Bouncing loft floor

[Rob]

Yeah, unfortunately the bounce can be felt all over. Its not too bad, just if you jump around you can feel a shudder going through the room. But reading peoples replies leads me to believe that a significant amount of flex is common in long wooden beams?
I didnt think about the risk to the ceiling plaster by screwing the joist together, you could have saved me a messy mistake there! It is lathe and plaster so will this make the risk of cracking greater or less?

 

[Stephen]

Without going into specific technical detail there is a simple method to reduce bounce in a floor and it has been mentioned already.

Take ny existing board up, replace with 18mm ply (the larger the sheets you can handle the better)and screw down to the joists using fully threaded scews that go at least 2-inches into the joist and use a screw every 6-inches.

This changes the way the floor performs as it creates a diaphragm structure. Once finished you can re-lay board on it if you wish or simply carpet.

I came across this solution some years ago and was told by engineer that they reckoned it doubled the strength of the floor.

A simpler solution I once used in an Edwardian house was to leave the boards in place and change all nails to fully threaded screws. This took all the bounce out of the floor.

The reason it works is that nails work by friction and usually the joists and board are separat elements. By using fully threaded screws you are positively fixing the board to the joist and creating a series of T-beams linked to form a diaphragm.

Using glue (as mentioned in one post) increases the general strength, but makes the work irreversible (a b****r if you later get a plumbing leak, etc).

Hope this helps.

 

[Penners]

the bounce can be felt all over. Its not too bad, just if you jump around you can feel a shudder going through the room.

Rob - judging from that description, it doesn't sound to me as though you've got too much to worry about.

As has already been mentioned, wood flexes - and it can flex a lot before it breaks. There's a considerable margin between a "shudder when you jump around" to a pile of debris in the room below.

If I were in your shoes, I think I would follow Stephen's recommendation in creating a diaghram floor. That will give you a nice level (if not necessarily flat) floor, and add considerable strength the structure. It certainly sounds a great deal easier than all those noggins and metal straps.

I do also agree with Frank that screwing the overlapping ends of the joists together won't achieve any significant increase in strength, and could cause considerable damage your lath & plaster ceiling below, by dragging the joists and laths sideways. The membrane idea may lift some of the laths a bit as the joists align themselves on the underside of the ply, but unless there is a lot of misalignment joist-to-joist, this shouldn't cause much more than a hairline crack or two in the ceiling plaster. Easy to rectify.

 

[Rob]

hello.

cheers for the positive replies, Im feeling a lot more relaxed about the whole thing now.
I am going to screw large T&G boards accross the whole attic floor, but due to the unevenness of the floor do i need to lay thin wood strips accross the existing beams to bring them all to a standard height before fitting the boards? Will this make it stronger by ensuring I am not adding and more stress to the rafters by pulling them up to meet the boards, or weaken the structure as the floor boards are not in direct contact with the joists.

 

[Penners]

When you talk about large T&G boards, do you mean flooring chipboard? If so, I'm not sure that this will have such a beneficial effect as Stephen's suggested plywood. Chipboard just doesn't have the structural strength of ply.

Hopefully Stephen will add his more expert view.

However, whatever material you end up laying as a floor, I think it will be to the benefit of your ceiling below if you do try and level up the top surfaces of the joists with firring strips.

I'm not sure that this will add a great deal in terms of the strength of the structure, but it will reduce or eliminate (depending on your accuracy!) any movement of the joists, and hence of the underneath ceiling lath & plaster.

 

[Rob]

I was planning on using chipboard as this is T&G on all four sides and would create stronger joints between boards than plywood. unfortunately not only are my joist nov evenly spaced, they arent parallel with each other so joining boards on the joists would be extremely difficult.
I have a router and the necessary bits, is it possible to route plywood to give a T&G effect?

 

[Stephen]

If the sag is not too much I am not sure you wll put too much stress on the joists by leaving the sag. Otherwise you could use firing pieces to level the floor slightly, but the screw must go into the joist a good depth to provide the positive fixing.

I prefer plywood and if screwing completely the tongue and groove edges do not make much difference. You could put nogging timbers in between the joists under the lines of the board edges to provide a better fixing. I would not bother routing the edges, etc.

If you use chipboard T&G I think you will need to glue in place as well as screw.

I think if you simply screw large sheets of plywood down you will be pleasantly surprised at how much stiffer the floor becomes.

 

[Penners]

If you use chipboard T&G I think you will need to glue in place as well as screw.

And bear in mind the warning above about this. It's hunky-dory until the day comes when you want to lift a board!

 

[Gervase]

How true! I had a job recently which involved replacing a floor which had been so fixed. I might have muttered a few rude words under my breath...

 

[Penners]

I find that almost impossible to believe.

Almost.

 

[Rob]

thanks for your resdponses.
One last question, Is the increase in strength provided by using plywood instead of chipboard worth the additional cost? Ply sheets are almost 6x the price of equivelant chipboard.

 

[Penners]

Definitely well worth it. Chipboard would be OK if you just wanted to store stuff up there, but considering what you're trying to achieve for a habitable room I wouldn't consider it. Plywood will give you a proper job.

 

[chuckey]

Ply is a lot stiffer then chipboard flooring. In your case if you go for chipbooard, go for the tanalised 22mm not 18. I seem to have forgotten, what is your joist spacing? it looks big wide in the pics.

The boards must be screwed to a flat surface so each joist must be packed out. I find that really old and degraded plywood delaminates easily and offers packing in 1/16" increments.

Noggins should be fitted under joints, but its very time consuming/difficult. Your panels will be fitted at right angles to the run of the joists with the joints staggered. This means that at some point intermediate to a pair of joists a joint must be made. If the joint is 2" of a joist then some pieces of wood can be screwed to the joist. If the joint is in the middle of a joist space, then theorecticaly 3 noggins are required, one to support each side and one to carry the centre fixing.

A nice way of making noggins is to cut the main noggin 1" shorter then the space, nail a piece of 1/2" ply on each end so the thing looks like a capital I. Drill the "ears" and fix to joists with screws. When wielding a hammer to put nails through the side of a 4" joist or tosh nailing it is difficult to put a nail lower then 2" above the ceiling.

As to glue or not, the amount of effort to lift panel , even if you have a "magic" strip to clear the skirting board, is tremendous. Every sheet has to be labelled and stacked. Gluing adds a significant amount of strength. If access is required just cut out your panel and glue and screw butt straps, before screwing you panel back. MARK the top of the boards to show pipe runs/wiring so you can find the right spot first time.

Around here every chipboard floor I see is badly done!!!

Frank

 

[NT]

In your situ I'd go with nice and thick T&G chipboard. Its plenty tough enough for your job and cheap compared to ply, but more importantly its T&G on all 4 sides, unlike sheet ply. This makes a real difference when you've got joints that arent over joists.

I'd glue the T&G ends together for improved strength & rigidity. Access is no easier if you dont glue them, either way for access you'd need to cut a panel out and refit it after putting noggins under.

I wouldn't glue the chip to the joists though.

I'd pack the joists level first, otherwise the L&P ceiling might break and collapse.

Screws rather than nails all round, more strength and less risk of ceiling breaking up.

Long plasterboard screws have thin shanks for easy pilotless driving, bugle heads that dont need countersinking, and twin threads for faster driving. Plus theyre cheaper than regular wood screws.
http://www.screwfix.com/app/sfd/cat/cat.jsp?cId=A331389&ts=76960


NT

 

[Rob]

Hi guys.

Cheers for the responses, I feel much happier now.
As it is an old house there arent any pipes etc running under the floor so i am happy to glue when and if necessary.

Now all I need to do is to decide whether or not to go with the tri iso foil type insulation or not.........

Rob.

 

[Penners]

Now all I need to do is to decide whether or not to go with the tri iso foil type insulation or not

Serious questions are being asked about this type of insulation. My daughter is building an extension to her house that was originally going to use multifoil isulation in the roof. However, her Building Inspector has now said she should use something else - Kingspan or similar.

 

[Rob]

unfortunately due to my roof being previously thatched, the rafters supporting the tiles a at uneven distances and arent parralell.
They are made from 4x2 joists, some laid 4" deep, the others 2" (if that makes sense?)
Therefore I am not able to install anything between the rafters and still maintain the required air gap for ventilation.
I plan to lay the foil accross the rafters and then fit 25mm battens above this to screw plasterboard to.

 

[biffvernon]

What, precisely, do you expect the 'foil' to do?

 

[Rob]

by foil I mean the foil type insulation such as tri iso super 10 etc.

 

[biffvernon]

Yes, but what do you want it to do?