Floor joists connection to eaves

Question

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:

Update: I have followed this question on here:

How do I fit a joist in this small space?

Answer 1

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.

Answer 2

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.

Answer 3

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).

Answer 4

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).

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.