I have a downstairs space, about 8m x 4.5m. The floor above it is constructed from 4"x2" timbers (17" centres) hung from the walls and a central oak beam. The oak beam (dating from about 1650) spans the 8.5m and is supported at half way. The ceiling is just the upstairs floorboards.

This arrangement is unsatisfactory for several reasons, the most annoying of which is the way the 4x2s are hung off the oak beam - it really is a mess! It's also going to be quite difficult to run services as the joists are so shallow, the sound insulation is non existent, and the floor is a tad bouncy. I had resigned myself to ripping the whole lot out and putting in a new floor. I don't want to box in the beam as it's one of the few period features I have, and there's not much headroom as it is.

Then it occurred to me that I could just put another floor on top of the existing one. I could mirror all the joists and glue and screw both floors together. This would allow me to improve the aesthetics of the beam/joist join. Then I would get a less bouncy floor, have room to run services and save lots of money and time, and there would be much less waste. If the wife objects to the floorboard look of the ceiling (which I think would look quite nice painted white), I could stick something onto it even if it's only paper or do the usual plasterboard thing (eventually).

Does this seem a sensible idea, or is there something I've not thought of?

Thanks for the reassurance and advice. Due to the bizarre nature of the floor construction, the top of the present joists are an inch or so above the top of the oak beam. I was intending to put 4 6"x2" timbers above the beam (it's about 8" wide) and hang the new (and old) joists off them. Due to some internal alterations downstairs, I'll also be giving the beam a bit more support from below. I was hoping that decent sized screws every 6" or so plus glue should hold everything together.

Regards

Tom

You do not say how deep the oak beam is. For a beam its deflection is directly inversely proportional to its width ( 2 X wide, 1/2 X deflection). For the depth the deflection is inversely proportional to the cube of the depth( 2 X deep, 1/8 X deflection). The material right along the centre of a beam (the neutral axis) does not contribute to the deflection at all, it just keeps the top and bottom apart.
If you go ahead, I would use a ceiling prop to actually raise the oak beam's centre by a few mms. This is to ensure that when you put your bits of 6 X 2 upstairs and cause the beam to bend you don't go ahead and fix this bend in permenantly. Don't use PVA glue, use a proper structural glue. I would use screws to fix every thing with proper pilot holes. Nailing into this oak beam will be quite a task!
Frank

Yes chuckey, thats exactly what I was gonna say. A new floor on top wont sort out bouncing, but a bit more support from below would help. No, oak doesnt like steel, brass is better, a few coach bolts for the joists are a better idea than glue??

The ancient funcrusher has been there – as he has most places technical – though I deny rumours that I was around in 1650.

Firstly, check the condition of your timbers. Most oak dating from 1650 has disappeared through providing nutrition for the deathwatch beetle, especially in southern England. The holes are larger than woodworm, the creature is more voracious, and often hollows out oak beams! Check especially the built-in ends. You mention that the cross joists are only 4 x 2 and built in. Are they oak? Usually in ancient houses they are 4x4 oak. Often in old buildings they are supported on ‘half beams’ along the walls. If not, they may be supported on a horizontal timber wall plate embedded in the walls. Whatever the construction, the ends will quite likely be eaten/rotten/insecure – which would account for the bouncy floor, at least in part.

It is quite likely that the floor above is badly out of alignment ie slopes all over the place – and quite likely the spine beam has sagged. Check this – easily with a laser level, or otherwise by traditional means

You state that your spine beam is supported mid-span, so the functioning spans are only 4.25m (14ft) which is quite modest, especially with a room width of only 4.5m if the beam is deep enough. Oak beams were often 12x 12 spanning 20ft square rooms.

Obviously your actions will be influenced by the condition of the timbers, but I would not recommend the action you propose. If you have plenty of headroom in the floor above,(and presumably you have as you propose to raise it by 6+ inches) then I would put in a complete new floor above and retain your existing as an historic ceiling. If the upper floor loadings are normal bedrooms, then 9x2 transverse joists on 400 centres would do; or you could put in 4 transverse steel beams and run 7x2 joists longways; if need be sitting the joist ends into the web to save height. Sit the ends of the steel beams 6” into the wall resting on 12 x8 x4 concrete pads (unless the wall is in excellent stone )

If you don’t have headroom and are really stuck, you could turn your oak beam into a flitch beam: a dodge used in olden times. This involves inserting a steel plate in a vertical slot you have carefully sawn down the centre of the beam – preserve the appearance by making the slot an inch shallower than the beam. Contact me for further details if interested.

I've known it done with a chain saw but other saw rigs are possible