Ply as part of Floor Structure

[pearce_jj]

Firstly, Hi

Got a springy floor to sort as much as possible without disturbing downstairs. Upstairs is being redone and room is empty so broadly anything goes.

House is c.1928 in the UK. Joists measure 2"x7", unknown type. Span is 4.8m max, see rough diagram here:



Currently original T&G boards nailed, been pulled up in quite a few places. There's a 2.5" deep notch on the trimmer where indicated, and no noggings. Could well be other notchings, I've not had all the boards up yet.

Obviously this isn't a strong floor but it's not sagging noticably and the ceiling below is not cracked (boarded and skimmed about 7 years ago). Plan is to mount wardrobes over the small brick pier to take the load, so it will only really be supporting carpet, and bed, and a few sundries like a chair.

I'm considering:

• adding 2x7" noggings staggered across the middle all the way across, cut tight and screwed with 6x100mm screws;
• similar in front of existing trimmer around fireplace; and
• replacing boards with 18mm hardwood faced plywood, glued to joists using thick coat of PVA and screwed all the way along at 150mm centres using 5x75mm screws.

Please help - all comments greatly appreciated! Ply is quite a time consuming job for me so would appreciate comments on that in particular.

Cheers
James.

[pearce_jj]

BTW the field-glued ply idea came from a wikipedia article on strengthening floors and this book:
http://books.google.co.uk/books?id=b...um=2&ct=result

I could probably accomodate 25mm ply as well, if that makes any difference!

Thanks!

[snapman007]

Floor joists seem to be way undersized. Minimum size should be and assuming it's not supporting roof load is 240x45 F27. Even this may exhibit excessive bounce.
You could try bolt fixing an additional HW joist (170x45 F27) up the existing joists and add noggings.
Another thought, you can get engineered floor trusses. These will be a lot lighter and easier to manage.
Take your drawing into a pre-fab framing mob, they generally have an inhouse engineer who will sort you out.
Good luck with it.
Paul

[joe greiner]

I agree the joists seem somewhat flimsy. The existing floorboards contribute nothing to the strength or stiffness, as they're likely perpendicular to the joists.

Adding plywood will convert the structure to a composite system, with drastically increased stiffness and strength (T-beams instead of single joists). For maximum conversion, the face plies should be parallel to the joists. The thicker plywood will make it even better, but probably un-needed extra capacity. The bottom fibers of the T-beams are farther from the new neutral axis, and the noggings will help to stabilise them.

Unless the roof above, or an end wall, is removed, new engineered joists seem a lot more complicated than adding plywood through stairway access. Lay out the sheets in a quasi-running bond, like brickwork, to minimise concentrations of weak points.

I'm not licenced in UK, so you didn't read any of this from me.

Joe

[Honorary Bloke]

Joe's got it in one. I don't know why plywood should be particularly time-consuming. Actually it is a relatively easy fix.

[Manuka Jock]

I'm considering:

• adding 2x7" noggings staggered across the middle all the way across, cut tight and screwed with 6x100mm screws;
• similar in front of existing trimmer around fireplace; and
• replacing boards with 18mm hardwood faced plywood, glued to joists using thick coat of PVA and screwed all the way along at 150mm centres using 5x75mm screws.

Please help - all comments greatly appreciated! Ply is quite a time consuming job for me so would appreciate comments on that in particular.
.

James ,
What size are the sheets of ply ?
What way are you planning on orientating them to the run of the joists ?
How many rows of dwangs are you planning on putting in between the joists ?

Our practice here in NZ , is to screw at 150mm centers along the edges of the sheets ,
and 300mm elsewhere along the joists and dwangs ( far less screwing than 150 all over) ,
and with continuous glue fixing on every contacting surface .
On structural diaphrams , the mechanical fixing (screwing) can be as close as 100mm , and 200mm .


Go for the 25 mm ply , every bit helps eh

[snapman007]

Like Joe said, He's not allowed to get into trouble for helping you. Best advise is to take it a pre Pre-Fab Timber joint and talk to them direct. They will ask many more questions about your situation that you can answer direct.- Shouldn't cost u anything.

[pearce_jj]

Hi All,

Thanks you so much to everyone for all the input

Re noggins, I originally was thinking of one row, but perhaps three rows would be better (see diagram below).

Re ply, so 25mm ply laid long end parallel to the joists with the grain running along the joist at the interface would be best? T&G ply isn't widely available here so I guess it would be just 8x4ft sheets, cut such that the edges are directly over a joist.

Would adding 25mm ply to the underside at some point in the future add more strength? There must come a point where the weight of the ply starts becoming a limitation!

The room actually has a bay, not too sure whether to run noggings into that as well, here's a very rough diagram (propsed noggins indicated by green strips). Not too sure how the bay bit works yet, presumably it just supported by the window below it (it has dropped a little over the years, probably when the windows were replaced I guess).



Thanks again, I really appreciate your help.

James.

[Manuka Jock]

Yep , ply the bay too . Considering the fact that it is starting to drop , it needs it . Start your sheets there , to tie it into the main floor .
As to the number of dwang rows , consult the specie that comes from the ply floor manufacturers , also the local building authority , and most importantly , a few reputable Certified Carpenters , who may be able to identify the joist timber .
Do not have to do with 'cowboys'

[tpconman]

kiaora
have seen herring bone sprockets work well as a floor stiffner might be easier than dwangs/noggins , in conjunction with ply diaphram
cheers

[Manuka Jock]

kiaora
have seen herring bone sprockets work well as a floor stiffner might be easier than dwangs/noggins , in conjunction with ply diaphram
cheers

Thats true .
'Struth , I haven't put them in for years

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KiaOra Tpconman ,

welcome to the forum ehoa .

cheers ,
Jock

[joe greiner]

I had assumed there was a ceiling in place. Adding sheets on the bottom would both eliminate the noggings (= dwangs?), and convert to a box structure - even stronger than T-beams. With either T-beams or box structure, the sag due to weight of the sheets can be offset by propping the structure BEFORE fastening the sheets. We do this in bridge construction by introducing "camber" (curved upwards) in the main beams to offset the weight of the "future" concrete slab or similar running surface. The computed amount of offset isn't particularly difficult for simple spans; carpenters' handbooks should have formulas.

I was only half kidding about the rules and regulations. USA and UK might, or might not, have an extradition treaty covering such a weird situation. And extradition might accelerate my passport renewal application. I've never visited Merrie Olde; gummint expense could sweeten the pot.

Joe

[Manuka Jock]

Joe ,
apparently the ceiling is lined ,

"Obviously this isn't a strong floor but it's not sagging noticeably and the ceiling below is not cracked (boarded and skimmed about 7 years ago). Plan is to mount wardrobes over the small brick pier to take the load, so it will only really be supporting carpet, and bed, and a few sundries like a chair."

I reckon you got it right .....
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James ,
strip the plaster board (Gib Board ) off down below , and ply the ceiling , remembering to run the sheets in the appropriate fashion , in relation to those on the floor above ....IE. Stagger the Joins

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and ...


A dwang is , as I understand it a Gaelic word .
a short member fixed between framing timbers.


Nog , I think is Germanic in some way , a masonry term .
a temporary lintel , to lay stone/brick over a narrow opening .

Here in NZ , we Mainlanders use Dwangs , and them on the wee North Island use Nogs

[pearce_jj]

Evening all

Just to thank everyone for the input on this thread

For interest (and the benefit of the search) I had a structural engineer do some calculations for me.

The existing floor 'deflection' was way over the general guideline of 0.3% of the span (about 14mm for 4.6m) at about 30mm.

But by replacing the existing t+g boards with 20mm plywood glued (using a two part glue) and screwed every 6" using No.6 screws the deflection is reduced to about 9mm, within the spec and a huge improvement

Anyway, cheers!

[Harry72]

Redo the ceiling downstairs with ply, add heaps of noggins make a torsion box