How to prop up floor joists

I want to support my ground floor joists. This is partly to take the weight of a piano and also because the joists do deflect rather a lot anyway when weight is put on them (such as bookcases, cookers etc. The joists in these houses (8"x2" at 18" centres) are, apparently, of rather poor quality and have shrunk and bent in every house in eth development. They were built in 1964 - part of the "Span House" movement. When the floor deflects the walls that are built on it (of block) move and crack which is not nice.
Under the house the gap between the earth to underside of each joist is about 50cm. This is too big for a car scissor jack - I had imagined using a row of scissor jacks. It's too small for an ACROW prop.
One simple solution is to use some wooden 4"x2" props with carpenter's wedges to adjust the height. But I am worried about creating a damp- conducting path up to the joists even if I stand the base on damp proofing membrane.
Does anyone have any other suggestions?
Robert
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House bricks and put a piece of insulation in between the joist and the top brick ie plastic sheeting doubled over.
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George wrote:

Might also be worth putting some noggins/dwangs in between the joists at regular intervals to stop the joists twisting as they get loaded. Our builder did this when sorting out our wonky dining room floor.
Tim
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RobertL wrote:

Robert,
It is not clear to me whether you are talking about temporary or permanent support. I can imagine jacks as a temporary measure (e.g. while you address the other problems) but not never as permanent. But the 4"x2" props sounds more permament as do your worries about damp conducting.
My thoughts shout "piles of concrete blocks" and inserting a DPM as they are built up. But I have no idea of what should be done to the earth beneath them - or whether this is something that requires Building Control involvement.
There may well be ways of reinforcing the joists (e.g. slap an extra piece of timber either side and bolt through) that are much better ideas.
Clearly you should also be considering a) insulation under the floor; b) airflow; c) anything else you might want to do that could/should be done at the same time (e.g. extending electric circuits, checking/insulating/changing central heating pipes, network cables).
--
Rod

Hypothyroidism is a seriously debilitating condition with an insidious
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On Tue, 20 May 2008 07:11:46 -0700 (PDT), RobertL wrote:

The normal solution would ne to build a sleeper wall under the joists and pack up to the underside of the joists with a DPC (a bit of slate is strong in compression). Props will work but you'll have to think of a way of stopping the wedges working loose over time and possibly some means of spreading the load on the soil so they don't sink in.
--
Cheers
Dave.




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Dave Liquorice wrote:

How much does this piano weigh? A Yamaha grand is only 250kgs. Ten bags of sand isn't going to do much to any floor. People are quite heavy these days as well.
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On Tue, 20 May 2008 17:44:37 +0100, stuart noble wrote:

Not if you spread 'em out but pile 'em all in a heap at one side... The OP has already stated the the floor and walls move enough to cause cracking in normal use. 8 x 2 at 18" is, IMHO, a pretty light weight bouncy floor. 10 x 3 at 12 would be a good strong a floor.
--
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Dave.




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Dave Liquorice wrote:

its way bigger than I'm used to, I'm surprised the OP is having problems. A sleeper wall is good, halving the span makes a world of difference, and a dwarf block wall is inherently proof against localised sinking, which may happen.
NT
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On May 21, 12:04am, snipped-for-privacy@care2.com wrote:

I should have added that the span is 5 metres. These houses were notorious when built in teh 1960s. Some of the original bueyrs still live here so they can tell the tale. It is said that the builders used cheap, undried joists imported from scandinavia and within 10 years they had all shrunk by 1/4 inch (i.e. 8" became 7.75". This left a 1/4" gap under all the skirting boats plus curious effects where some internal walls stayed up (where attached to the outder walls) and others sank. in addition to the shrinkage, the joists sagged where internal (block) walls rest on them and they are very springy.
Robert
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RobertL wrote:

Right. Halving span should work nicely. If you want to see for yourself how much difference it makes, get a small bit of scrap wood, support it at its ends and push down to see it deflect, then add a central support and try again. Big difference.
NT
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On Tue, 20 May 2008 07:11:46 -0700 (PDT) RobertL wrote :

You want trench props. http://www.toptower.co.uk/builders_props.htm was the first source thrown up by Google but there may be cheaper options. As you only want to use them the once, a hire shop might sell some used ones.
--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk


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thank you all for your suggestions. Tony was right on the nail though. TRENCH PROPS are exactly what I need. they come in just the right sizes.
It is a permanent installation, but I might decide to move them around if the piano is moved.
Rod's comments about doing otehr work down there at the same time it well taken, but I have good access via a trap door so I can get down there without too much effort.
Many thanks,
Robert
Robert
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In article

The ground floor in my Victorian house was changed from sitting on wood wall plates to 'floating' as part of damp and wood treatment. They built brick piers and columns in the cellar with minimal footings which the joists now rest on - via a cross joist or two. A simple damp proof membrane between brick and wood.
--
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Dear Robert
You do not state what span these joist are at. In a house of that age the oversite "should" have been concrete. If it is not then you need to found whatever you decide to use on a suitable base. I am guessing that with 8" x 2" joists you have a decent sized room say 20' long... so you need to halve the span. This really can only be done sensibly as follows: get yourself some decent sized concrete blocks and bed on the oversite or if that is friable on the ground dug up under the oversite (ie dig down to a solid-ish substrate) and fit the blocks at appropriate centres - say 6' or so accross centre span. This is likely to be say one each side of the room and two or possibly three in the middle. Use the blocks to build up brick piers or if you so wish to use trench acrows - which would be more expensive and prone to rust but much quicker If you do use them - liberally cover in red lead or the equivalent now that has been banned. Surmounting the piers or on top of the trench acrows should be a decent sized wall plate say 4" x 4" which in the case of the acrows can be screwed up but if you accept my and the others' suggestion of a brick pier 9" x 9" - once fully set and strong (7 to 10 days with 3:1 sand cement mortar) then use slate wedges to pack up each plate to the joists. Have a layer of dry pack in between slate lamina so there are no slip planes
Lastsly, screw or nail joists to plate and supplement this with solid strutting further to stiffen the structure Chris G
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On May 20, 11:24pm, snipped-for-privacy@atics.co.uk wrote:

Thank you for this very detailed suggestion Chris. I'll try diggindigging down to see if i find concrete or earth. I already removed a lot of old rubble and 1960s cigarette packets etc. I suspect the underneath of the houses was used as a dump by the builders for old materials and that there may well be a conrete layer a little further down. Clearly a form bedding is needed for whatever solution I adopt.
The span is about 15 feet. ironically the style of housing is called "Span House". Here's an example of the type of house (though mine has a pitched roof on it and looks much nicer).
http://www.propertyfinder.com/cgi-bin/rsearch?a=o&id=502649078
Robert
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On Wed, 21 May 2008 00:51:28 -0700 (PDT), RobertL

Oh dear, Eric Lyons will be turning in his grave. His best know development in partnership with Span was New Ash Green. So it's not just the concrete monstrosities by the likes of the Smithsons in the fifties that have problems :-(
Maris
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On Wed, 21 May 2008 13:19:20 +0100 Maris wrote :

There are similar houses near me (Fieldend, Teddington): my grandfather's house was one of those knocked down to build the estate. The thing that would really put me off is that when you buy such a house you have to sign up for a whole load of rules.
From the rules of Span, Weybridge
"Most two-storey houses have a small area in front, which is part of the estate grounds and not the owners property. Maintenance of these areas is the responsibility of the contractor and no unauthorised planting or sowing is permitted. In particular, the landscape design for the estate specifies that these areas should be planted with non-flowering shrubs and plants. Flowers and bulbs are not, therefore, appropriate or permitted"
though
" as a result of a referendum held in 1990, the Society now permit the replacement of wooden windows and doors by UPVC ones on the condition that only authorised profiles, materials and colours are used. Failure to use specified styles and materials could render the householder liable for violation of the lease agreement leading to enforcement action,"
http://www.weymede.co.uk/resident_society_fact_sheet.htm
It might suit some people, but not me.
--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk


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We have no such rules in our little estate of Span houses. The houses are actually great with huge windows and a real feeing of the outside being connected to the inside.
Robert
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wrote:

I know those conditions sound draconian but these houses were designed for a particular clientele, who were, of course, middle class and very design conscious. I would have been even more draconian and allowed only wooden windows! As for the planting I've always favoured architectural planting as opposed to pretty flowers. It's like comparing the Royal Crescent, bath (and the discipline of its uniformity - the whole is greater than the sum of the parts) with council blocks where the tenants have changed their uniform front doors with 'Carolina' style Magnet (when they were still making joinery) doors. Maris
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yes, I am inclined to agree. As long as peopple clearly understand what they are buying. It seems good to me that a variety of control levels is a good thing.
Although our Span hosues (in Cambridge) have a rather cheap construction they do have nice features such as solid beech flooring and mahogany tongue and groove wall panelling.
Our only rule (completely ignored by all) is a prohibition of externally mounted TV aerials.
Robert
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