Building the Floor Structure of my Large Extension

It’s been a busy few weeks working on our extension. With the upper main walls now rapidly taking shape, for the first time you can really get a feel for its size — something that everyone who visits seems obliged to comment on. “You could fit a couple of terraced houses in here,” observed Godfrey, our builder.

The previous week I’d missed the building control site visit but the builders seemed to think it all went OK. To be on the safe side I phoned Andrew, our building control officer, and he raised no concerns but remarked on the impressive size of the twin 6.6m long steel beams, adding: “Basically what you’re doing is putting a new house on the side of your existing property.” He also informed me his next visit would occur once the roof structure is in place.

Earlier in the month our build programme had nearly been sabotaged because the firm of scaffolders booked by Godfrey did a ‘no show’, wasting everybody’s time. But ever resourceful, the builders temporarily deployed a series of scaffold planks on trestles as an improvised first lift, having earlier backfilled the ground around the main walls where it had been excavated for the rainwater pipework.

The following Saturday the scaffolders’ truck finally made an appearance, the first lift erected single-handedly by the owner of the company because his labourers hadn’t shown up for work.

Coordinating Deliveries

I’ve had to devote a surprising amount of time to ordering materials and coordinating deliveries, so I’m beginning to appreciate the extent of the logistical work that main contractors normally carry out.

Running a building project at close quarters has also provided an insight into the problems faced by delivery lorry drivers. Accurately offloading all manner of heavy, bulky materials via truck-mounted Hiab cranes into awkward, often wet and muddy sites is a hugely underrated skill that we take for granted.

Ceiling Heights

Normally, setting the ceiling heights is a straightforward part of the design, typically ranging from about 2.3 to 2.5m from finished floor level to ceiling. But on our planning drawings the height of the front wall was a bit short because the extension roof had to be set 200mm lower than the main house roof.

Without a dormer poking up through the roof to buy some extra window height, I was worried that the window on the first floor might appear comically low as you walk into the room.

In most rooms the heads of the windows align roughly with the tops of the internal doors, although in period cottages you can sometimes get away with quirky bedroom windows set a little lower. But in our new extension anything lower than eye level is going to look a trifle odd to say the least.

Fortunately, this being a self-contained annexe extension, the upstairs floor could be positioned a fair bit lower than the existing landing in the main house, so adopting a reduced 2,300mm ceiling height downstairs has allowed us to gain a bit more headroom in bedroom above.

Choosing Floor Joists 

For the first floor structure I’d originally planned on using engineered timber I-joists. These have been a standard component in mainstream housing developments for more than a decade because they’re far less prone to shrinkage problems and squeaks (although creaking can also be caused by poorly-fitted joist hangers, missing noggins or cheap chipboard flooring).

Although I-joists are dearer than traditional softwood, they’re quicker to fit with consequent savings in labour. But with the manufacturers quoting two to three week lead-in times following approval of drawings, this was looking a bit too tight for comfort.

After running the options past our structural engineer I decided on balance it would be better to stick with traditional treated softwood which offers greater versatility and flexibility for cutting and fitting on site.

The Floor Structure

I handed Colin, our chippie, a freshly printed colour drawing depicting the required positions of all the joists and the stair opening, including some to be doubled under internal walls, and he promptly set to work.

On the question of how best to support joist ends, I was under the misapprehension that they need to be hung from metal joist hangers rather than being slotted into masonry walls or supported on ledges in the time-honoured manner.

But although this holds true for new homes where air leakage and sound transmission are more of an issue, when it comes to extensions apparently it’s still acceptable to build them into the wall, although building control will pull chippies up where any joists extend into (potentially damp) cavities.

According to Howarth Timber, a leading timber supplier I spoke to, it’s more common in the north to build joists into walls, whereas down south it seems we have a greater fondness for hangers. Another detail that building control often check is that all the pre-formed nail holes in timber-to-timber joist hangers actually have nails in them because, as they like to remind us: “They’re there for a reason”. 

Colin polished off the floor structure in a couple of days, taking care to position the noggins where possible so they corresponded to standard plasterboard dimensions, making life easier for the following trades. A few hefty sheets of 18mm OSB were then carefully placed over the floor deck to work from internally.

This is one of those stages in a build when you can stand back and take pleasure in witnessing your drawings coming to life. That evening I rewarded myself with a leisurely stroll around the upstairs floor and tried to picture how the finished rooms would look.