Terraced house loft conversion floor joists

30 Apr 2011
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United Kingdom
When reading this bare in mind I'm on about a typical 2 bed mid terrace - here goes.... Is it acceptable to fix joist hangers to the walls to either side of the house for the floor joists. The current ceiling joists run front to back with a load bearing wall approx half way. To make the floor joists run the same direction as the ceiling joists would, I'm told probably require steel beams under the eaves with the floor joist resting on these and the load bearing wall.
Surely if each floor joist (across the house) is attached to hangers the load would be spread more evenly across the walls, steel beams would have fewer points of contact and be a nightmare to install. The max span across is 4.2m meaning, if I remember right 9x2 joists will be needed. Head room is currently 2.6m so this depth of joist shouldn't be a problem with the floor joists entirely above the ceiling joists.
Any advice would be appreciated thanks.
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It is pretty normal to do a lofty as has been suggested to you, 2 steels spanning from side to side around 1m (or so) from the eaves to support the purlins. Often the purlins take some support from the structure of the roof you wish to remove and this needs to be replaced with the steels, there will also be more loading on the existing rafters from the insulation/plasterboard so sometimes these need to be beefed up and utilising that purlin can help to minimize the upgrade required. Then span between these with your joists taking support from a structural wall if available or sometimes another steel. The steels can (if the existing house lends itself to this method) be slid into the roof through a hole made in the tiles and then manoeuvred into position. However far more common than that is that the steels arrive in sections of 3 or 4 members which are then bolted together in the loft.

As you rightly say the joists can sit between the existing ceiling joists which can help to maximize headroom, the joists may be smaller than spanning from side to side although we don't know what the spans would actually be. Also bear in mind you will likely have to add typically around 120mm to the underside of the existing rafter line for additional insulation and plasterboard. Also bear in mind that adding additional height to the floor level can make the stairs ever more difficult to get in as you may end up adding another step or two. Stairs and their associated fire door at the top or bottom can be a nightmare on loftys.

Having said that, 9x2 are about right for a span of 4.2m at 400 centres. It may also be difficult getting these up into the loft and that problem may occur if you were installing them from side to side anyway.

Your structural engineer (and you will need one to get through Building Regs) will be able to sort you out and justify if and why you need steels (which you may well do).

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