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Apologies if this is a duplicate.

I'm attempting to floor my loft space. It currently has 47mm x 75mm ceiling joists, not enough for a floor. I am going to add something like 47mm x 200mm along side each existing ceiling joist, resting on the existing brickwork.

I am not going to be sistering these together, the existing joists are old and already bowed. I may perhaps bracket them together in the centre span to avoid any more sag on the old joists. The old joists used to have vertical planks nailed to the rafters above to achieve this, but they have since degraded.

The problem I have is this: I have flush eaves, so the vertical space over the outer brick skin (at the eaves) is limited and certainly not enough to accommodate an 8 inch square joist end. Are there any specialist joist extension plates (steel) I can use to rest on the brickwork, or is it safe to cut a triangle off the end of the joist without affecting the shearing resistance?

Extremely crude drawing: enter image description here

Update: I have followed this question on here:

Fitting a joist in this small space

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have you got approved Building Regulations for this work? –  Mike Perry Jul 26 '11 at 0:49
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@Mike Perry Not yet, but I'm not making it a habitable room, so it only needs to pass inspection. Either way, I'm in contact with building control in my council, so they will advise me where needed. –  Adam Houldsworth Jul 26 '11 at 7:38
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4 Answers 4

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This will impact your shear resistance, but it significantly depends on how much of the joist is cut away and how much is resting on the brick. If the roof is so steep that the entire width of the joist is supported several inches deep, it shouldn't have any impact at all. But if it's so shallow that the joist is only an inch or so wide over the support, then of course it will fail. Unfortunately I don't have the specifics on how much support you need (I'm not a structural engineer) but it wouldn't hurt to get the opinion of an expert on this.

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I would have to disagree. Once it is properly nailed to both members of the truss it becomes a friction issue. Which is why a lot of times, when people build these rooms they cut off the angled end completely and put an upright to the top of the truss. As long as you nail it to both members of the truss, the shear on the wall is a handled by the truss. –  Tatton Chantry Jul 25 '11 at 15:57
    
@Tatton: from the question, Adam said he wouldn't be sistering the joists together. If that's the case, then there's no transfer of load to the existing truss. Unless I'm misunderstanding the question. –  BMitch Jul 25 '11 at 16:58
    
@Mike Perry I was trying to avoid attaching the new joists to the rafters at all. The existing roof structure is not being touched. –  Adam Houldsworth Jul 26 '11 at 7:41
    
@Mike Perry I've decided to go for raising the joists (now to the height you specify) and connect the outer end to the rafter via a bolt (as advised by my local building control). I'm still undecided about connecting them. The existing joists are fine for the ceiling, and the new joists are fine for the floor - I see no reason to connect them as they won't help each other. –  Adam Houldsworth Jul 26 '11 at 14:55
    
@Mike indeed that is hopefully what I'm gunning for. I will be connecting the joists to the rafters, as you say - is bolting them with some washer plates is the way to go there? –  Adam Houldsworth Jul 26 '11 at 15:08
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After reading the question and all the comments, I have to add a few things.

If in fact what you have are trusses not joists, then you need to know if they are load rated for a floor. Even if the space is only for storage, a rating of 60 to 80 pounds per sq ft is required. Storage can often become a heavier load that a person just walking on the floor. Also if they are trusses, then the horizontal 2X4 or whatever is used is called a collar tie, not a joist. The key here is find the load bearing surface and size the new joists for the span. In this application with a limited support area will involve the height of the joist end supported. Often a ledger and post may be needed to bear the weight of the joist end. What is needed to really answer the question are accurate dimensions of vertical angle area above the supporting wall, span of the proposed floor etc. The trusses in this question are probably designed to carry a roof load only, not a live load from a floor. Practically speaking, often sistered joists to collars will carry substantial loads, but the real question is how much and will they meet your local codes. A real concern is the fact that you have an existing ceiling, and any flex from weight above will effect it.

My advice is to get your facts, measurements, and a professional opinion together before you do anything. Photos would help, but without doing the span math etc., no one can give you a viable solution from the info we have.

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Thanks for the info. The timber beams are definitely joists, not trusses. The existing roof frame and ceiling are being left untouched, the new joists will sit along the same brickwork. As for the stress on the ceiling plaster work, I was going to raise the joists to sit a handful of inches higher - there will effectively be an air gap. With the loading calculations, I have local authorities helping me determine what size joist I need. My main question was simply about the shearing stress changes of having to trim the joist to fit. –  Adam Houldsworth Jul 26 '11 at 12:50
    
@Mike thanks, I'll be able to figure out how much material needs removing soon (hopefully tonight if I can get girlfriend approval for the mess I need to make) If not much is removed, I won't concern myself further with it. If I end up needing to chop a lot off, I'll be back here with another question :-) –  Adam Houldsworth Jul 26 '11 at 15:05
    
@Adam, just a heads-up, I've removed my comments and put them instead into an answer. –  Mike Perry Aug 9 '11 at 16:55
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Q. Have you got approved Building Regulations for this work?

Adam: Not yet, but I'm not making it a habitable room, so it only needs to pass inspection. Either way, I'm in contact with building control in my council, so they will advise me where needed.

Re: "Side view, double skin brick, ceiling joist rafter" detail, is it possible to post photos showing this existing detail, both internally and externally? Also some dimensions would be helpful.

Adam: I'm attempting to floor my loft space. It currently has 47mm x 75mm ceiling joists, not enough for a floor. I am going to add something like 47mm x 200mm along side each existing ceiling joist, resting on the existing brickwork.

"Generally" speaking, 200mm x 47mm floor joists are considered minimum size in the UK. Of course if the span is (very) short, it's possible to go down as small as 150mm x 47mm, but that really is considered nowadays to be the absolute smallest size (for floor/ceiling joists) that should be used (in the UK domestic construction). Regardless, you should check and confirm new floor joist size with your local Building Control Authority before proceeding.

"Standard" practice in the UK is to raise the new floor joists 12.5 or 19mm (½inch or ¾inch) above the underside of the existing ceiling joists. It's also "standard" practice to place the new floor joists along side the existing roof raters and connect the two together, using carriage bolts, dog-toothed washers between existing and new, square plate washers...

Adam: I am not going to be sistering these together, the existing joists are old and already bowed. I may perhaps bracket them together in the centre span to avoid any more sag on the old joists. The old joists used to have vertical planks nailed to the rafters above to achieve this, but they have since degraded.

Seeing as you have removed the existing ceiling joists binder(s) your local Build Control Authority may require you to do something like the following:

  • Along the length of the new floor joists at 1.8m (6ft) centres, put in new timber noggins that sit above the existing ceiling joists and span between the new floor joists. Fix the new timber noggins to the new floor joists, then fix the existing ceiling joists to the new timber noggins.

  • Of course the above goes out of the window if you run the new floor joists butted up against (a face of) the existing ceiling joists. In that situation, local Build Control Authority may require you to directly connect the existing with the new.

Adam: The problem I have is this: I have flush eaves, so the vertical space over the outer brick skin (at the eaves) is limited and certainly not enough to accommodate an 8 inch square joist end. Are there any specialist joist extension plates (steel) I can use to rest on the brickwork, or is it safe to cut a triangle off the end of the joist without affecting the shearing resistance?

"Standard" practice in the UK is to place the new floor joists along side the existing roof raters and connect the two together, via carriage bolts, dog-toothed washers (sandwiched between the existing and new), square plate washers... Your local Building Control Authority should be able to tell you the size of carriage bolt (12.5mm /½inch is "normally" the minimum) they want to see and if they prefer 1 or 2 carriage bolts per connection.

The shearing stress shouldn't be a concern as the amount of material removed (triangular piece) shouldn't be that great in the area where the new floor joists sit on the inner face of the blockwork (100mm) or brickwork (102.5mm). It is "standard" for the inner face of blockwork/brickwork to take the load of the new floor joists, unless of course there is something "funky" going on that I'm not aware of, or if your local Building Control Authority insist on you "floating" over the inner blockwork/brickwork and instead bear down onto the outer blockwork/brickwork.

I say the above with some certainty as I've designed (calculated) this detail at least half a dozen times (submitted to and passed by UK Building Reg's) and I've also actually done this work on site a number of times (during 5 years working for a building as a Saturday/Holiday boy and 3 years full-time with the same builder).

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thanks for collating the comments into an answer. It, like many jobs around the house, is currently on the backburner while I make progress on other things. –  Adam Houldsworth Aug 9 '11 at 17:58
    
@Adam, no worries, I'm slowly learning it's considered best to do that kind of thing here on SE. If you have any questions when you get back to tackling this job, post back here, and we will try our best to guide you in the right direction... –  Mike Perry Aug 9 '11 at 18:03
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If you are asking what I think you are asking, then you should be able to cut the new floor joist/bottom of the truss and nail it in alongside the existing one (just like my green dotted line).

new truss bottom/joist

Just get a speed square and find the pitch of your roof. Then go 90 degrees off of that. Then you should have the exact angle and you can cut them and nail them in place.

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Hi, yes that it was I think needs to occur, however does that then alter the strength of the joist against shearing on the brick face? –  Adam Houldsworth Jul 25 '11 at 14:25
    
versus it being square? No, because it is nail to the horizontal and vertical parts of the truss. Then, the forces will be acting on friction more than the upright support. I am not an engineer, but I have seen this done a LOT, and that is the way it was explained to me. –  Tatton Chantry Jul 25 '11 at 14:35
    
Not sure I follow. By shearing stress I mean the forces acting downwards. The wood supported by brick is fine, but the wood hanging over the edge will try to shear away from the wood on the brick - all from the downward pressure from the floor contents. If I take a triangle out of the end of the joist, I alter the thickness of the part of the joist resisting the shearing. Not sure if I'm worrying about nothing :-) –  Adam Houldsworth Jul 25 '11 at 14:37
    
I understand what are saying but it's hard to explain what I am saying without pointing to it. The basics is that the new joist will be supported more by the existing truss than the wall. The wall will just hold up the whole thing. So, no that little triangle will not matter. –  Tatton Chantry Jul 25 '11 at 14:46
    
Ah ok. I can't rely on the existing ceiling joists as they are old, thin, and already supporting the ceiling plasterwork. What I'm going to do now is expose my eaves and see exactly how much space I've got, then draw a profile plan of the joist sat on the brick in relation to the existing woodwork. –  Adam Houldsworth Jul 25 '11 at 14:49
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