Big joist notch - worried

PZero · 12 Dec 2008

We have an old victorian tenement flat and are in the middle of having out flat renovated and at the moment the plumbers are in. I prepared the building warrant drawing and got the building warrant myself, and when the plumbers started work I gave them several copies of the drawing and hung one on the wall to make sure they could refer to it when needed.

It's quite obvious they are not following it as there are a few things wrong, but one thing I am a bit worried about. Building control would not allow me to run the condensate drainage indoors because it meant notching the joist tops. So I moved it on the drawing to show it going through the wall and along the outside wall to join into the svp. Now they have ignored this and run it inside. One of the joists is heavily notched. In fact it looks more like its been hacked away with drill and a flat drill bit.

The joist is about 240mm high by about 75mm wide. On on side it is notched about 30mm deep, sloping to about 80mm deep on the other side. Building regs state here that a notch is allowed to 1/8th of the joist height, which would be 30mm. The joist span is about 5500mm.

Is this dangerous? Should I be looking to have it repaired? Should I be shouting at the plumbers?

seco services · 12 Dec 2008

as you say should be 30mm max and between 380mm - 1375mm along the joist.
80mm is a bit over the top

jackthom · 12 Dec 2008

That sounds like a hefty and important structural joist!
I'd say yes, yes and yes to your questions.

If you need a bit of moral support will the building control send out an inspector?

PZero · 12 Dec 2008

I think building control would probably send an inspector.

I'm more annoyed that this could have been avoided if they had bothered to read the drawing.

So I assume that it is their responsibility to put this right, ie; reinforce the joist and run the condensate where it should be?

namsag · 12 Dec 2008

WTF have they gone 80mm down , condensate waste inside only usually needs to be 21mm diameter pipe.

But regards structure and rules on notching its amazing some of the hatchet jobs you see with joists absolutly butchered and very little left of them and it does not affect anything .Alot depends on what walls etc are under them distances etc supporting them.

But if you specifically asked them not to cut then you have a genuine greivance and should be asking for answers

PZero · 12 Dec 2008

namsag said:
WTF have they gone 80mm down , condensate waste inside only usually needs to be 21mm diameter pipe.

They've gone 80mm down because the 50mm waste that the condensate connects to runs down the side of the joist, so they have cut the joist to make space for a tee in the 50mm.

Its not the first thing they've done wrong to be honest.

namsag · 12 Dec 2008

They could have got down to that depth by using fittings no excuse at all to cut down that sort of depth , just lazy and saving about 50p an elbow/45 bend

PZero · 12 Dec 2008

Theyve done the same in another room. There are two radiators on the wall and a joist runs directly below, in line with them both. They have cut about 80mm deep away for each of the 4 pipes to feed the rads, leaving about 10 - 20mm of the original joist top intact. Luckily this joist will not be subject to much load, but this surely is not good practice?

bengasman · 12 Dec 2008

any half decent plumber knows you can't just chop in joists at will, and 80 mm deep is a complete cowboy job.
you really should get whoever is responsible for this lot to explain to you how he is going to rectify it.
the strength of joist is the height square, 90% of height is 80% of strength, 80% height is 64% strength etc
in your case that means the joist is now half as strong as it used to be.

kstandsteve · 12 Dec 2008

Firstly I am a qualified structural engineer so I guess am best placed to give you a practical reply.

Secondly it makes a big difference where along the span the notch is. At midspan its at its worst location as the timber stresses are at a maximum. Notching towards the support is much easier.

The Building Regs give guidlines as regards notches so that the layman has guidance and knows this will not cause structural problems.

Its not always the case that exceeding these limits results in an unsafe structure. It purely means that you will need structural calculations to justify the notch.

Now this maybe where you fall down (not literally!) as "new" timber is stress graded and subject to British Standards. Therefore calculations can be provided with some certainty. Older timbers were not, so an engineer providing calculations will have to air on the safe side and downgrade the allowable stresses.

Basically what I am saying is it might be safe it might not. You need to get a structural engineer to check it for you.

Steve.

bengasman · 12 Dec 2008

kstandsteve said:
Firstly I am a qualified structural engineer so I guess am best placed to give you a practical reply.

Secondly it makes a big difference where along the span the notch is. At midspan its at its worst location as the timber stresses are at a maximum.

maybe you are the man who can finally fill me in Steve.

the above makes perfect sense about stress being at its maximum in the middle, at the furthest from the support. seems common sense.

what i never quite have been able to work out is why prefab steel roof supports, or bridges and that kind of stuff often have the lowest profile in the middle of the span.
seems to go against the grain

sorry for the highjack

kstandsteve · 12 Dec 2008

Ha yeah.

What you have seen is a typical 3 pin arch or to a lesser extent a portal frame. Both these are stiffened by the column with the highest stresses at the eaves or connection between rafter and column. Structures such as these can be "pinned" at the apex with no bending at this point.

A different thing all together.

Goldspoon · 13 Dec 2008

kstandsteve said:
Basically what I am saying is it might be safe it might not. You need to get a structural engineer to check it for you.

Steve.

Even if "safe" I would still be very concerned. The plumber/s have chosen to ignore you and to ignore building regs (or do not know them). As the person paying them this would really **** me off.

kstandsteve · 13 Dec 2008

Agree totally. These people are not structural engineers and should not be doing things to contradict BR's.

Does smack of incompetance.

bengasman · 13 Dec 2008

cowboys, no 2 ways about it.
even if what they did does not compromise the structure, which is quite possible as old houses tend to be well overdimensioned, it shows they take no pride in their work.
if they cut corners here to finish the job quicker, they will do so at other points too.
it might be an idea to engage a quality engineer to pop around a couple of times after hours to see if they don't mess things up and hide the problems.

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Big joist notch - worried

namsag

namsag

Goldspoon

Similar threads