Cutting ends of new loft floor joists

[antonyc]

Hello everyone!

I'm planning to redo the floor of the loft so that I get the first step of a loft conversion done. I've worked out using Trada tables that I need 7" x 2" joists but the issue is, when I try to put them into place on top of the wall plate, the top corner of the joist knocks against the roof!

How do I deal with this correctly? Do I cut the corner of the joist off or would this be going against the regulations? What should I do?

Thanks, Antony

 

[cjard]

I dare say you could physically cut the corner off with no ill effects but you shouldn't. Have a look to see if engineered joists will fit and do the spans you require. Google for JJI SPAN TABLES and look at the results from James Jones. The 145 joists may fit under your rafters, and even the bigger ones like 195 May fit if there is enough bearing on the wall plater. My house has lots of these joists and they bear on 40mm sometimes (party wall is 90 wide but has a 45 wide glulam ring around the outside)

 

[antonyc]

I dare say you could physically cut the corner off with no ill effects but you shouldn't. Have a look to see if engineered joists will fit and do the spans you require. Google for JJI SPAN TABLES and look at the results from James Jones. The 145 joists may fit under your rafters, and even the bigger ones like 195 May fit if there is enough bearing on the wall plater. My house has lots of these joists and they bear on 40mm sometimes (party wall is 90 wide but has a 45 wide glulam ring around the outside)

Thanks... There isn't enough space between the wall plate and roof to fit anything in. Surely, most people have this issue no?

 

[RonnyRaygun]

Depending on how much you need to cut off there should be no issues with trimming the joists a bit.

You will reduce the shear capacity of the joist but shear is never the governing factor on uniformly loaded joists. You can often reduce the size of the joist by 50% near the bearings and still have no problems with shear.

How much do you need to trim off?

There are advantages to engineered joists but they are will cost you more, and I don't see the benefit on this occasion.

 

[antonyc]

Depending on how much you need to cut off there should be no issues with trimming the joists a bit.

You will reduce the shear capacity of the joist but shear is never the governing factor on uniformly loaded joists. You can often reduce the size of the joist by 50% near the bearings and still have no problems with shear.

How much do you need to trim off?

There are advantages to engineered joists but they are will cost you more, and I don't see the benefit on this occasion.

Well the wallplate is 4" wide/deep and the angle of the roof seems to be about 45 degrees - straight off the wall plate. So I'd need to cut right from the very end of the joist. Pretty much, a 7" x 7" triangle would need to be cut out of the joist. I think maybe its a bit much because only 4" stays on the brick.

I've just found a document which says that the bearing needs to be 90mm on masonry work. I assume that means full height bearing as it looks like it in the picture.

I think my original plan is a bit too mad.. I'm going to drop the ceilings instead I think.
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[RonnyRaygun]

The bearing size is the area of bearing, not the height.

Do you mean that the roof rafters bear directly onto the wall plate with no overhang and no birds mouth? That's an unusual detail isn't it? How are the rafters tied by the ceiling joists?
It does sound like you will struggle to fit the joists on the wall plate, if the detail is as you are describing.

If you do want to lower the ceiling you will still have to think about how you tie the rafters, or if you remove the rafter ties then you will need a ridge beam to prevent roof spread.

To be honest it would help a lot if you could sketch a section through what you have at the wall plate. At the moment I'm still a bit confused.

 

[antonyc]

The bearing size is the area of bearing, not the height.

Do you mean that the roof rafters bear directly onto the wall plate with no overhang and no birds mouth? That's an unusual detail isn't it? How are the rafters tied by the ceiling joists?
It does sound like you will struggle to fit the joists on the wall plate, if the detail is as you are describing.

If you do want to lower the ceiling you will still have to think about how you tie the rafters, or if you remove the rafter ties then you will need a ridge beam to prevent roof spread.

To be honest it would help a lot if you could sketch a section through what you have at the wall plate. At the moment I'm still a bit confused.

I'll have another look tomorrow... I couldn't see any bird's mouths but then again, the birds are the other side of the roof. Lol, joking, there are no birds mouths that I could see and the rafters seem to shoot straight off the wall plate! Although, I'll admit, it doesn't sound right! The rafters are indeed tied to the ceiling joists though.

Well, what I'm going to do is drop the ceilings, add joist hangers in-between the current joists (100mm lower), lay the new joists and then tie the rafters to the new joists.

I suppose really, I might aswell just replace the joists completely and pull the old ones out one by one as I go.

 

[tony1851]

As RR said, this is a bit confusing. Don't drop the ceiling yet!!!

As advised, put up a rough sketch section. There may well be much easier ways of solving this without dropping the ceiling - doing that you will merely be swapping one problem for a much bigger one.

 

[antonyc]

Right, ok, I've had a really good look up there.

Thankfully, it is quite normal. There is a wall plate and there is a birds mouth in the rafters. The reason I couldn't see it was because there was a brick filler and you can't actually see the wall plate without first knocking some of the filler bricks out!

Anyway, all is good.

Now, there are two methods which i'm considering:

1: REPLACING the 3x2 joists completely with 7x2 notched joists WITH hangers also. I'd have to re-nail the rafters to the joists, etc. This would keep the same structure but beef it right up!

2: ADDING new 7x2 notched joists WITH hangers in between the current 3x2 joists.

Whichever of the two, I'd hang the new joists 100mm lower to meet up with where the old ceiling joists used to be so I'd drop the cielings 100mm.

 

[tony1851]

Is your wall plate on the outer half of the wall by chance?

If it's as this sketch, you would be OK cutting a triangle off the top corner sufficient to clear the battens. As RR said, there is only shear stress to consider and it will be low.

If you can get left with 3" or so vertical at the very end, you should be well-OK.

(sketch doesn't show ceiling joists -leave those in).

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• tony1851
• 5 May 2015
• 1

 

[antonyc]

Is your wall plate on the outer half of the wall by chance?

If it's as this sketch, you would be OK cutting a triangle off the top corner sufficient to clear the battens. As RR said, there is only shear stress to consider and it will be low.

If you can get left with 3" or so vertical at the very end, you should be well-OK.

(sketch doesn't show ceiling joists -leave those in).

Thanks Tony,

The wall plate is in the inside. The sheer stress would be very high in my case because the pitch is almost 40 degrees. Of the 7" height of the new joist, there would only be a 4" bearing and at no point of that bearing would it hold the full 7" height, it would hold only 4" height and even after it has cleared the wallplate/bearing, it still wouldn't fit.

here is a pic...

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• antonyc
• 5 May 2015
• 1

 

[tony1851]

Why are you notching the underneath of the joists???

Just sit the joist on top of the wall plate, and cut the triangle off the top. With the roof being fairly steep, that will help your situation.

Can't see why you want to take out the ceiling???

 

[antonyc]

Why are you notching the underneath of the joists???

Just sit the joist on top of the wall plate, and cut the triangle off the top. With the roof being fairly steep, that will help your situation.

Can't see why you want to take out the ceiling???

Hi Tony,

I can't sit the new joist on the top of the wall plate because the triange is way too large. It wouldn't be a case of cutting a small triangle out of it. I'd have to cut a huge triangle out of it.

Here's a pic which is to scale and you can see what I mean. Red is the wall plate, yellow is exisitng joist and green is the new joist.

Look at how servere the cut is!

 

[tony1851]

Why can't you cut it like this?

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• tony1851
• 6 May 2015
• 1

 

[antonyc]

Why can't you cut it like this?

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• tony1851
• 6 May 2015
• 1

Hang on a sec. I've not drawn the image correctly.