Concern that ceiling beams/floor joists aren't strong enough

[Minkeycat]

Does anyone know of a calculation that exists to get an idea of the size and thickness of oak beams required to support an upstairs room/
I'm concerned about the strength of the beams in our dining room. Above it is a spare bedroom. The dining room is about 4.5m across with a good, chunky central beam spanning the width about 2.5m out from the wall across the middle(ish) of the room. The smaller beams running from the external wall to this beam are what's concerning me because they feel a bit bouncy in the room above which has a very wonky floor.
They're not evenly spaced and I think there should, in an ideal world, be a couple more of them. There are 7 of them in total. They range in thicknesses of about 10-12cm wide and, I'm guessing/hoping about the same high but I can't gauge this from below due to the ceiling. The gaps between the beams range too. The two that run along the wall have a gap of about 16cm between them but others are 55cm apart. I think this is because there used to be an oak screen dividing the room until a few decades ago and we think the stairs were in a difference location in there in its history so this would have dictated where the beams are today. Should they be evenly spaced?
Another concern is that where these beams join the main beam, they've been reduced in size by 3-4cm vertically and then slotted in to the main beam. In my mind, this makes them less strong.
One or two of them have been reduced in thickness (vertically) along their length and you can see these flex when someone walks above.
The floor above has been chipboarded with carpet on the top. We'd like to change this at a later date back to floorboards. I know these beams been there for hundreds of years and many generations of occupants but I want to make sure that the house is structurally good. It occurs to me that people are bigger and heavier these days and so are we put more "stuff" in our houses so I wonder if this particular floor needs attention?
Any helpful thoughts are welcome.

 

[Cubist]

Hi Minkeycat,

Sounds like we have similar homes and similar challenges. I have replacing the chip-board floor in the master-bedroom of our place on my to-do list but will need to level it as part of the process - up to 200mm variation in height in places - but not due to weak or sagging joists fortunately. Sounds like you have some simple deflection issues on some of the joists but you will need to know the dimensions of each of the joists and the type of timber as the basis of a deflection calculation and to secure any guidance on maximum loading.

While I was conducting my preliminary investigation to get rough measures I drilled a small hole in the ceiling next to one of the joists and pushed a pencil through until it met the floor above, marked off where the edge of the exposed joist met the pencil and measure. Turns out ours are square section and my guess would be that yours are too. Before the advent of Building Regs it was not uncommon to see anything up to 60 cm centres between these and typically they are either mounted in square section or dovetail housings in the main beams. As a general rule a timber framer would not make reciprocal cuts in the joists to produce a flush floor as this can significantly weaken the joists. To achieve a flush floor, while retaining the full strength of each joist, a good framer would have cut pockets into the side of each beam to receive the full depth of the joist and the tenon/dovetail above would be cut to about one third of half the width of the beam.

You should be able to see from below whether the framer did indeed cut pockets in the beam or simply halved the joint. If he halved them, then your deflection/loading calculation has to be based on the halved depth of the joist.

Finally, the condition of the joists will have a bearing (no pun intended) on their ability to resist deflection but you will only be able to fully assess this when you lift the floor.

Hope this helps.

Cheers,

Cubist

 

[88v8]

People bigger, heavier. Went to Aldi yesterday, the aisles full of waddling lumps of blubber. My 8st wife looked like a refugee from Lilliput.

Thoughts;

Anything over 18" spacing is not ideal as it leaves the floor boards more exposed to load.
Depth matters more than breadth, so beams reduced in depth, or notched by idiot electricians/plumbers, are a weak point. The nearer to the middle that they are notched, the worse.
Flex is OK. Flex due to woodworm is not.
If your beams are 4 - 5" wide and the same height, they are pretty chunky. Way chunkier than the elm beams I'm sitting on.

If the joists are oak, Prof Henry Adams writing in 1913, gives a safe ultimate tensile strength of c600 lb/square inch cross-section.
At the ends the safe bearing pressure circa 1,100 lb. Unless you have worm, I would not worry about the ends. It is quite normal for the ends of a framed floor to be significantly reduced in cross-section.

Assuming the spine beam is central, your span is about seven ft.
Roughly ascertaining the load capacity of each joist; the square of the depth, times the breadth, so assuming them to be 4" square that's
(4x4) X 4 = 64, then divide by the length of 7 ft = 9.15 cwt distributed, then divide by 2 to get the safe load if applied centrally - such as two fat people standing side by side - = 4.5 cwt with a factor of safety of twelve for English oak and seven for spruce fir.
4.5cwt / 500lb is a lot.
So for a bedroom I wouldn't worry, but I would want to know whether the joists have been notched and if so where and to what extent.

Redo the calculation for the joists that have been reduced in depth. Square of the depth in inches, times breadth in inches, divide by length in feet = safe distributed load in hundredweight. Divide by two for central load. Gives safe load in cwt with factor of safety as already mentioned.

Consider whether you can restore any missing depth by nailing and gluing an oak strip along the bottom of the joist.

If it were a bathroom, the standing load of a full iron bath would be of concern.

Some floors in our c17 cottage are bouncy. This bedroom, the main bedroom, the kitchen. They used enough timber but not too much. It's part of the difference from a modern house. We live in Period houses because they're different. Resist always the insidious temptation to make it more like a modern house.

Ivor

 

[Minkeycat]

Thanks both, much food for thought and savvy advice.

Glad I'm not alone, Cubist! You're right in that condition is everything. One of our beams has been reduced in height by a mad adze-man, probably to gain room height but it's only on one beam. That beam's certainly a bit bouncy. It's going to be a matter of getting the floors up above and looking at what state they're in. I suspect there'll be some form of historic insect dinner damage.

I'll work out the calculations. Thanks for those Ivor. Maths isn't my strong point but we do know a couple who, I suspect, go over the 500lbs limit so I think we'll put them elsewhere if they stay! Happily I'm on a diet myself so I can feel smug that my beams are probably strong enough for now!

I'll likely be back on this thread when the time comes. For now, we'll keep it as a single room!

 

[MatthewC]

Depth matters more than breadth, so beams reduced in depth, or notched by idiot electricians/plumbers, are a weak point. The nearer to the middle that they are notched, the worse.

That was a problem I had on the back bedrooms. I had to remove all the plaster from the laths anyway (and there were some wet holes in it!) and I found that the size of the joists varied. Some were only 2 inches square, and they appeared to have been re-used after being wrongly cut, so there were notches one inch deep (in two inch wood!) which were in random places between the middle beam and the wall plates. I did a lot of strengthening with doublers. It was still all a bit wobbly but stiffened up nicely when the plaster was applied.

Matthew

 

[Cubist]

I found that the size of the joists varied. Some were only 2 inches square,

Hi Matthew,
Thats just downright dangerous, theres no way the floor above can be safe.

Hopefully I've got the wrong end of the stick here but plaster cannot provide any structural integrity. I think I would be ripping the floor up and replacing the joists as the safest and best solution even at the expense of a few more inches lost off the room height.

Please tell me I have misunderstood.

Cheers,

Cubist

 

[88v8]

...we do know a couple who, I suspect, go over the 500lbs limit so I think we'll put them elsewhere if they stay!

Perfect!
Au contraire!
Ask them to stand together at a certain point in the spare bedroom for the purpose of, umm well you'll think of some excuse, and if they fall through the floor** then you'll have identified the weak point, and if they don't, there isn't a problem

Ivor

** unfortunately if the beans are oak you'll probably be denied that hilarity, the 500lb is, as I mentioned, a figure with a twelve times factor of safety.

 

[livias]

and to add another wrinkle to the decision ... The floorboards may well be stronger than the beams

In my last house I was mystified by my medieval joists (4" high 6" wide oak) which at one end were levitating three inches above the beam they were meant to be sitting on. Eventually I decided that the elm floor above was actually holding up the joists. I packed the gap between joist and beam just to be on the safe side in case the floorboards decided one day to weaken ... but it has been fine for over 500 years so ....

 

[Zebra]

My floor is bouncy.
Giraffe and I are alright up there, but if my hippopotamus friend comes to stay I make sure she sleeps downstairs.

 

[malcolm]

One of my bedroom floors was bouncy near the outside wall. It turned out that when altering the 17C structure to put in the huge bay window downstairs the Edwardians had supported one end of the existing joists only by a bit of 2 by 2 nailed to the bottom of one of the new lintels. I have since put in joists hangers which has made the floor much less bouncy (but still not quite to modern standards as the timbers aren't very tall).

They did tend to under-engineer joists in the old days. Strength tends to be fine if they haven't been messed with, but they didn't seem to regard stiffness with the importance we do today.

 

[CliffordPope]

I found that the size of the joists varied. Some were only 2 inches square,

Thats just downright dangerous, theres no way the floor above can be safe.

It does depend how long they are. I've seen infilling joists that size spanning the space where the slate slab of an upstairs hearth used to be, about 18" long.

 

[Cubist]

I found that the size of the joists varied. Some were only 2 inches square,

Thats just downright dangerous, theres no way the floor above can be safe.

It does depend how long they are. I've seen infilling joists that size spanning the space where the slate slab of an upstairs hearth used to be, about 18" long.

True enough Clifford and fair comment but can such reasonably be called joists?

 

[Cubist]

I clearly have not got the hang of how these 'Quote' functions work - ho hum.