. Also, given the close proximity of the extension to our sizeable garden pond, I was worried that building control might chuck the book at us and insist on expensive pile foundations.

It was reassuring that Anne felt conventional trench fill foundations ought to be sufficient and that it was unlikely we’d have to excavate much deeper than the minimum trench depth of 1.2m stipulated by Aylesbury Vale Building Control. Given that the clay was of a reasonably firm consistency, she also felt a slip membrane probably wouldn’t be necessary.

There were a couple of other issues I wanted to ask her advice about, too. The main roof structure was designed with vaulted ceilings, following the line of the rafters. I wanted to avoid the need for visually prominent purlins supporting the rafters at mid-span.

A few swift calculations confirmed that installing a steel ridge beam would remove the need for purlins. The rafters would need to be of 150mm-thick C24 timber spaced at 400mm centres.

The adjoining rear roof over the back bedroom was a simpler proposition. Due to the roof’s conventional A-shaped design, we wouldn’t need purlins or any special ridge treatment thanks to the ceiling joists act which act as collars tying together the rafters on the opposing roof slopes.

Finally there was the question of the bi-fold doors to the flat-roofed single-storey kitchen extension. Should we run them the full width of the wall like a glass curtain, or frame them with a rendered surround? Full-width glazed doors would necessitate construction of an expensive hidden steel supporting framework.

The simpler option would require a masonry corner return column either side, 650mm minimum in width to comply with Building Regulations. Inevitably, in any project, a lot of design decisions involve weighing up aesthetics and the desired look against the practicalities and cost.

After mulling this over for some time, and soliciting comments from friends and family, the consensus was that a ‘pure’ full-width expanse of bi-fold glazing would clash with the traditional architecture elsewhere.

My own feeling was that a white-rendered surround would suit the overall design better and would certainly be a lot lighter on the budget. I think it’s fair to say that unless you happen to be blessed with innate streaks of architectural genius, there often remains a lingering trace of post-decision doubt; only when it’s built and staring you in the face can you be certain that you made the right call.

Anne set to work calculating the key components required for the Building Regs application. This meant the drawings could now be finalised and submitted with a summary of the main structural design points, with the detailed supporting documentation to follow shortly afterwards. With all the key parameters now clearly defined we would soon be able to send out specifications and drawings to invite firm quotes from building contractors.

Preparing for all Eventualities

Before renovation began Ian dug a trial trench

Before work began, Ian dug a trial trench in close proximity to the pond before pre-empting the building control officer’s misgivings about the distance and reclaimed space back by infilling the overgrown pond edges

Notwithstanding the engineer’s reassurances, in my mind there remained a nagging worry about the pond being located not much more than 1m away from the foundation trench. Although the pond is shallow, the dark water can give a misleading impression.

In the final analysis, regardless of the engineer’s advice and what’s on your drawings, the ultimate decision on foundation depths rests with the building control officer on the day of excavation.

To pre-empt this risk, I sought to extend the distance to the pond by infilling the overgrown stepped pond edges. This reclaimed stretch of land could then be buttressed with a series of cobblestone-filled gabion baskets.

A few hours’ hard graft with a concrete breaker made light work of demolishing redundant dwarf garden walls and the patio located in the area earmarked for development; the rubble was then recycled as hardcore to build up the pond edges.

The extra space we’d reclaimed between the pond and the extension front wall meant it would be wide enough to drive across en route to the newly gravelled parking area to the side.

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