“One of the builders came to the door this morning holding something really disgusting,” said my wife as I came in from work. Curiosity having got the better of me, I popped outside to quiz Godfrey the builder, who was busy toiling over the drainage run.

It transpired that the grizzly item in question had already been discarded, but with scarcely concealed glee he thrust his camera phone in my direction.

What at first sight appeared to be an image of a huge white blob was, upon closer scrutiny, identifiable as a stupendous fatberg he’d extracted from the kitchen waste pipe.

This was probably the root cause of a longstanding niggle with our waste plumbing; despite our house only being a few years older than its NHBC warranty period, the kitchen sink suffered irritating bouts of bad tempered gurgling and retarded discharge, requiring occasional rodding, not helped by the paucity of inspection chambers. Clearly the previous owners of our humble abode had enjoyed a richly calorific diet!

drainage run to be modified

Ian’s plan is to modify the existing drainage run to serve his new extension

The plan for the extension drainage was to make use of the existing pipe run, modified with new branches serving the extension bathrooms, with the addition of some internally accessible rodding points, but remaining in situ under the suspended ground floor structure.

To improve access we planned to install a large new 450mm diameter inspection chamber outside the front wall to intercept the pipes before they joined those serving the main house and out to the main sewer in the road.

Fortunately the underground drainage system served no other properties and being located entirely within our boundary meant there was no requirement for costly CCTV scans, building-over agreements and form-filling bureaucracy.

The Next Phase

The groundwork and foundation concreting had all been completed the previous week in the first three days (along with the new driveway opening). This was rapid progress indeed. But on the morning of day four an eerie silence descended upon the site — in stark contrast to the frenetic activity of the previous 72 hours.

Sites tend to be one extreme or the other — either all guns blazing or models of monastic tranquillity. Just when I was wondering what had become of the workforce, Godfrey phoned to say he’d be back on site next Monday with a team of brickies to start the next stage constructing the lower walls.

This short interlude allowed time for the concrete to fully set and was a useful opportunity to check the programmed delivery dates for materials. It also gave me chance to produce additional 1:50 working drawings showing details of the drain run and the positions of the air bricks in the lower walls. One of the advantages of learning how to produce your own CAD drawings is that it makes it relatively quick and easy to print out modified versions with detailed guidance for the builders.

Delivery Schedule

brick delivery on site

Bricks are delivered and moved into position by the digger on site

It’s essential to keep your eye firmly on the ball when ordering building materials or you can quickly find yourself hemmed in behind mountains of bulky pallets, a victim of inconveniently bunched up deliveries.

I was trying to coordinate deliveries so they’d be used up promptly rather than sitting around for ages blocking everyone’s access — a logistical concept known as ‘Just In Time’ in the world of manufacturing.

This of course is easier said than done, with some suppliers more professional than others, and you run the risk that late deliveries could result in trades starved of materials sitting around doing nothing.

We already had sufficient supplies on site of concrete blocks and semi-engineering bricks for the imminent foundation wall construction, but the huge quantities of bricklaying mortar we’d be consuming clearly demanded something more than a few bags from B&Q. So I opened an account with local merchant AW Mobbs to get bulk deliveries of loose sand and gravel at trade prices.

The following day, seven tonnes of soft building sand duly arrived on a tipper lorry which reversed nimbly up the drive, cascading the load onto a fabric sheet laid on the grass. Covers were draped over the mini mountain of sand before the local feline population could eye it up as a giant litter tray.

Marking Out

The next task was to mark the wall lines on the surface of concrete trenchfill foundations, in preparation for the lower wall construction. This was accomplished the old fashioned way, with lengths of bricklayers’ luminous string lines pinned into the surface of the concrete using a surveyor’s tape and a builder’s square.

wall lines marked up foundations concrete

The wall lines were marked up on the concrete using a
builder’s square and tape

Transposing the measurements from the drawings was a relatively straightforward process with no real need to use more exotic hi-tech kit. However one area where technology can now achieve markedly quicker and more accurate results is at the design stage when measuring existing buildings to get dimensions for the elevation and plan drawings. Here, achieving accuracy can be challenging due to height limitations, awkward shapes, irregularly proportioned additions and walls out of true.

Where walls are rendered (as with our property), readings can be especially inconsistent, with the resulting measurements out by a centimetre or two.

Conventional practice is to use tape measures (where accessible), brick counts and other rules of thumb to calculate dimensions for more awkward areas, backed up by desk research piecing together any existing 2D elevation drawings against OS plans.

To improve accuracy, professional designers are increasingly employing long-range laser scanners such as the Leica ScanStation or Faro Focus to get digital readings about the size and shape of buildings. These highly accurate devices capture 3D geometry using a laser to measure the distance between the scanner and the structure being surveyed, and the ‘point cloud’ produced is then traced over to form the model. Unfortunately such wizardry comes with a hefty price tag of £20,000 or more. Maybe we’ll hire one for the next project.

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