I'm fairly new to joinery on a builders scale, this is a bit beyond putting shelves up! so please forgive what may be an obvious question to some...
How to fix joists so they are stable in the middle of their span without wanting to "wobble"? In other words should they be fastened to the supporting beams underneath them somehow or just left to "float" on them? More detail...
I'm replacing the attic floor that is 6 metres wide by 9 metres long. After removing the ancient floor (tons of clay and shingles) I'm just left with four massive oak beams (literally tree trunks) spanning the 6 metre width. These beams are built directly into the stone walls. On the advice from a joiner friend I've bought 30 joists (5 metres x 50mm x 150mm) to lay at 40cm spacing. The joists will overlap by approx one metre in the middle of the 9 metre span. The far ends of the joined joists will be cemented directly into the supporting stone/clay walls, so this should hold them upright at the ends. Between the walls the joists just rest on the four big beams, with a little packing here and there under them to get them level. Should the joists actually be fastened down to the beams some how? Perhaps some noggins nailed between the joists would keep them "upright" and wobble free in the middle of the 9 metre span? Any other suggestions?
look at herring bone or solid strutting,we use solid strutting most of the time,have seen flooring fail where plumbers ,have dislodged a herring bone to put in the waterpipes
The message from David in Normandy contains these words:
When I replaced the hay loft in my 18' x 33' barn many years ago the joists ran longways so I put in a couple of RSJs to replace the rotten wood transverse beams which had been propped in the centre. IIRC the joists are 7" by 3" with a fee span of about 12' either side of the entrance. The joists overlap on the RSJs and I used off cuts at right angles to keep the joists upright. Gravity holds the joists down. The only doubt I had was that the RSJs were a bit flimsy but so far at least they show no sign of buckling.
Hi David, there's quite a few points to raise, so one at a time...
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OK. I'm assuming this is going to be a habitable space?
Floorboards on top of the joists and plasterboard below?
Sounds about right. I would check the sizes/spans against the tables in the (old version of) Part A structures of the Building Regs. I know this is in France with different regs, but the approved documents contain a lot of good sense.
I'd be inclined to fix them together before placing them, so they're dead straight (i.e. use a 3rd joist as a straight edge) by putting in lots of screws (for example 6*80mm turbogold - buy a cordless impact driver). Then place them, then pack underneath them as necessary.
I'd guess the oak beams are pretty uneven, and you have to pack the joists both for beam contact and to bring the joists level with each other? You might want to go round with a lighter (straight) timber to figure out the high and low points both across and along the beams.
Hopefully you've bought regularised joists (very straight, rounded corners), with an appropriate strength grade (C16 or C24) - and they've been preservative treated. Sounds like you won't have to cut your joists at all, so treated end can go against the walls. In the UK you would be required to fit plastic boots to the end if they're going into masonry as additional rot protection - or use steel joist hangers.
Maybe you have existing provision in the masonry for the joists? If not, joist hangers will be easier.
OK
Yes! Fix the packing under the joists to the beams, and/or fix the joists (through the packing) into the beams. 2 screws at every intersection, from either side, and skewed so they can't tear out easily.
Yes! 3 methods. Steel herringbones (cheap, very easy - recommended), wooden herringbones (requires a bit more work/skill), solid noggins (must fit tight, useful on odd joist spacings).
If joist twists even a little out of upright it looses a lot of it's strength and rigidity.
For that sort of span, probably 3-5 rows of herringbones (these really help stiffen the floor and kill resonances - as well as the essential anti-racking. Use the steel ones, quick, easy - and you'll be glad you put in lots of rows).
Floorboards should be nailed to the joists with nails 3x the thickness of the boards. 2 nails at every intersection.
Remember wooden floors brace the masonry walls, as well as the walls supporting floors. They need to be firmly fixed together for overall structural stiffness.
I'm a bit concerned that you're started on something of structural importance without being aware of relevant building regulations or common practice. A good place to learn more would be the standard city and guilds books on carpentry and joinery - along with the building regs (or appropriate French equivalents).
Dave I agree with Dom that you do not put the joist into the walls. Use joist hangers and keep them clear. If you insist then Dom's plastic boot (IE any thin poly membrane to isolate timber masonry contact is essential. You will need lots of packing and I would not worry too much about levels - go with the flow of the timbers under! Chris
Yes. But putting down floorboards is likely to a year or two down the line.
I've checked against the table in the link posted by ALex and the timber and spacings seem to be well within tolerance.
Is a cordless (or mains powered) impact driver different to a drill? Up to now I've been putting in screws for all DIY with a standard Bosch mains drill, though it gets a bit more difficult to hold when putting in large screws, it tend to jump out of the screw slot without lots of force.
I've used a laser level to find the highest point on the highest beam and will use this as the "base" point and pack all the joists to the same height as this. The beams are very uneven, in places the packing will need to be up to around three or more inches.
As far as I'm aware the joists are building regs compliant. They are preservative treated already. The plastic boots sound like a good idea, I'll look to see if they're available in France. Joist hangers aren't really an option because the walls are made of a mixture of rocks of different shapes and sizes including very hard granite and the mortar between them consists of a mixture of clay and straw. Difficult to put any sort of plugs into the wall, the clay is too soft and granite too hard. There are existing holes in the wall so I intend to use those and use cement mortar to fix the joists tightly into place.
Solid noggins sound easier to me.
I acknowledge I'm a beginner, but I'm trying to do a good and structurally sound job and seeking advice with every step. I have a reputation amongst family and friends in DIY for very solid and over the top construction. If in doubt I tend to over engineer, which is perhaps not the most economical approach but certainly the safest and strongest.
Good idea about the C&G joinery books, I'll see if I can find any to buy online. This sort of work is beyond the DIY books I've got.
I hate skew nailing, so I'd look at some "builders metalwork" solutions, e.g. Toolstation's
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the proper nails, e.g.
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mention Toolstation rather than Screwfix because the last lot of framing anchors I bought from Screwfix were very flimsy affairs - much lighter gauge than the ones they used to sell. No guarantee that Toolstation haven't gone the same way.
See the diy faq. Cordless impact drivers are designed for *exactly* what you're doing. They will be a revelation and mean you won't hesitate to add extra fixings.
Yup. Seen that sort of thing. You might find it easier to cut out some "saddles" (i.e. jigsaw to cut out an appropriately weirdly shaped piece of wood) and fix these in place. Then use flat shims to get the support under the joists just right.
Fair enough.
Traditionally, solid noggins were considered inferior to herringbones. And they must fit tight. However with large screws pulling up an irregularities, maybe it's neither here nor there.
Sorry if I sounded sharp previously. I'm sure you'll make every effort to achieve maximum structural integrity.
The other option to noggings is to use herringbone straps - either smaller wood offcuts (say 2x1") or the shop bought metal straps. The metal straps are easy to fix since they just nail on.
the mortar between them consists of a mixture of clay and
If you make a very dry pack mortar mix (just damp sand and cement is usually enough) and ram it well in, then you should get little if any shrinkage in the mortar.
Note that rather than skew nailing them, you can just stagger them a few inches such that you nail through the joist side into the end grain of the nogging.
Steel herringbone is the easiest. Unless noggins are cut very accurately, and the joists don't shrink(very unlikely), herringbone strutting greatly stiffens the floor, much more so than noggins.
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