Once the 52 pre-stressed concrete floor beams had been delivered, the next job was converting the big stack into the skeleton of our new ground floor. Correctly aligning heavy prefabricated joists is a challenging enough task with a small rectangular extension. But our design is considerably more complex, comprising seven distinct ‘bays’, with each allocated different lengths of concrete floor beams varying from 2,440mm to 4,300mm. The finished floor is calculated to weigh a whopping six tonnes and manoeuvring beams comparable in length to a family car is no mean feat.

To help guide the builders I printed out an enlarged set of floorplan drawings (produced by our supplier TT Concrete). It’s not entirely unknown for the finer details on drawings to get overlooked on site, so I decided to aid the process by colour-coding the various concrete beams with spray paint and marking the positions of the beam ends on the inner leafs of the lower walls which were built up to damp-proof course (DPC) level.

floor beams in place extension project

The floor beams are manoeuvred into place for Ian’s 100m2 extension project

Testing the Drains

Before the foul drain run could be covered up and buried forever in the void under the new floor it needed to be tested for watertightness and the falls checked (the recommended minimum is 1:40). Because we’d opted to retain the existing drain run and modify it with a couple of new branches to serve the new bathrooms (with added stub-stacks, Durgo valves and rodding points) it was still in everyday use. So during the test period the family were banished from the kitchen.

Testing a simple drain run is not a complex operation; all you do is temporarily seal off the underground pipe at an inspection chamber and then run the indoor taps until the sinks start to fill up; then after a five-minute tea break simply check that the levels haven’t dropped. To dam up pipes you can buy special rubber ‘donuts’ which are inserted into the pipe, turning a central wing-nut screw by hand to expand and seal it.

At least that’s the theory; in practice I’d give these a ‘chocolate teapot’ rating for usefulness because they have an annoying tendency to start leaking once under pressure, or even worse, they become firmly lodged — and you’d feel a bit of a plonker confessing to the plumber that you’d bunged up your own drains!

A better tool for the job is a purpose-made inflatable rubber ball which can be attached to a car pump and deflated afterwards. Thankfully the test proved satisfactory with stable water levels. It’s sometimes argued that a small drop in levels can be acceptable because unlike incoming pressurised water supplies, seepage in new waste pipes is likely to ‘self-seal’, thanks to the fat and impurities contained in the wastewater. But this isn’t a risk worth taking, because if pipes aren’t firmly supported at the outset any gaps already present will only tend to get worse over time and leaking drains are a major cause of tree root ingress and subsidence.

The jury is still out on the longer-term performance of modern plastic push-fit drainage components which rely on integral polypropylene seals for watertightness; will prolonged exposure to aggressive cleaning agents and bleaches eventually cause them to break down and leak?

Once satisfactorily tested, the pipes were liberally sprinkled with pea shingle to bed them in place and a weed-suppressant fabric laid over the remaining exposed areas of the oversite, topped with shovel-loads of sand and gravel. Now the floor construction could begin in earnest.

Installing the Floor Beams

blocks placed between floor beams

The spaces between the floor beams are filled in with blocks

The advent of affordable mini-diggers for use on small sites has been a real game-changer, helping to mechanise a wide range of laborious and dangerous tasks, including transporting heavy materials. But shifting cumbersome concrete beams around the site would surely require the use of a crane?

As I was mulling over this conundrum, the builders set to work lifting the concrete beams into place with the aid of strong fabric straps hung from a wide digger bucket. Working as a small team, Daniel (the son of our builder, Godfrey), veteran brickie James and labourer Eddie methodically navigated each beam onto the sub walls.

It’s essential to space the beams accurately at defined centres, some doubled up to support internal walls (which is arguably a case of over-engineering for internal stud partition walls), and to achieve this level of precision positioning it’s hard to beat ‘grunt power’.

For designers and builders alike this is one of those moments of truth when you discover whether the beam lengths were correctly specified and the lower walls built in the right place! The beams are designed to rest on the 100mm-wide inner leaf; if they’re a bit too long they can overshoot slightly into the cavity and will need to be trimmed back with an angle grinder, which, although not ideal, is a far better outcome than discovering they’re too short! So I was pleased to see things taking shape in accordance with the drawings.

Once all the floor joists are correctly aligned, the next stage, infilling the spaces between them by dropping in concrete floor blocks, should be plain sailing.

Some more advanced floor systems on the market substitute standard concrete floor blocks with special thick insulation slabs, allowing you to significantly reduce the thickness of the internal floor insulation later on, but such wizardry comes with a sizeable price tag.

The Inspection

The next day the building control officer appeared on schedule. It was a different guy this time, standing in for our usual contact, Andrew, who was on holiday. He fired off numerous questions about drains, surface water discharge and ventilation to the sub-floor. He queried the level of the beam and block floor structure, so I explained this was set slightly lower than usual so that the finished floor level could match that of the existing house while accommodating about 180mm depth of floor insulation and screed (to comply with Part L).

periscope vent fitted into brickwork

With the go ahead from the inspector, Ian was able to crack on with jobs including fitting the periscope vents

The discrepancy came about because the developers who built our house in 2001 completely omitted the floor insulation. I also explained that the space around the finished lower walls would be filled with gravel (a ‘French drain’) as an aid to moisture dispersal, and the generously sized DPC would overlap with the floor membrane, in effect ‘tanking’ the lower walls.

The officer seemed satisfied so I asked when they wanted to do the next visit: “When the steels are in,” came the reply. This was the green light we’d been waiting for to crack on and finish the job, grouting the floor, and fitting periscope vents, air bricks, and slip bricks between the joist ends.

After living for so long with a moat of mud around our home, it felt great to see a clean and dry expanse of concrete decking. That evening, with a glass of wine in hand, Ewa and I could toast the world’s most expensive patio! 

  • Post a comment
    You must be logged in to comment. Log in