Sagging Rafters in loft conversion.

[MarkWoody]

Hi,
We are in the middle of a loft conversion and are in the process of removing the existing purlins and fitting a new 'purlin wall' lower down the roof slope. (the purlins are out and the roof is currently supported by 24 temporary vertical supports, one on each rafter)

To take account of the new longer unsupported span of the rafters the existing old 100x50 rafters are having new 150x50 rafters nailed alongside.

However, the existing rafters have sagged slightly over the past 80 or so years and the new 150x50 timbers do not fit alongside neatly.

Is this a problem? - is it OK to nail the new and old together even if the new rafters do not lie tight against the roof for the entire length.
I suppose that we are still giving strength to the old rafters.

Thanks Mark

 

[amwc]

Not really sure what you mean but the rafters have to go from the ridge board to the wall plate, to support the battons that the slates/tiles sit on. If you are taking out the existing purlins and moving the support further down, the new rafters will need to go from this support to the ridge board as long as it does this to support the roof covering, then strapping it to the existing rafters further down the roof than the old purlins were,will ensure the span between the wall plate and the new support is smaller than the distance between the wallplate and the old purlin.

 

[MarkWoody]

Thanks for the reply.
I know the new rafters have to go from the ridgeboard to the new purlin wall, but the problem is that the existing rafters have sagged over the years so when the new deeper (straight) rafters are placed alongside they dont line up all the way.

Ie. if they meet at the ridge board they are about 2 inches out at the purlin wall and vice versa.

Maybe I'll place them alongside at the middle of the span and accept a 1 inch difference at both ends.

Does this matter so long as I can nail them securely together.

Thanks
Mark

 

[amwc]

Its hard to say without seeing it obviously,but as long as the last,new, support up the rafter creates a smaller span than the old distance from purlin to ridge board it should be ok.

 

[noseall]

Sounds like a classic case of re-build the roof and do away with the old, or is this the case?

If it were me i would be scaffolding the outside, removing the entire roof and rebuilding the thing from new, thus removing all the old tired roof members.

 

[amwc]

Yeah in the perfect senario this would be the best option. Especially if it needs re tiling as well.

 

[^woody^]

You'll just have to try and slot the new rafters in as best you can. The existing rafters wont push back up now.

Don't just wedge the new rafters up tight next to the underside of the felt. Try and line the bottom edges of the new rafters up, or the plasterboard will be all wavy. So there will be varying gaps

 

[Deluks]

The new joists are to stop the 4x2 from sagging further. This happens mid span, so if the ends aren't quite connected up it shouldn't really matter, as there will only be a very short section of the 4x2 taking the loading, which shouldn't bend or sag at all.
If you have to sit them further in and lose an extra inch of room space, then so be it.

Will there be collar ties at the top beneath the ridge? 'chopping off' the point where rafter meets ridgboard and giving a flat ceiling?

 

[MarkWoody]

Yes there are collars to be fitted.

I've got one new rafter in now and tried to centre it and all looks ok - slightly out at the top and slightly out at the bottom. Its still nailed to the old rafter along its length so it should give the extra support.

The biggest problem is going to be lining up all the new rafters and ensuring the new sloping part of the ceiling it level.

Thanks

 

[Deluks]

Yes there are collars to be fitted.

I've got one new rafter in now and tried to centre it and all looks ok - slightly out at the top and slightly out at the bottom. Its still nailed to the old rafter along its length so it should give the extra support.

The biggest problem is going to be lining up all the new rafters and ensuring the new sloping part of the ceiling it level.

Thanks

Get a couple in at each end, then string lines across the middle, 3rd of the way up from the bottom, and 3rd way down from the top. Fix the rafters in line with these and keep checking with a straight edge to make sure they line up. ie: hold straight edge horizontally against rafters and make sure all of them touch it.

Or: If you have a straight length of timber to hand, you could temporarily clamp or screw it to the floor up against the bottom of the existing rafters, leaving say a 60mm gap. Then the new ones can sit flush against this so that they are all flush at the bottom. Any that won't quite squeeze in can be trimmed or planed until they do (within reason)

Then at the top, offer up another section dead horizontally until all the rafter tops are touching it. Probably best to do this where the collars are going.

 

[GaryJ]

Hi,
Im converting my loft and although I ain't removing the purlins I have noticed nothing up there is inline or level at all. the rafters have sagged a little over the years, probably only half an inch or so, but none of them appear to be inline at all. I embarked on an idea of getting 12mm ply cut into 70mm wide strips. I then screwed 10mm x 25mm timber to the ply making it L shaped in cross section and fitted them on either side of the rafters(sistering). This provided 2 functions, I would have a straight underside edge under the rafters eliminating the sag. Also the 60mm kingspan fitted snug in between and couldnt be pushed to far back towards the tiles so maintaining the air gap. And then because the rafters aren't vertical I was thinking this could be a nightmare trying to get backed plasterboard to line up with a rafter when butt jointing so am now thinking cross battening with 25mmx35mm underneath the rafters/ply which will provide perfect horizontal battens then fit 25mm kingspan between and then foil backed plasterboard underneath, everything should then be hopefully flat and level.