floor span

[tiling spread]

as im replaceing the joists and the old floor say the room is no more then
1700mm(w)/2700mm(L) at 400mm centers what size joists do i need i was trhinking 50mm/100mm(2+4ins or is that too small im puting in nogins too as its supporting a bath its a ground floor flat and all the joists and floor boards are roten so its new joists and T&G chipboard flooring going down i dont want bownce in the floor as im tiling it soon after

if i dont get a ts60 bythen ill be using my ts40 ha ha

[whitebeam]

Why use chipboard rather than ply

[tiling spread]

I can its just is T&G thort its stronger thats all i just dont know the size of joists too use its doing my sweed in so i can price the floor in

[peckers]

What size are the joists you taking out? is there any deflection in the floor before they were removed I would say 5x2 would be better (but im not sure) noggins at 300 centres use ply as white beam has said as chipboard is not stable enough to tile onto

[tiling spread]

so its 5+2 3m lenths what sort of price are 5+2 joists then

[mikethetile]

you require 2x8 for floor joists and you need to ring your local timber supplier for a price

[tiling spread]

woooooooooooooow that thick thats a bit big is that 600mm centers think im looking to get 10/ 3m lenths

[peckers]

you require 2x8 for floor joists and you need to ring your local timber supplier for a price

I knew someone would know

[Colour Republic]

Exactly how do plan to remove the joists and replace? You saying these joist only form part of this room?

[Rob Z]

The four S's are what you need to consider when sizing floor joists: span-species-spacing-size.

Span-what is the unsupported span of the joists?
Species-what is the species and grade of the lumber?
Spacing-what are centers for spacing the joists?
Size-what is the size of the joist (depth)?

I agree with Mike-I can't imagine any floor joist being adequate that is less than a minimum 2 X 8 and 16" o.c.

There are "Deflecto-meters" available on the web, and you can input the data for your floor and get the calculated deflection rating. The number used here in North America is a minimum of L/360 for ceramic. I've never seen this discussed here on the site but the same principles must hold true in the UK for floor framing.

[jay]

its amazing just how much a tiler has to know to do a job

[faithhealer]

Only replaced joists the once and called a mate in to give a hand because I would have been there all week trying to figure it out. Learnt a lot about wood worm and rot that day

[Rob Z]

John, one of my first paying jobs was framing houses, so I have a bit of experience with this...it helps out because when a floor needs work it keeps the income coming to me and my employees rather than another contractor.

[mikethetile]

the states and canada are streets ahead of the uk on timber frame building and understand timber and how it behaves

that knowledge is limited in the uk as we are mainly masonry built, even timber frame is masonry clad which has caused all kinds of problems over the years

both Robs are right, its not as simple as ripping out and replacing. its structural work and needs to be treated as such

one question that hasnt been answered is why the joists need replacing

are the undersized and cant take the weight of a bathroom or have they rotted

if rotted ...........why

it the walls are lathe and plaster , have they been built across the joists

will cutting the joists out jeapordise the integrity of the building

this is work for a fully insured registered builder that understands structural work and has the experiance to deal with this kind of work safely

[TF Ed]

they seem to frown at 8 x 2 floor joists in canada, they always recommend 10 x 2's
is the us the same rob?

ps, agree with mike on the us / canadian framers

usa / italy / australia/ all have great tiling reputations too...

[Rob Z]

Ed,

I'm not too keen on 2 X 8's, but they might be OK on a short span. I have done floors on 2 X 8 joist systems, but only after checking out the span. In my area, there were a lot of houses built in the 70's and early 80's that had 2 X 8 joists, and that was a time that had a lot of cost-cutting in construction.

From my time on Harry Dunbar's Canadian site, and having a lot of correspondence with three of the guys on that site, I think it's safe to say that much of the engineering and codes that are in place in Canada are in place here in the US.

[Rob Z]

the states and canada are streets ahead of the uk on timber frame building and understand timber and how it behaves

that knowledge is limited in the uk as we are mainly masonry built, even timber frame is masonry clad which has caused all kinds of problems over the years

both Robs are right, its not as simple as ripping out and replacing. its structural work and needs to be treated as such

one question that hasnt been answered is why the joists need replacing

are the undersized and cant take the weight of a bathroom or have they rotted

if rotted ...........why

it the walls are lathe and plaster , have they been built across the joists

will cutting the joists out jeapordise the integrity of the building

this is work for a fully insured registered builder that understands structural work and has the experiance to deal with this kind of work safely

Hi Mike, In the older houses here, the floor joists were "let into" the masonry walls, which was/is essentially the only thing that kept the brick and block from moving laterally (and still it did). Is it the same way in the UK now?

[mikethetile]

we use 10 x 2 here too

but I was going on the dimensions of the room

very limited on info here and theres no point in asking

im assumng from the other construction that the joists will be supported mid span by dwarf walls if its ground floor and a supporting wall if its ustairs

with the age of construction its unlikely thay 10x 2 were used originally

[mikethetile]

Hi Mike, In the older houses here, the floor joists were "let into" the masonry walls, which was/is essentially the only thing that kept the brick and block from moving laterally (and still it did). Is it the same way in the UK now?

we use exactly the same method to tie buildings togethers and prevent walls moving outwards

hence the care needed when altering joists in buildings

[mikethetile]

we use exactly the same method to tie buildings togethers and prevent walls moving outwards

hence the care needed when altering joists in buildings

we have a system now where the ends of the joists are given plastic boots that are built into the masonry to prevent end rot

in some instances you can hang the floor on joist hangers so the joists are not tied in but are cut tight between the walls

this can only be done if the floor isnt designed to tie the building together

[Scott]

If its lathe and plaster then they will be let in to the walls for sure. No modern galvanised joist hangers back then!

[mikethetile]

If its lathe and plaster then they will be let in to the walls for sure. No modern galvanised joist hangers back then!

your absolutly spot on there, nor will you get away with fitting joist hangers

[Rob Z]

Good info, Mike!

We start work soon on a whole-house remodel in Washington DC, circa 1900. I'll be seeing lots of masonry detail on that one. The other interesting thing to see on those old houses is that there in no structural steel of any sort-all headers and so forth are big pieces of timber spanning the openings.

The only thing holding these houses together is gravity and habit.

If there is ever an earthquake here these houses will collapse.

[Ian Harris]

If there are problems with a deflecting floor due to undersized joists the situation can be improved by gluing and screwing plywood to the underside
of the joist which creates a stressed skin floor
This may be easier than replacing the floor joist in some instances
but of course you will need to re board the ceiling
A structural engineer will advise on what improvement can be obtained
Gluing and screwing the plywood floor instead of just screwing also improves deflection