Loft project Posi joists attaching to rafters?

[arborexplora]

Hello all, I'm new to the forum and this is my first post so apologies if this has come up before.

I am not a builder by trade but last year completed a large extension on my house without any trade assistance (apart from elec, I did the CAD drawings and BC spec myself also). I added three rooms and am very happy with the result, so I would consider myself a competent diy'er with a big emphasis on preparation and planning.

Now that is complete I am planning a loft conversion, which again I intend to do myself (obviously a BC notice will be submitted). I want to avoid steels if I can.

To get the headroom I am going to drop the first floor ceilings

The existing 2" x 4" joists that hold the ceiling do not sit on wall plates and are ties approximately 1/8th of the way up the rafter (probably so the could use 4.8m length timbers). As the clear span is large (5.2m) I am intending to use Posi- Joists etc for good strength to weight.

I want to sit the new Posi joists onto the wall plate which will mean dropping the ceiling 200mm, by the time the Posi 's are in I should gain 100-150mm headroom, which would be acceptable for a teenagers bedroom.

So far straight forward, question is once posi's are in, is there a fixing I can use to attach them to the foot of the rafter, so I can remove the existing 2" X4" ties? Or can I nail the Posi as with timber joist?

 

[Deluks]

Sounds tricky, I don't think you will be able to prevent rafter spread once you've taken the ties out, I think steels are an inevitability without things getting messy.
You might end up with some sort of cranked steel replacing some of the rafters.

 

[arborexplora]

Thanks Deluks,

I'm not sure I explained it properly, rafter spacing is 350-380mm centres with existing ceiling joists/ ties attached approx 200mm above wall plate all on same side of rafter (every rafter full width of roof). I would like to attach the Posi joists to the opposite side of the rafter (every rafter) bearing on the wall plate and fixed to rafter, then remove original beams. So theoretically all I am doing is replacing the existing 2 x 4 with Posi beams. The beam would take the load for the floor and if attached to rafter foot would also prevent rafter spread. I am just unsure how the Posi can be fastened to the rafter, nails or special metalware bracket?

Simple solution is I could use 9 x 3, sit it on wall plate, nail to rafter the remove 2 x 4 (other ride of rafter), but if I could do the same with Posi then much easier to handle and manoeuver, the span is 5.2m.

 

[freddiemercurystwin]

How is the roof supported? Are there no supports for the roof that will need removing then?

I've never done a loft conversion without steels, not saying it can't be done, just thinking aloud!

 

[arborexplora]

We moved in about 10yrs ago, the previous owner part boarded it out and did remove some supports. As far as I can make out he took out 3 pairs of struts that left the purlins and attached to a binder at half span. I was obviously concerned but as there is no supporting wall below where the binder would have been the load wouldn't have carried well anyway? As the purlins sit in the block gables, I added vertical struts every second rafter onto new binders directly below the purlin. Appears pretty solid and no cracks since I installed them soon after moving in.

If BC is happy with existing roof structure and no additional supports required for the roof, I could get away with leaving all as is, drop ceiling to wall plate level, install new deep floor joists onto wall plate and as long as the top of the beams are above roof structure ceiling/ tie beams I would have two independent structures. This is probably the route I will pursue, any ideas, comments? Liked the idea of Posi or I joist for man handling.

freddymercurystwin- In your conversions do you usually drop the ceilings? I know in lots of conversions steels are used for great strength and breaking up wide spans so smaller timbers can be used so ceilings don't need to be dropped. I have the height at first floor approx 2.6m and don't mind replacing the ceilings as they are currently awful artex.

 

[freddiemercurystwin]

I would be surprised if BC did not want to see qualified evidence (ie structural engineer) to support your view that removal of the existing support consequently replaced with some studwork is structurally sound whether it has cracks or no cracks at this point in time.

I have never done a lofty where the ceiling was lowered but that is more because most people think that is a step too far and don't want the inconvenience/mess and their ceilings were too low to be lowered further, lowering the ceilings is certainly plausible if you have the necessary height and the inclination/will to do it.

 

[arborexplora]

I would be surprised if BC did not want to see qualified evidence (ie structural engineer) to support your view that removal of the existing support consequently replaced with some studwork is structurally sound whether it has cracks or no cracks at this point in time.

I'm sure you are right, but as I am proposing to to comply with the regs for a new habitable room, the plans I submit along with structural engineer calcs should correct all errors previously made. My point was I can't see that the previous system would have worked well, the struts fastened to the binder in the centre of the beam spam and with no support below surely the force of the roof is directed to the centre of unsupported beam which must be the weakest point. The 3 pairs of struts that were removed have been replaced with 13 pairs of vertical struts off purling onto new binder (26 beams- pair every other beam).

I'm not necessarily opposed to steels, I am just trying to weigh up various configurations and suitable systems, as it is very much in the planning stage. As previously mentioned I have now concerns about dropping the ceiling, but I don't want to loose any headroom in the attic. I would welcome any suggestions. If I can figure out how I will try and post a sketch of how I see it working.

 

[Static]

You will probably find that the posi-joists will be about 450mm deep.. which may cause detail issues when you hit the wall plate.. steels are uses to break down the spans so you get shallower timbers..

 

[arborexplora]

Hopefully the image below will explain better what I would like to do. I knocked it up quickly so its not to scale but I have put on the timber dimensions. The new joist member should be big enough for the span, according to span tables.

Obviously this is a solid timber joist, my original query was if I used a Posi instead is there a way of fastening it to the rafters in addition to sitting on a wall plate? (To prevent rafter spread).

Untitled

• arborexplora
• 9 Sep 2010
• 1