Stiffening Joists

I have an existing loft where the joists are Imperial and measure 7" x

2". The unsupported span being 4 Metres. I would like to raise the loft roof and use this area as a new room. The 7" x 2" joists seem to be right on the limit for a 4 Metre span. How does it sound if I stand 4" x 2" timber on top of the 7 x 2's to make a composite joist of 11" x 2". I would use 'gangnail' plates at say 400mm. spaceing to join the two joists together. I can afford to loose 4" in room height with the 4" x 2" stiffening joist. Does this sound a kosha method of doing it?

Regards Chris.

Reply to
mcbrien410
Loading thread data ...

Why? I'd just enjoy the extra ceiling height and the fact that you are lucky enough not to need to spend time/money fixing the floor. If you want to use up excessive height, stuff more insulation into the ceiling. At least this provides a substantial benefit.

Christian.

Reply to
Christian McArdle

Christian, you rekon that the existing 7 x 2's are OK for a 4 metre unsupported span? Chris.

Reply to
mcbrien410

BCO says I need 8x2's for 3 metre spans at 400mm centres. That makes your floor or mine wrong.

Paul

Reply to
Paul

AFAIK, should be 9 x 2 to pass the deflection requirements. You could add more rafters between the existing ones. Only works to a point, but should be OK just to stop the plaster flaking off the ceiling below when you take the piano up there. Disadvantages is the floor and ceiling below are not isolated as they would be with a normal loft conversion type flloor. Simon.

Reply to
sm_jamieson

I need the exactly length, width and the spacing of the joists, as well as the exactly depth of the timber. Tables are published for 170mm and 195mm depths.

For example (assuming light flooring material and no imposed loads, like walls):

At 400mm spacing,

50 x 170 is OK to 3.91m 50 x 195 is OK to 4.47m

Superbeam can be used to interpolate. If you have exactly 7" by 2", (51mm x

178mm) on 400mm spacing, then it will probably be over 4m. You need to get the measuring tape out and be exact. You also need to be really exact on the span. "About 4m" is not enough!

Even if the span is too long, there might be solutions if the failure is really marginal. They may include pleading with the BCO, or adding additional joists between (especially if current spacing is 600mm).

Christian.

Reply to
Christian McArdle

Assuming no imposed loads like walls or water tanks, a 8x2 (50x195 SC4) joist is good for a span of 4.47m or for lower quality wood 4.31m (SC3).

A 3m floor joist with no imposed loading with 400mm spacing only requires

50x147. (2" by 5.5").

There's no harm in having a larger joist, but if headroom is limited, you don't want to waste it.

Christian.

Reply to
Christian McArdle

I should mention that these are for SC4 wood. I probably should have suggested the SC3 figures, which are slightly shorter.

50 x 170 is 3.77m. 50 x 195 is 4.33m.

Christian.

Reply to
Christian McArdle

| I have an existing loft where the joists are Imperial and measure 7" x | 2". The unsupported span being 4 Metres. | I would like to raise the loft roof and use this area as a new room. | The 7" x 2" joists seem to be right on the limit for a 4 Metre span. | How does it sound if I stand 4" x 2" timber on top of the 7 x 2's to | make a composite joist of 11" x 2". I would use 'gangnail' plates at | say 400mm. spaceing to join the two joists together. I can afford to | loose 4" in room height with the 4" x 2" stiffening joist. | Does this sound a kosha method of doing it?

I haven't done the calculations. but would extra 4*2s between the existing

4*2s work and give you extra height. Joists do not *have* to be 16 inches apart.
Reply to
Dave Fawthrop

My unsupported span is 4025mm. thats allowing for 12.5mm. plasterboard on both walls. The distance between joists is 400mm.

When I've been in the loft and bounced on a single joist I can feel a bit of springyness. That's why I thought of putting the additional 4" x

2" timbers on top of the existing joists. The loft is NOT floored at the moment and the 'dwangs' (noggins) are not full height either. I can rectify that OK.

Chris.

Reply to
mcbrien410

The usual way to deal with the floor joists in a loft conversion is t install suitably sized new joists alongside the existing. You would si them on 18mm ply packers on the wall plate to avoid any interferenc with the existing ceiling. Will you need to remove any struts o bracing from the roof, if so these will need to be replaced wit something else, usually low stud walls built of off the new floo joists. Adding 4" timbers on top of 7" ones doesn't make an 11" joist,the tw timbers aren't acting together and dont have the same load bearin capacity, a fairly "normal" joist size for an "average" loft conversio might be 9" x 2". You may well need to get a structural engineer to have a look at it i you are doing a "proper" job, if you just want to lash up a bit of pla room in the loft, well who knows

-- Nick H

Reply to
Nick H

Christian, the existing joists are 6.75" x 2" rough sawn and the unsupported span is 4025mm. As I said it is no problem loosing 95mm of height as I'll be raising the roof anyway. Another alternative would be to put a 45mm x 170mm alongside the original 7 x 2 and bolt them together at say 600mm steps.

Chris.

Reply to
mcbrien410

The message from Paul contains these words:

How standards change. I have a 1965 architect and builders diary that gives 10' 3" (3.1m) as the allowable clear span for 6" x 2" joist at 16" (406mm) centres, 12' (3.6) for 7" x 2" and 13' 11" (4.2m) for 8" x 2".

Reply to
Roger

Nick, that is what I would prefer to do. By bolting another 7" x 2" or metric equivalent, alonside the existing 7" x 2" joist to make a sort of 7" x 4" joist. I tend NOT to use nails, except for facings/skirtings, I would use M8 bolts at 600mm spacing to join the two timbers. The difference in height between the imperial and metric joists would allow the use of 18mm ply, as you suggested, on top of the existing wall plate.

Chris. .

Reply to
mcbrien410

Of course they had better timber in those days ...

:-)

Reply to
Rob Morley

Rob, you are dead right there. This C16 stuff seems so light. Recently I had some short 45mm x 175mm off cuts. I was able to just split them with my bare hands along the length. It would have made a great Karate stunt. The other Redwood that I've used is so much stronger and straighter.

Chris.

Reply to
mcbrien410

I'd look at putting another set of 7x2's in the gaps between the existing joists.

The joist size is a function of both length and spacing.

Rick

Reply to
Rick

Yes, the existing aren't quite up to it, then. A bit less than 7" and a bit more than 4m is not quite good enough!

You could bolt 195mm x 50mm to the sides of the existing. They're strong enough to carry the load on their own and it means that you only gain about

30mm in height and don't need to disturb the ceilings below.

You may have guessed that I believe you can't have too much height!

Christian.

Reply to
Christian McArdle

I'd probably use 6 x 2's so that I wouldn't interfere with the present ceiling and wiring etc. Next question is how to get them up there and in place!

Reply to
Fred

6x2" might not be deep enough. The span needs 195mm. A 170mm backed up by a 147mm probably doesn't provide similar deflection resistance.

Christian.

Reply to
Christian McArdle

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.