Another loft floor strengthening question

[Zipadee Doodar]

I need to use my loft for storage and want to fit an independent floor to avoid the ceiling cracking. I have read the various posts on strengthening joists, and would like the panel's opinion on the following: new 7x2 joists, 14ft long, supported at centre on dividing wall, and at ends by timber straps from purlin, and a new cross member from party wall to supporting wall. I am thinking that the stresses will be taken up by a kind of cantilever effect (I have NO tech knowledge at all btw). The new joists would be a smidgeon above the ceiling. Any advice gratefully received, especially if it avoids the ceiling crashing down.

Bad ASCII art below to explain.......

10x6 purlin [ ] [ ] [_] [_] ! ! ! ! ! ! proposed 3x2 dropper ! ! ! ! ! ! ! ! ! ! _____________7ft_________________________________________ [ ]__________________________________7x2 new joist_______[ ] 7x2 timber ! ! ! !supporting wall

ZD

[BigWallop]

So you literally want to hang a new floor from the roofing timbers? Or are you just needing more joists set between the existing ones?

If you place more joists of the same dimensions as the existing ones, then you will increase the loading capability by just the act of literally decreasing the spacing of the joists. Why go to all the bother of trying to install a new sub-floor when it isn't needed?

[John Rumm]

Not sure I quite follow this....

Firstly, what sort of house? Sounds like a Semi from the mention of party wall.

Where do the supporting walls run? (i.e. Front, middle, back, and non party side)?

What are you storing? How big are the existing joists?

Is that 14ft total length, one 14ft joist from front to centre, and another from centre to back, or, is it 14ft front to back with support in the centre at about 7ft?

Purlins are strong, but may not be that strong.... Think of how they stay where they are - typically it is with the aid of one or two smallish beams that prop them up. These beams acting in compression - with one end on a supporting wall. The purlins often pick up the mid span of the rafters to stop the slope of the roof sagging. It is often the weight of the roof plus the fixings to the rafters acting against the push of the prop that keeps the purlin in equlibrium. Load it too much and it will want to pull down and away from the slope of the roof.

If you are running a joist perpendicular to the others from supporting wall to supporting wall (i.e. a stringer that will carry the new floor joists), why to you also want to strap it to the purlin?

How long will this stringer be?

Would you be able to run right up to the front wall instead (even of it ment cutting the ends of the joists at a slant so as to fit under the eves of the roof?

A tad above the ceiling sounds good. (1" is often used). The loading on the purlin sounds a bit doubtfull.

[Zipadee Doodar]

Hmmm. didn't come out the way it went in......

ZD

[Zipadee Doodar]

Yes, semi, about 1930s

supporting wall between 2 bedrooms would be at centre of the 14ft joists, i.e. the distance from joist to purlin would be about 7 ft. Another supporting wall runs (opposite and parallel) to party wall. This wall separates bedroom from landing which is 3 ft wide. The another parallel wall for bathroom. Finally, external wall another 5 ft away. Don't know if this will come out right but....

external wall [] bathroom 5ft [] landing

3ft [] bedroom 12ft [] party wall The intended usable area would be above the bedroom.

strengthening

general household junk, plus stacks of videos that wife wont part with. joists are 2.75 x 1.25 inches

No other beams. the purlins run straight across with no intermediate support.

Thought it might give extra support, but if not necessary......

Would this be necessary, since the usable area would terminate above a dividing wall anyway. This would mean the stringer would be 12ft, and supported at one end at the party wall, and at the other over a dividing wall.

Sorry if this is clear as mud,but a diagram would make it all clear. No luck with ascii art though, sorry.

ZD

[John Rumm]

OK, 3 or 4 bed? what sort of layout?

Hmmm - not conviced the diagram helped much ;-)

So you are only doing a new floor for about a quarter of the total loft space?

You may find 7x2" is larger than you actually need in this case.

So you have a gable wall on the non attached side of the house rather than a hipped roof? (i.e. the purlin sits on/in a gable at each end, and the roof slopes front and back, but not on the non attached side))

It may, but you only need "enough". Once you have enough adding "more than enough" if it requires extra construction is just over complicating things and putting the price up.

The simplest extra floor joist is one that sits on a load bearing wall at each end.

|| || || ==||==||==||===

[Christian McArdle]

For successful ASCII art,

1. Don't use tabs ever, anywhere.
2. Don't use > 70 characters wide
3. Ensure you have a fixed width font (i.e. Courier New) selected.

Christian.