Getting rafters past the purlins

[Tony Bolger]

Our loft conversion is at the planning stage, architect has submitted the p lanning application, structural engineer doing calculations for building co ntrol.

I will be doing most of the work myself and employing various tradesmen as required.

The architect has specified deeper rafters spiked to the existing 50mm deep rafters to go from wall plate to new steel ridge beam without purlins, to be done without removing the existing slate roof.

I assume the new rafters will need to be in place before the old purlins ar e removed but I am looking for ideas of how this can be done without the ro of collapsing.

Is it a case of propping up the purlins from ground level, bracing the exis ting rafters and removing a sections of the 3 purlins to fit one new rafter at a time, gradually moving the props along and removing sections of the p urlins as I go ?

Or is there another way to get the deeper rafters past the purlins before t hey are removed, that doesn?t involve stripping off the roof ?

I will of course speak to the architect and SE about this but was looking f or info from anyone who as done this before, I will probably employ a roofe r and a joiner to do this part of the job but want to know how this can be done.

[John Rumm]

How much deeper are the new rafters? How big is the purlin?

If its say only an additional 2" of rafter, and the purlin is substantial (e.g. 9x3), then you could probably just notch out the back of the purlin. Once the new rafter is in place, it will support the notched bit of purlin anyway if the fit is reasonably good.

You may be able to add additional propping - many purlins have a strut that conducts some of the load to a central spine wall. If yours is like that, then you could add additional struts where required.

(Its hard to say without being about to see what the existing construction is like etc)

You presumably want the tops of the new rafters level with the old ones

- so they "extra" depth needs to intrude into the purlin's space, not rise above the existing tiles / slates.

[harry]

planning application, structural engineer doing calculations for building control.

ep rafters to go from wall plate to new steel ridge beam without purlins, t o be done without removing the existing slate roof.

are removed but I am looking for ideas of how this can be done without the roof collapsing.

isting rafters and removing a sections of the 3 purlins to fit one new raft er at a time, gradually moving the props along and removing sections of the purlins as I go ?

they are removed, that doesn?t involve stripping off the roof ?

for info from anyone who as done this before, I will probably employ a roo fer and a joiner to do this part of the job but want to know how this can b e done.

There are several ways of doing this. I'm surprised you need new rafters. With this sort of conversion, it's more usual to fit additional rafters of same size between the originals. Or even beside and nailed to originals

Or additional supports off interior walls.

Not enough information.

There comes a point when it's easier to strip off and replace the roof (Dep ending on it's condition and the size of the conversion.)

You will probably get more sense out of an experienced builder than any dop ey architect. Ask yourself why he needed a structural engineer anyway.

Architects are in general a useless profession.

[dennis]

Why not put the new rafters next to the old ones and pin/bolt and glue them together then remove the purlins. After all the new ones and the old ones are going to be supported at the ends and an offset of an inch or two won't make much difference.

You could even build a composite rafter using ply rather than solid wood if you wanted to. A strip either side with the additional gap made up with timber would be very strong.

[Andrew]

But how are you going to insert a full length rafter anyway with the purlin in place ?. You have to somehow slide it up through the gutter area, or remove a tile at the ridge on the other side and drop it in ??

Inserting at more than 45 degrees will require a huge chunk out of the purlin.

Personally I would remove a complete section of the purlin where a newrafter is going, starting at one gable, but use acroprops to provide temporary bracing back to a supporting internal wall. Then cut the the next 18 inches of purlin off, and so on across the roof.

[John Rumm]

That's what he is trying to do... but the new rafters won't fit the gap without either removing the purlin first, or chopping a bit out of the back of it.

The main difficulty is that the OP states the existing rafters are only

2" deep - that is very skimpy for an unsupported rafter even if doubled up.

[John Rumm]

Yup, from the eves.

You won't be able to do it from inside...

That would be ok, although depending on the roof design you may have to support it in several places... A gable end design would be easiest - struts either side of the middle rafter, chop out centre of purlin, and do the first rafter - then work out toward the edges.

[Roger Hayter]

50mm rafters seem unusual - I wouldn't like to stand on such a roof, let alone hammer nails into it. But it wouldn't really change the problem much if they were an inch or two deeper.

[Tony Bolger]

Sorry for the late reply, had trouble logging on.

The property is link detached with gable each end, no party walls.

75mm x 50 mm rafters supported by 3 purlins each side, no joist/ceiling han gers, no internal supporting walls.

Architect has specified the purlins to be removed and deeper rafters to be installed, possibly 75mm x 150mm to be confirmed.

The question was how to remove the purlins and fit new rafters because the deeper rafters won?t go past the purlins which are approximately 20

0mm deep.

The architect says this can all be done from inside without stripping off t he existing roof covering, not long been felted and reslated.

I kind of assumed it would involve removing sections of the purlins in stag es, fitting the new rafters as I go.

I assume the purlins will be supported with acro props etc from the ground floor, possibly fit some temporary bracing etc.

I was interested in the two approaches of removing the purlins, start at on e gable and work across v start in the middle and work towards the gable, o f the two, I am leaning towards the start-in-middle approach so loads remai n fairly equal on each gable, but any constructive advice is most welcome.

I will of course seek guidance from the architect and SE and may employ oth ers to do this part of the job but want to know your thoughts on how it? ??s done.

[tabbypurr]

angers, no internal supporting walls.

e installed, possibly 75mm x 150mm to be confirmed.

e deeper rafters won?t go past the purlins which are approximately

200mm deep.

the existing roof covering, not long been felted and reslated.

ages, fitting the new rafters as I go.

as soon as you cut through a purlin anywhere its whole support is gone.

d floor, possibly fit some temporary bracing etc.

one gable and work across v start in the middle and work towards the gable, of the two, I am leaning towards the start-in-middle approach so loads rem ain fairly equal on each gable, but any constructive advice is most welcome .

thers to do this part of the job but want to know your thoughts on how it ?s done.

Getting timber behind existing purlins doesn't fly if there are 3 purlins. I presume the plan was to attach temporary bracing to the purlin as it's cu t away.

It seems unlikely that the plan was to just remove the purlins during work & count on the weather forecast. 2x3 would usually cope with that.

NT

[John Rumm]

So the existing rafters are actually 75mm deep already? (you suggested

50mm in the earlier post)

While you might be able to notch an extra 75mm out of a 200mm deep purlin, you are probably not going to be able to get the new rafter in behind three of them.

It sounds like that is what you will need to do in this case.

Is this a single storey place then?

As long as you are supporting the cut ends, then the loads at the gable ends will either remain the same or go down.

[TS]

Hey! I’ve got similar situation. I was wondering what you did at the end ?

[John Rumm]

Please quote some of the post you are referring to, so we know what you are talking about.

[Brian Gaff (Sofa]

Well, as soon as I see Home Owners club on the posting list, I know it will either be very old indeed or nobody will have the slightest clue, since the web site is not clear that it could be read without the web details seen on the sight. Bad design nobody cares a shit a the company which runs it and to my mind they should be excommunicated from Usenet until they these problems. Brian

[Andrew]

The OP was Tony Bolger, who posted this 3 years ago, so for Brians benefit here is the text -

<<< Our loft conversion is at the planning stage, architect has submitted the planning application, structural engineer doing calculations for building control. I will be doing most of the work myself and employing various tradesmen as required.

The architect has specified deeper rafters spiked to the existing 50mm deep rafters to go from wall plate to new steel ridge beam without purlins, to be done without removing the existing slate roof.

I assume the new rafters will need to be in place before the old purlins are removed but I am looking for ideas of how this can be done without the roof collapsing.

Is it a case of propping up the purlins from ground level, bracing the existing rafters and removing a sections of the 3 purlins to fit one new rafter at a time, gradually moving the props along and removing sections of the purlins as I go ? Or is there another way to get the deeper rafters past the purlins before they are removed, that doesn't involve stripping off the roof ?

I will of course speak to the architect and SE about this but was look> >> Hey!