Easiest way to do so that I have found is to lay additional strengthening joists on top of the existing ones. These timbers are glued and screwed onto the existing joists. The gluing does make a big difference, so don't skip this stage. You will get away with PVA but can use more exotic glues if you wish. Be liberal. Long screws to go straight through the new joist and set into the original joist are available for decking supplies. I have used this trick in a number of different instances in my house with considerable success. If you add 2 inches to a 4 * 2 joist, you will NOT end up with the same strength as a 6 * 2 joist, but it will not be much less. If in doubt, add more, you do not want plaster falling off your ceilings, or a joist collapsing in the worst instance.
If you glue and screw 3 * 2 on top of 4 * 2, you will get approximately the strength (resistance to bending) of a 6 * 2 joist. Now I do not know what your spans are, and hence what joists you would require, however you can check the required size from here: http://www.home-extension.co.uk/tech2.html So, work out what size timbers you have already (probably 4 * 2 inch), calculate what you need (from the measured span and joist spacing from your loft), then add the calculated difference plus an inch on top to each joist.
I get you. So the problems with lofts/ceilings are the joists actually bending? So increasing the height reduces it flexing? Not sure exactly what the spans are, I've just screwed in a load of loft legs & loft panels on those (due to the insulation thickness) just so I can store bits up there until I can do something better to it in a couple years.
you do realise that when you have laid the boards on the ceiling beams they will place excessive loading on the ceiling beams that are only meant for boarding and plaster you'll be transferring, the weight of whatever is on the table through the legs and concentrating it to points on the beams. I believe your best option is to install a floor over the existing
Which is the whole point of stiffening the joists; to make them capable of holding people and stuff, rather than just a ceiling. You can turn an attic into a useable room, and relatively easily, but yes, on a modern house, you WILL have to add timber, because they won't have used 8 * 2's, just the minimum for the job.
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