Loft Conversion

DIpY Loft Conversion*
(*Do It Partly Yourself!)

Don't Flitch

The first major job is strengthening the floor. Although the existing loft floor is comprised of 4x2" timber (which is quite good for a loft by modern standards), this is nowhere near strong enough for a new floor. Hence the need to play a game of inserting lots of 8" deep beams into a space while dodging all the existing beams.

Some of the longer joists also need to carry point loads from other joists or "stringers" that meet them. As a result, a simple timber beam is not always going to cope without deflecting too far (too meet limits imposed by building regulations), or being dangerously overloaded.

Hence in these situations "Flitch" beams are used. These are made with a layer of steel plate sandwiched between two timber beams, and the whole lot bolted together every 600mm.

The plans called for a 6mm thick plate - the steel supplier gave us a free upgrade to 8mm (fine except it makes the things a bit more difficult to manhandle). They also take care of drilling the holes and painting them with a (red) passivating paint.

Most of the windows in the house are of a span of under 2m with a nice concrete lintel over them. The main bay window at the front however is much wider. Rather than pull apart the rooms below to establish the construction of the lintel over these windows we decided to use another flitch beam as a stringer to carry the front floor joists. This first photo shows it being manoeuvred into position. It is deliberately placed over the existing loft floor joists since these are being used in a cantilever to hold up the front bonnet roof over the bay. The front floor beams will be hung from this flitch beam on long joist hangers.

The beam is supported at one end on the longitudinal flitch beam, and at the other in a heavy shoe rawl bolted to the party wall.

Note the original loft floor had "tie" beams that ran across the width of the loft to keep all the other floor joists ridged and add extra support for the weight of the ceilings below. Each time we crossed one of these, we needed to batten the beam to the purlins above before we cut it. This is to ensure that we do not introduce any more sag into the ceilings below. Once the joist was in place "noggins" could be added between the new joists to hold the old floor joists instead.

The new floor beams sit on a 1" thick wood plate laid on the top of the supporting walls. In this way the new beams sit a little above the existing ceiling level and do not come into contact with it. This way there is no danger of them causing cracks or damage to the ceilings below when they are loaded.

It is interesting to see however just how far the ceilings have sagged over the years just from their own weight (plus that of any stuff stored on them). The 1" gap at the end can be as much as 4" in the centre of the span. Not that this is actually noticeable from the rooms below.

Getting close to done.

The new floor joists are now in place. This picture also shows the noggins that replace the original tie beam that steadied and supported the original ceiling joists. These are nailed between the new joists and also onto the old ceiling joists. The only downside of this is that you now couple the new floor to the old ceiling slightly which will increase noise transfer a little. (in reality the effect is small and noise transfer is far less between 2nd and 1st floors than it is between 1st and ground).

The ends of the beams poke out through the rear of the roof through a strip of tiles we have removed. Ultimately this whole rear roof section will go and be replaced with a large dormer.

In total we only had to remove one of the existing floor joists close to the party wall so we could get the main rear flitch beam into its proper place. This was a delicate operation of chain sawing the original beam into short sections (without going through the plasterboard ceiling that is nailed to the underside of it), and then pulling up each section so that it rips the plaster board nails through the back of the board, hopefully leaving the finish to the ceiling below undamaged. Remarkably we only damaged one small section of ceiling when we hit a nail with the beast, and a 50p sized bit of artex and skim decided to jump of the ceiling!