Structural calculations in Hillingdon

London Clay and what carries a Hillingdon extension

A beam is only as good as the ground it finally bears on. In the council's surface water evidence base the entry for the borough's geology reads London Clay Formation dominant, with chalk near the River Colne and gravels in the south of the borough.

Clay shrinks and swells with the seasons and with nearby roots, so the foundation under a new bearing point deserves as much thought as the steel above it. Hillingdon records 731 tree preservation orders, which is a good reason to plot the trees on the survey before the calculation fixes where a load comes down.

We record trees, drains and the depth of existing footings on our survey. Where a padstone lands near any of them, the engineer sees it flagged and sizes the foundation to the site, not to an assumption.

Houses, extensions and the openings they need

Hillingdon is a borough of houses: 68.9 per cent of households live in a house or bungalow, and 56.6 per cent own their home. Typical structural jobs in houses like these include a through lounge, a rear extension opened up to the old kitchen, a loft over a semi-detached house.

The council granted 29 larger home extension prior approvals in its latest return, and an extension of that kind often has a wide opening at the back that needs a beam. A loft puts new load on the floor below instead; our loft conversion drawings in Hillingdon show how the stair and the floor structure are drawn together.

Green Belt covers 42 per cent of the borough. On that land the size of the original house limits what you can add, so we settle the extension's volume first and the structure follows from it.

How Hillingdon has decided householder applications

In its latest published year Hillingdon decided 1,147 householder applications, granted 90.2 per cent of them and decided 94.3 per cent within eight weeks or an agreed extension.

Those figures cover every kind of householder project and say nothing about a single house. They help you plan the timetable: planning, then building regulations, then the beam order. Our planning drawings in Hillingdon set out the first step, and the calculations belong to the third.

A typical Hillingdon job from survey to schedule

Take an interwar semi-detached house with a cavity-wall rear extension and a through lounge to be opened. We visit, measure both rooms on one datum, and note the wall construction, the floor above, the roof bearing on the party wall and where the drains and nearest trees sit.

The engineer then takes the loads: the first floor, the roof and the old back wall's own weight. The beam is sized for bending, shear and deflection, the padstones are checked against the brickwork they sit on, and the temporary props are specified for the builder.

What you receive is a package that makes sense to three readers. The council's examiner sees the figures behind each opening, the builder sees the steel to order and the order of work, and a future buyer's solicitor sees a document that proves the opening was designed, not improvised. Keep it with the deeds.

Fee, delivery and what you hold at the end

Our structural calculations are fixed at £295 with VAT included, confirmed after a constraints check on your address. You receive the calculations, a beam and padstone schedule and a marked-up layout, usually about a week after the survey.

A technologist who did not prepare the set checks it before it leaves us. If Hillingdon asks for a change to the figures we revise them without a further charge, and the numbers carry straight into our building regulations drawings in Hillingdon.

Next door and across the borough

For the wider picture start at architectural drawings in Hillingdon, or read how the same service works in Hounslow, which borders Hillingdon. The national guide to structural calculations explains Approved Document A and the simple rules behind the figures.