re-enforcing a wooden floor

[chrisfitter]

I’m renovating an old house in France and have a rather too flexible 7m floor in a bedroom. It seems safe enough but moves when you walk across the room. The beams are exposed in the kitchen below so I don’t really want to add anything to them and there is no handy place to put any vertical support, so I was thinking of adding another layer of 23mm floorboards directly onto it to stiffen it up as well as improving sound insulation a little. Is this wise?

 

[DAZB]

Could you give a bit more detail, such as the size of the beams ?
I would think that adding another skin of flooring will just add to the problem because of the extra weight involved but the beam size will tell us more.

 

[chrisfitter]

Hi- and thank you for taking an interest The beams are220mm x 70mm and span a 7m length and are about 60cm apart.The problem being that they flex as you walk across, I realise that more (or stronger) beams would be the real solution. However, as the movement is not too alarming and I am prepared to accept just a slight improvement, I feel that another layer of boards if well enough attached would add a little stiffness. I'm worried though that the two types and ages of wood may expand and contract differently and cause trouble. What do you think?[/quote]

 

[DAZB]

I would definately not put any more weight on that floor as it stands now.
I have checked out the Span guides within the Building Regs and the safe allowable spans for a joist of 75 X 220 at 600 centres are between 4.07m and a maximum of 4.93m depending on the grade of timber used and the deadweight loading of the floor. I can see your point about trying to add stiffness but is it worth the risk of it coming down ?

 

[David P]

there is one possibility - and its a long shot - but ill give it anyway.

structuraly the beams are dependant upon their shape - the top surface is in compression and the bottom surface is in tension - that is why a wider beam is more rigid, it has little to do with the material mass.

a classic beam only really needs to comprise of a top thats good in compression a base thats good in tension and a rigid fillet that joins them - this is why RSJs are I shaped.

there are two ways you could go with this:

1 add a tension strengthener to the bottom of the beams - ie a tightened tension cable fixed along the beam at points (could be hidden in a groove).
it should be tightened as much as physicaly possible.

2 add a fillet - such as high grade plywood to one or both sides of the beams - fixed along the top and bottom. with this you could pre-stress by attaching them when the beams were raised slightly with acro-jacks.

adding a top surface (floorboards) isn't much good as it wont add much to the compressive strength of the existing wooden floor, that is already strong in compression. It will as mentioned earlier add a great deal to the self load of the structure and could well be dangerous.

another option would be to interspace some beams with a similar appearance - in effect reducing the weight each beam supports - be sure to fix the floorboards to them as together they make a closed structure (stronger).

 

[chrisfitter]

Thank you so much for your helpful and very interesting thoughts. I particularly like the plywood option although I'm a little concerned that the fact that it's a material which only comes in relatively short lengths may make the method ineffective. Perhaps it would help to attach two layers overlapped? Glued, screwed and occasionally bolted right through on both sides of perhaps even just one pre-stressed beam would make for a pretty solid structure in the middle of the room, where the flexibility is at its worst. If this proved not to be enough, I could continue the treatment with other beams until I was happy.
When you say high grade plywood are we talking about marine ply or is there something even better?

 

[David P]

damn I just spent about an hour writing a reply and my connection dropped just before send - so here goes attempt no 2.

the short lengths arent really much of a problem - you can visualise it like this - the current beams are sagging evenly throughout their length, any added fillets will in effect be rigid sections that resist sagging except where they join - so the span begins to act as a series of hinged panels - now assuming the panels are firmly fixed to the existing beam the stress occurs at the joins (over a very short distance perhaps 2mm) this has the effect of making it more like a shear stress, which wood handles very well.

having said that, yes it would be nice to fix them all together - but adding an overlap would be doing a job that the existing beam could do just as well. It would be effort best spent finding the perfect fixing.

mechanical fixings (nails, bolts) tend to weaken beams where they peirce.
that is why you should only ever bolt a beam at its (horizontal) dead centre - which is not subject to any stress. the problem here is that you need to fix your fillets to the beam at the top and bottom or you arent adding ay strength.

I would suggest that you use one of the new polyurethane wood glues. they are incredibly strong and longlasting, being developed for boatbuilding. they foam slightly as they set so that you get a constant join even where the wood is slightly out of contact. unlike pva based adhesives - they can be got in a variety of setting times and use the moisture in the wood to speed the setting - they also set underwater for what its worth.

the only fixings i would use would be some nails as tacks to hold it in situ (along the centres) and lots of clamps.

another option is that you can buy constructed beams (my local builder merchant Travis Perkins stocks them) - in an I section with pine top and base joined with a thin fillet - they are incredibly light and dont cost too much.
as for what sort of ply - well the best quality that you can get is birch ply, but i think it would be overkill to use it - shutterply is prone to having breaks in the laminate. so perhaps a marine ply would be alright. although I have heard it said that the best board for dimensional stability along its plane is stirling board - that stuff that is made from very large chips pressed into a board.
this is what the comercial constructed beams are made from too.

finally a link to the glue I mentioned above - there are lots of different manufacturers but Bison where the first to introduce it in the uk - I just cant remember the name of the one I use so the link is to bison.


PU MAX TIMBER THIXOTROPIC


PU MAX TIMBER LIQUID

ther are others including bison risorcin but i cant find spec on the website.
here

 

[jeds]

chrisfitter, as has been said above, adding ply to the top of the floor will not help. It will increase the load and make the floor more bouncy.

You would actually be better off adding a layer of 18mm ply to the underside of the floor - not that I'm recommending that but it would be better.

Have you considered adding a cross beam to the underside? i.e. one that crosses the mid-span of the floor joists. Or you could add two to make it even stiffer and balance the look of the ceiling.

If you have the headroom below this would be a cost effective way of stiffening the floor.

 

[chrisfitter]

I'm overwhelmed by the generocity of your replies - thank you.
And yes; I am considering the cross beam option. Should I use an RSJ or would a 220mm wooden beam do the trick? The span in that direction would be 5.1m and I would very much prefer to put just one in as I suppose I'll have to build a brick pier at one end to support the beam where it meets a partition wall (and anyway I'm convinced one will be enough as the problem is not too alarming). If I use an RSJ what dimensions would it have to be and how much is it going to weigh? I have plenty of headroom by the way; 2.7m

 

[David P]

there is a chart appended to the british building regs that gives you min sizes for particular spans - but they are for when you are using the beams as joists spaced at regular distances.

i seem to remember that the regs also give you a typical domestic floor load weight that you can use to estimate the weight that the beam will need to support. it is not a difficult calculation - it is probably on the web somewhere try 'beam calculation' as a search term (there is also some good shareware called DrBeam for the mac - im sure more exist for pc).

I would stick with wood for the job. it is lighter to lift and fix plus will not force you to box it in and spoil the existing appearance. see if you can source laminated beams in france - they are popular on the continent and are a delight to work with, look good too

 

[jeds]

chrisfitter, Building Regs won't be help much here as it doesn't have a category for supporting beams. A timber beam will be more than adequate for what you want. Intuitively I would say bolt 2 beams together of approx 50x225. If you want to be absolutely certain post again and I will happily do a rough calculation for you.

As you say, you are only 'adding to'. You can't go wrong really.

I wouldn't bother with a steel beam - very heavy to handle on site and a pain if you need to trim 20mm off!

When you fit this beam, look along the length and make sure the camber is facing up. i.e. so when you jack the ends into place you will automatically put the floor in slight tension. With me?

 

[David P]

found this link to glulam (glued laminate) beams that might be of some help.

click the image

 

[nigelsz]

hi just done a loft conversion ...with building regs approval
i had to batten to top of joists to add strenth had to glue and srew at 100mm centres across every joist and add interspaced joists . think the guy who reccomended interspaced joists in on the right track / architects use structural engineers to do the calcs(i think thats what they call them ) cost about 100 pounds on top of the architects fees ( in northern irealnd ) we are cheap compared to uk
if you want to talk more i can give you the calcs on my plans if you dont want to go down the road of architects .