Floor joists

[Callum]

Hi,

Can anyone help me with the following problem. I'm currently renovating a 3 storey terraced house, and have moved onto the top floor (see diagram below). The top floor is in the roof, but isn't an attic conversion as there is an original dormer window to the front. The problem I have is I want to create more space by knocking down a wall (see * in the diagram below - it's only a lathe and plaster wall, non load bearing) and taking the room into the eaves. This should be easy, but where they used 6x2 joists (see 1) in the room, they used 3x2 joists (see 2) in the eaves. On top and across of the smaller joists runs a 6x2 joist, which I guess is to stop them sagging with the weight of the ceiling below. This joist is supported by the outer wall and the last 6x2 joist. I guess this joist will have to come out eventually as it's 3 inches higher than the floor in the room (3 inch joist below + 6 inch joist = 9 inches vs 6 inch joist in the room)

How can I re-inforce these smaller joists so they can take the weight of people moving around (although they are in the eaves, there is enough head room for a bed, desk, etc). I'd also like to be able to keep the whole floor at the same level.

I really appreciate any help.

Cheers

_________________________ | | |22222222221222222222222| | | |2222222222122222222222_| | | |222222222212222222222| | | |222222222212222222222|__ | | |22222222221222222222222| |________ | |**1************| |111111111111111| | | |111111111111111| STAIRS | | |111111111111111| | | |11111 etc, etc | | |

--DOOR--| | | _| | | | | | | | |__ | | |________ ___| | | | | | | | | | | |___________| DORMER WINDOW

[Callum]

Sorry diagram should look like this:

_________________________ | | |22222222221222222222222| | | |2222222222122222222222_| | | |222222222212222222222| | | |222222222212222222222|__ | | |22222222221222222222222| |________ | |**1************| |111111111111111| | | |111111111111111| STAIRS | | |111111111111111| | | |11111 etc, etc | | |

--DOOR--| | | _| | | | | | | | |__ | | |________ ___| | | | | | | | | | | |__________| DORMER WINDOW

Callum wrote:

[Callum]

or not....anyone get the gist from the rubbish diagram? Try clicking view original, it seems to be properly formatted there.

[Set Square]

I think I've got a fair idea from the diagram - although ascii art isn't a very good way to represent it.

What's the max unsupported span of the 6 x 2 joists?

Am I right in thinking that the joists are at right angles to the roof rafters in plan view - rather than the more usual parallel arrangement?

What is actually holding the roof up - are there purlins, supported in the end walls? If not, are you *sure* that this stud wall is not structural, and is not helping to support the roof?

Assuming that the stud wall *isn't*structural, and that the span is ok for 6 x 2's, could you fix a new 6 x 2 to the side of each 3 x 2 - and support the ends off the end walls with hangers? You'd have to remove the 6 x 2 running the other way - but that shouldn't be a problem.

As always, the advice has to be to consult a structural engineer - but it doesn't hurt to have your own ideas first, as a basis for discussion.

[Rob Morley]

Just because it's a stud wall doesn't mean that it's not holding the roof up.

I think the simplest solution is to fit more 6x2 joists alongside the single one at the rear, then build up the floor at the front by three inches. But you probably don't want to lose the headroom. The best solution is to replace the 3" joists with 6" joists, but that would require new ceilings downstairs. An alternative is to build up the 3" joists to 6" by gluing and screwing additional 3x2 along each one and fixing the ends by bolting metal plates down the sides. Would this meet with building regs approval? Probably best to ask a structural engineer.

[John Rumm]

You may well be right, but don't assume that just because it is a stud wall it is not load bearing.

Oh, I think I have played this game before... ;-)

The way it is normally done is to ignore the smaller joists and insert ones of the required size interspaced between them so that the new ones run parallel to the old ones.

That does mean that you will need to cut the old tie beam however. The way to do this is one section at a time. before you cut a section you strap what will be the cut ends to something solid so that it can not move. After you have at least two of the new beams in place, you can then place a noggin between them, which you fix to the old joist to replace the function of the tie beam for that joist.

Here is one I did ealier:

and second photos on this page show what it sounds like you are describing. The tie beam was only a 4x2 in my case, but the same principle applies. The last photo shows the floor with the tie beam gone and its function replaced by noggins.

Two things spring to mind. Firstly doing this will probably need building regulations approval (and hence submission of a building notice etc). The BCO may want to see calculations to show the new floor will meet the required deflection limits etc.

Secondly, it is usual to insert new beams with a spacer under their ends so that they keep clear of the existing ceiling. This saves and chance of damaging the ceiling when they deflect. In your case however you may not want to do this since it would take the floor level up a bit from the rest of the joists. You may be able to get away slightly less deep (but perhaps wider) joists - although you are not starting with much.

[Tim Lamb]

In message , John Rumm writes snips.

Having lived in a bungalow converted to have dormer bedrooms I can confirm that secondary joists not fixed to the originals causes problems with the existing ceiling. In our case this was limited to the plaster skin popping off the nail heads but still a nuisance.

regards

[Dave Plowman (News)]

This is more or less exactly what I did to my existing attic room - the only difference being the dormer was at the back.

Ended up putting in a steel at one end, and new joists front to back sitting over the existing.

Think your best plan is to get a structural engineer in now, rather than later. The studding wall you describe as non load bearing almost certainly is, as it forms a triangle between floor joists and rafters.

[Callum]

Thanks guys for all your useful messages. I'm pretty sure the wall is non load bearing as the 'studs' are attached to a 6x2 beam running across the rafters (at about head height) which is bedded into the walls each side of the house.

I think probably the best course of action will be to install the 6x2s in between the 3x2s and attach them to the walls with hangers. Would I require an inspection from the builder officer for this, even though it matches with the rest of the room (and the house come to think of it).

Though to be on the safe side I'll probably be looking in the yellow pages for a structural engineer tomorrow!

Thanks again.

[John Rumm]

The trickey bit is that it will need to comply with current building regs. The old floor may not meet them. There would be no need to bring the old floor up to spec, howevfer the new one would have to meet the spec. Hence if calculations show that you actually need deeper beams to keep the deflection under the required limits you may have a matching problem!

You could have a play with the demo version of superbeam:

see what sort of answers you get first - that may give you a good indication of how difficult it is likely to be.

(You can just model one beam to start with - enter the dimensions, and a uniform load of say 0.8kN/m, turn on "load sharing", and see what happens. If you are having difficulty getting in spec then you can experiment with wider beams, better timber (i.e. C24 instead fo C16), or even flitch beams (i.e. with steel plates in them).

[Set Square]

How far apart are the end walls? 6x2 doesn't sound to me to be a very large section for a purlin. Mine are 9x3 and are supported by load-bearing walls half way along. Do you have any load-bearing walls *under* your joists? It wouldn't surprise me if one of the studs doubled as a strut to support the middle of the purlin.

[Callum]

Thanks John I'll take a look at the link.

Set square...the party walls are about 3m apart. There is a plaster and lathe wall directly below the joist supporting the studs. It's a very small house (2 up, 2 down with 3 floors!) so not sure wether the roof would require much support. Although the rafters look particulary flimsy!

[John Rumm]

OK, I just did a quick test beam in superbeam for you: 3m span, C16 timber of 50x150mm configured as a load sharing system. Assuming a uniform floor load of 0.8kN/m (i.e. the floor and no other dead load structural elements like walls etc)

Surprisingly that does actually meet the required spec. It shows a max deflection of about 7mm (the limit would be 9mm for a 3m beam according to building regs). The problem here would be potential damage to the ceilings. So I tried another test with 50x140mm timber. This just scrapes through with a 8.7mm deflection! However that would allow you to space the beams 1 cm higher than the ceiling level and hopefully they would keep clear of it.

Personally I think I would either switch to C24 timber of 50x140mm (7mm deflection) or alternatively stay with C16 but go for 75x140mm beams (under 6mm deflection) to allow more margin for error.

(Note you may have difficulty buying 140mm high beams since this is not a standard depth - however 150 could be thicknessed down if required)

This may make life easier if it is currently load bearing since by the look of your diagram is splits the span in half.

As long as you don't go hacking out purlins or any other stays that support the rafters your ought to be no worse off. You will need to insulate and clad the underside of the rafters, which will add a little weight though.

What depth are the rafters? Is there under tile felt (i.e. sarking)? Are there any soffit / ridge vents?

[Set Square]

I would be more interested to see a superbeam calc for the purlin - having to hold the roof up all on its own, once the stud wall under it has been removed. [I may be wrong, but I suspect that the purlin is currently supported all along its length by load-bearing walls which go all the way down to ground level].

[John Rumm]

Yup, re-reading the OPs description I think you could be right. If so, then yanking the wall away will leave the purlin carrying a uniform load of something like 4kN/m (assuming a tiled roof). According to the program, that will be a fail in bending, shear, and deflection (deflection of something like 4 inches!).

The simplest fix seemed to be to turn the purlin into a flitch beam by slapping a 6mm steel plate against it, followed by another 50x150 C16 timber beam and then bolting the lot together (12mm bolts, every 600mm, alternate sides, 50mm in from the edge).

[Dave Plowman (News)]

Wouldn't that be a bit unusual in the normal two rooms per floor house?

[John Rumm]

It is quite common to have a stay somewhere in the middle of the purlin to brace it, however that function may be being satisfied by the wall transferring the load to the floor of the loft (or more accurately a beam spanning the width of the house under the floor).

It would be interesting to know what kind beam is under the wall marked with "***1*****" on the diagram.

I had a similar problem when I did my loft, there were purlins on three sides mid way along the span of the rafters. Two of these were going since the pitched roof was also going (gable wall on one side, and dormer on the rear). The front one however needed to be replaced with a dwarf wall to support all the front rafters.

If you look:

original purlin can just be seen here (with the fluorescent light on it). The stay runs obliquely down from roughly the middle of the purlin to a the supporting wall in the middle of the house.

In cross section you had:

/\ / \ / \ / \ / \ /P\ /p\ / \ / \ / \ / \ / \ / \ ## ## ##

Where "p" indicates the purlin running into the axis of the page.

The replacement was (with d indicating the dwarf wall):

/\________________ / # # / # # / # # / # # / # # # / # # # / # # #\ / #_____________#_________#__\ ## # ## ## d

See the first piccie after the "Finishing the Front" section:

[Callum]

I had a chat with a builder who's doing the same thing to a house a couple doors down. It's slightly different as they dont have the dormer window, it's flat fronted so theres slightly more headroom on the top floor.

Basically they were told by the council that the minimum size beam that could be used in their case was 8x2. These were run parallel with the

3x2s and doubled up round the chimney (to support the beams directly in front).

You were right in thinking the 6x2 purlin was supported by the studs so they had to strap on another 4x2 beam to strengthen it. The actual span of the purlin was in fact 3.6m (from party wall to party wall).

Unfortunately all this means I'll have a two level floor but without taking out the ceiling below I'll have to accept this.

I forgot to ask the builder about bolting beams together, is there a standard way of doing this (i.e. what size bolts, the spacing of these, etc)?

[Dave Plowman (News)]

What they did with mine was to put in two new purlins (approx 1/3 way along) and remove the old to give a 'flat' ceiling down to the new stud wall.

[John Rumm]

If you can show calculations to the council to demonstrate that your choice of beam is sound then you should be able to use whatever "works". Using 8x2 however would not be a major hassle in the scheme of things since you could simply lift the existing floor and add a 2x2 on top of the beams to rais the height to match the new bit.

The would suggest that either the roof construction is lighter that I assumed, or it has some extra support you have not mentioned, or, they have gone for a solution that is not really up to spec!

According to the calcs I did, even a 12x2 is not big enough, and that was assuming a 3m span not 3.6. Hence why I suggested a flitch beam to get the required strength.

Unless you prove the smaller beam is ok in the back, or pad the beams in the front.

If you are bolting, then 12mm bolts every 600mm on alternate edges, 50mm in from the edge. Square plate washers under the nut and bolt head. If it is a timber to timber joint (i.e. no flitch plate) then a dog tooth timber connector goes on the bolt between the two bits of wood.

Sometimes when you just have a double side by side joist you can simply nail them together.

If you go for any other bolt size then the costs go up rapidly because the dog connectors and plate washers that are commonly available are only in 12mm usually. So you would end up having stuff made or at least adapted.