Straight crack in the ceiling (Ed.)

[liftfan]

I've got this crack. In my ceiling. Should I be worried?

The crack is along the line of plasterboard. There is a lightweight block wall in the room above, about 200mm to the right of that crack. The wall is supported by the floor joists above which run 90 degrees to that wall ie. left to right in the picture below.

I've been in the house 10 years and the house is 50 years old. The crack has changed little in the ten years I've been here. But, it's time to decorate this room. So, should I do something about it, or just leave it?

Oh yes, that wall also takes "some" roof loads down it as well. I replaced all the clay tiles with slate a few years ago.

Any thoughts?

(Bonus points for the first person to say you need to get a structural engineer in).

 

[Ivor Windybottom]

Given that it's randomly messy artex with a central blobby feature centre where I guess a 1980s rose has been removed, I'd say it's not worth caring about.

Unless you're going to get rid of the artex, in which case find the joists and screw the plasterboard up firm either side of it before skimming it. Mesh scrim tape across it would be a good idea too.

 

[Koolpc]

If its old artex get it tested as with that age, artex had asbestos in it...

 

[liftfan]

Thanks for the reply. I'll probably board it over rather than remove the artex.

I was also wondering about structural issues.
I just asked ChatGPT about it.


It says:

1. Initial Problem:​

• You have a domestic house where the first-floor joists (8-inch deep, 400mm centers, spanning 3.5m) support a lightweight concrete block dividing wall in the master bedroom.
• This dividing wall runs parallel to the 4.8m span and is positioned 1m from one end of the joists.
• The floor joists have deflected by approximately 10mm over 50 years, raising concerns about whether this exceeds Building Control limits.
• The wall also carries part of the roof load, adding additional stress to the joists.

2. Building Control Deflection Limits:

According to UK Building Regulations (Approved Document A) and standard structural engineering guidelines:

• Maximum Allowable Deflection for Floors: Span/360
  • For 3.5m joist span:3500/360=9.7mm
  • Your measured deflection: 10mm
  • Verdict: Slightly exceeds the recommended limit, but not critically.

Since deflection is only slightly above the limit, it’s not an immediate risk, but reinforcement is recommended to prevent further sagging.

3. Structural Analysis & Load Considerations:​

• Loads Acting on the Joists:
  • Wall Load: 5.8 kN/m
  • Ceiling Load: 0.2 kN/m
  • Roof Load Contribution: 5.88 kN/m
  • Total UDL: 11.88 kN/m
• Deflection Calculations:
  • Without reinforcement (timber only): 10mm (slightly over limit)
  • With flitch beam reinforcement: 5mm (well within limits)
  • Flitch reinforcement reduces deflection by ~50%, preventing further sagging.

4. Recommended Reinforcement Solution:​

Flitch Beam Reinforcement on Every 3rd or 4th Joist

• 10mm thick steel plates, 200mm deep, bolted to both sides of selected joists.
• M12 or M16 bolts at 400mm centers for even load distribution.
• Ensures compliance with Building Control deflection limits.

Alternative Options Considered:

• Adding a steel or timber mid-span support beam under the joists (most effective but visible in the room below).
• Sistering existing joists with additional C24 timber members (cheaper, but less effective than steel).

5. Deliverables Provided:​

Full Structural Calculation Report (PDF)
Deflection Analysis Including Roof Load
Reinforcement Recommendations
Updated Structural Drawing Showing Flitch Beam Locations

 

[liftfan]

Scary.

 

[tony1851]

OMG!
Where to begin?
It doesnt tell you that, over time, timber under load deflects and takes on a permanent 'set' - its called timber 'creep'.
For that reason, you would never fit 200mm plates to the side of bent 200mm joists
Does CrapGPT tell you how to lug 10mm steel plates 3.5m long upstairs; how much you'll have to pay to rip up and replace all the boarding, and how you won't in any case get the joists straight because they are permanently bent?
Just overboard as you suggest and hide the artex at the same time - and forget about it.

 

[liftfan]

Just for a laugh I put your response Tony, into ChatGPT to see what it would say. In the next post I'll put in word for word! I just asked it to reply in the style of your response.

This is just a bit of fun!

 

[liftfan]

Ah, brilliant! Gotta love a bit of DIY forum wisdom, delivered with the grace of a bloke who's spent too long arguing with a plasterer about whether "it’ll be alright with a bit of bonding."

Oh, OMG indeed, mate! Where to begin? Let’s unpack this like an overstuffed skip outside a dodgy refurb job.

1. Timber Creep – The ‘Bent Joist’ Problem​

Ah yes, the classic timber ‘creep’ argument—because, obviously, floors behave exactly like pensioners’ spines. You’re half right, but let’s be fair: timber does creep over time, but not all deflection is permanent. A flitch beam won’t magically ‘unbend’ the joists, but it will stop further movement, which is the whole point. Unless, of course, you prefer the approach of “ah well, it’s bent now, might as well leave it.”

2. Lugging 10mm Steel Plates Upstairs​

Now, this is where you’ve got me! I should have included a full risk assessment for the perilous journey up the stairs with a 3.5m flitch plate.

• Should I have hired a Sherpa?
• Perhaps fashioned a pulley system using an old washing line and two blokes named Dave?
• Or better yet, called in a structural engineer, just to give it the ol' ‘looks fine to me’ nod?

Yes, steel is heavy, but let’s not pretend getting a few plates upstairs is akin to hauling the HMS Victory into dry dock. Two lads, a bit of sweat, job done.

3. Ripping Up the Boarding & Getting the Joists Straight​

Now, here’s where you’ve swung the hammer at the wrong nail.

• Nobody said flitch plates ‘straighten’ joists – they reinforce them so they don’t move further.
• Will the boarding need to come up? Probably. Welcome to structural work.
• Would overboarding work instead? Sure, if you fancy a solution about as permanent as a cheap pint at Wetherspoons.

If the joists are already too far gone, then yes, overboarding might be the ‘lazy but passable’ option. But if you actually want to do it properly, strengthening the structure before it gets worse isn’t the worst idea.

Final Thoughts:​

If you just want to paper over the cracks, go ahead—stick some boards down, pretend the floor isn’t sagging, and hope the next owner doesn’t notice. But if you’re actually doing structural work, reinforcing the load-bearing joists is the correct approach—whether with flitch plates, sistering, or a mid-span support.

So go ahead, overboard it if you must—but don’t be surprised if, in 10 years, you’re back on the same forum asking how to fix a floor that’s suddenly got ‘a bit of a wobble’.

Cheers, and don’t forget to treat yourself to a well-earned cuppa after that engineering masterpiece of a reply.

 

[liftfan]

As you say though, it's wrong about the 200mm plates. Here's what the numpty chat said:

3. The 200mm Flitch Plate Mistake – My Bad!​

Right, I’ll take this one on the chin—you’re absolutely right about the 200mm flitch plates. If the joists have already taken on a permanent sag, you can’t just bolt a rectangular steel plate onto a curved surface and expect it to fit like a glove. That’s like trying to panel a wobbly wall with a dead straight sheet of MDF.

So, fair play to you—if the joists have significant sag, I’d need to either:
Use thinner steel that can flex slightly to match the curve (say, 6mm instead of 10mm).
Jack up the joists slightly before fitting the plates.
Forget flitch beams and go for another solution, like sistering with timber or installing a mid-span beam.

 

[JohnD]

What a load of verbose nonsense.

Has there been any building work that has altered the construction since it was built.

 

[liftfan]

The only building work that could have affected it is when I replaced all the roof tiles with slates. So the roof was unloaded then reloaded again. I'm pretty sure the crack had started to form before that though.

Here's a picture of when the tiles were off. The relevant part is right at the far and of the photo and you can't really see it but you get an idea of how the roof is constructed. The horizontal timber in the middle foreground, in the insulation, is similar to what I'm talking about. I don't think there's a huge amount of load going down there though. Maybe more just for bracing?

 

[JohnD]

There is a lightweight block wall in the room above,

Where is this room in your picture of the roof structure?

 

[liftfan]

Thanks JohnD.

Here is a plan of the roof showing the direction the photo was taken. The room, which is below this area but on the ground floor, is in the circled area.
The second picture is the first floor with the lightweight block wall circled in red. Floor joist direction in blue dashed.
The third picture is the ground floor room with the ceiling crack shown in dashed green.

 

[JohnD]

If the room with the cracked ceiling is not directly beneath the roof, the roof is less likely to be the cause, unless it was bearing on an internal wall, that was supported only by the cracked ceiling.

I wonder if the joists of the first floor (the cracked ceiling) are too long or too thin? Can you measure them? Can you measure the deflection?

The timbers of the roof itself look to my unskilled eye to be quite generously proportioned, although I don't see ironwork in tension that I have seen in comparable roofs.

 

[stevie888]

As you say though, it's wrong about the 200mm plates. Here's what the numpty chat said:

3. The 200mm Flitch Plate Mistake – My Bad!​

Right, I’ll take this one on the chin—you’re absolutely right about the 200mm flitch plates. If the joists have already taken on a permanent sag, you can’t just bolt a rectangular steel plate onto a curved surface and expect it to fit like a glove. That’s like trying to panel a wobbly wall with a dead straight sheet of MDF.

So, fair play to you—if the joists have significant sag, I’d need to either:
Use thinner steel that can flex slightly to match the curve (say, 6mm instead of 10mm).
Jack up the joists slightly before fitting the plates.
Forget flitch beams and go for another solution, like sistering with timber or installing a mid-span beam.

I'd love to see the ChatGTP (whatever it is) trying to bend a 200x6 plate on its major axis. What a crock!!!!!