# quick question - 47 by 72 ceiling joists at 300 centres 2.15m span - that ok?

Discussion in 'Building' started by StephenStephen, 15 Jan 2016.

Tags:
1. ### StephenStephen

Joined:
1 Apr 2008
Messages:
395
23
Location:
Devon
Country:
I've got a ceiling span of 2.15m
Trying to keep ceiling joist depth to a minimum
So am thinking 47 by 72 at 300 centres
Do you think I'll have a happy, or saggy, ceiling on these figures?

btw:
I'm assuming I won't get away with smaller than 72 depth?
I can noggin or cross brace if necessary
Is there a table or formula for joists at less than 400 centres?

Thanks muchly

Stephen

2. ### jeds

Joined:
16 Apr 2004
Messages:
4,085
526
Country:
It won't fail but you'll have too much deflection. A rough calc gives about 11mm which is a lot over 2m. I reckon the minimum you'll get away with is 50x97 but even then it will be close.

• Thanks x 1
3. ### StephenStephen

Joined:
1 Apr 2008
Messages:
395
23
Location:
Devon
Country:
I know I'm slightly missing the point here, but.. is there a smaller joist spacing that I could use 47 by 72 at 2.15 span that wouldn't give too much deflection?

4. ### ^woody^

Joined:
3 Sep 2006
Messages:
26,271
3,238
Location:
West Mids
Country:
Double the joists up

• Thanks x 1
5. ### cjard

Joined:
6 Sep 2008
Messages:
2,334
250
Country:
11mm over 2 metres is the standard warp in joists from some DIy stores!

Joking apart, are you seriously telling me that people would walk into a room (and what sounds like a box room or hall at that) and look up and say "gosh, the centre of the ceiling looks to be 11mm lower than the edges"
No chance.. Even if it does sag that much, you'll notice it when you first put it up and get a ruler out, then it will just fade into the background of your life, your wife will never notice (she'll just be delighted that the utility room ceiling is finally not dropping dust all over the washing), the housewarming guests will make no comments about bowls, pillows or other curved shapes..

If it's genuinely going to bother you forever, or if it's going to have mirrors mounted on it and hence you'll see the distortion of it's not millpond flat.. you say the floor above is separately supported, why not strap off that midspan?

Last edited: 17 Jan 2016
• Thanks x 1
6. ### garyo

Joined:
11 Apr 2006
Messages:
1,842
166
Location:
Berkshire
Country:
What about dropping some mid span hangers from the floor joists above? Or does that defeat the object of what you're trying to achieve?

• Thanks x 1
7. ### tony1851

Joined:
23 Feb 2012
Messages:
9,099
1,355
Location:
Manchester
Country:
Eh?
With just plasterboard, no live loading, and 300 centres, I reckon deflection will be <3mm.
(span x 0.003 = 6.45mm)

• Thanks x 1
8. ### ^woody^

Joined:
3 Sep 2006
Messages:
26,271
3,238
Location:
West Mids
Country:
The ceiling won't crack all over the place then after a few heating seasons?

• Thanks x 1
9. ### StephenStephen

Joined:
1 Apr 2008
Messages:
395
23
Location:
Devon
Country:
Thanks ...yes - trying to isolate the 2 sets of joists to cut down sound transmission a bit, and the joists are offset, so a bit of a technical challenge also...

10. ### StephenStephen

Joined:
1 Apr 2008
Messages:
395
23
Location:
Devon
Country:
Thanks all - very much appreciated - I think I'll reduce the centres to something around 250-270, put in a few cross braces, and happily live with the possibility of a little sag - I'm not skimming and the plasterboard is running full span without break, so imagine no cracks.

11. ### cjard

Joined:
6 Sep 2008
Messages:
2,334
250
Country:
Just make the ceiling out of joists. You know you won't be happy with anything less. I'd also avoid taking a straightedge or level to anything else in the house

So.. why can only be height X when you've implied there's nothing directly above them to string them from? Just curious..

12. ### StephenStephen

Joined:
1 Apr 2008
Messages:
395
23
Location:
Devon
Country:
Bit of a misunderstanding here methinks...
- I'm new to putting up ceilings, and I'm happy with some deflection. Frankly a flat level ceiling might look a bit out of place in this house!
However on reading: It won't fail but you'll have too much deflection (above) which was written in good faith and appreciated by me, I assumed that this would be a problem that needed solving. I now understand that it's an aesthetic consideration, as long as I weren't to skim.

You're right to be curious - it's an odd one: Over most of the ceiling I'm installing the ceiling joists between the floor joists, with the bottom edge of ceiling joists at least 50mm below the bottom edge of the floor joists - I can then weave insulation between the two - under the floor joists and over the ceiling joists. There is one area of the room with a hearthstone in the floor above, so I need to get the ceiling joists under the trimmer (if that's the right word?) at the front of that hearth.

On reading this, I'm thinking I can actually use deeper joists, notch them under the trimmer and slap a metal plate on either side - fewer joists, less end fixings, less pain, less deflection.
I wouldn't have got there without this conversation, so thanks again all.

• Thanks x 1
13. ### cjard

Joined:
6 Sep 2008
Messages:
2,334
250
Country:
Even skimmed, the skim layer is a few mm thick and weight wise contributes little so your construction will have done most of its sagging by the time you get around to finishing it. Other an the odd drying out crack which is pretty much a universal affliction in housing we're not looking at much.

If you've yet to handle a plasterboard then you'll be in for a surprise. It's insane how much they can bend and show no signs..

As with any job you've never done before, start with the least conspicuous area, or the one you least mind redoing.. Chiefly because your technique will change and improve and come the end of boarding you might well think for half a day and a few quid on materials you'll scrap your first room's efforts and start over

• Thanks x 1

Joined:
11 Jan 2013
Messages:
2,080
358
Location:
Durham
Country:
Ain't that the truth

• Thanks x 1
15. ### jeds

Joined:
16 Apr 2004
Messages:
4,085
526
Country:
But with live load it'll be a lot more. 8 or 9mm is enough to crack britdeflection will crack brittle finishes.
You can't just ignore live loads though. Live loads push deflection up to 10 or 11mm, which will be a very bouncy floor and will likely crack brittle finishes. And you know was well as I do tony that this isn't about having a bit of a bow in the ceiling that nobody's really going to notice. Codes of practice and design standards are there for a reason.

1. ### Quick Question: Block or Brick to Attach Summer House Base to?

ILikeDIYS, in forum: Building
Replies:
2
Views:
160
2. ### Quick question about air bricks

Jim_cam, in forum: Building
Replies:
4
Views:
492

Replies:
4
Views:
505
4. ### Quick set cement moisture question

Bobharford, in forum: Building
Replies:
6
Views:
606
5. ### Quick question, is it possible to mix insulation types?

dagoof, in forum: Building
Replies:
6
Views:
741
6. ### Sleeve Anchors - quick question

robJ71, in forum: Building
Replies:
8
Views:
1,313
7. ### Quick cold/warm deck question

indus, in forum: Building
Replies:
2
Views:
240