Getting rafters past the purlins

[Tony Bolger]

Our loft conversion is at the planning stage, architect has submitted the planning application, structural engineers are doing the calculations for building control.

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

The architect has specified deeper rafters spiked to the existing 50mm rafters to go from wall plate to a new steel ridge beam without any purlins and to be fitted without removing the existing slate roof.

I am looking for ideas how this can be done, is it a case of propping up the 3 purlins from ground level, bracing the existing rafters and removing sections of the purlins to fit one new rafter at a time, move the props along as I go, fitting the new rafters ?

Or are there other ways to get the deeper rafters past the purlins that doesn’t involve stripping the roof ?

I will of course be speaking to the architect and structural engineers again, and will probably employ a roofer and joiner to do this part of the job but I want to know how this is done and if anyone else on the forum has done it without stripping off the roof.

 

[^woody^]

This will be fun. Mention to the architect that under CDM (2105) he is the designer and he needs to detail how the work is to be done safely.

But yes, it's more or less how you describe. What you dont want to happen is for the whole purlin to be removed at once and the roof drops a little.

 

[Ian H]

Sounds like it would be easier to bin the existing rafters and start again with the bigger ones.

 

[Tony Bolger]

Architect is away for a week but will mention that to him if he seems in any way vague.

I couldn’t think of any other way to do it, star at one end and work my way along, ensuring the remaining sections of purlins are supported until the last rafter is fixed in.

 

[Tony Bolger]

Being done without stripping the roof

 

[Ian H]

How will you get the new rafters over the wall plate at the bottom end?

 

[Tony Bolger]

Good question

 

[Tony Bolger]

Architect said he has specified deeper rafters to go from wall plate to ridge beam without purlins, fitted from inside, without removing existing roof covering.

Hence my post on here, how does anyone else do it without a full re-roof ?

 

[garyo]

Even without the purlins that's a pain because you'll have a plumb cut at the top and seat cut at the bottom, so they want to rotate in sideways but you'll struggle for wiggle room because of the existing neighbouring rafters.

 

[Tony Bolger]

The whole job will be a pain, but so long as it’s possible without a full re roof, I was concentrating so much on getting past the purlins I never thought of having enough rotation to actually slide the rafters in. Really want to avoid a re roof, if poss

 

[tony1851]

You don't have to go full length from wall plate to ridge with the deeper section - you can start about 200mm or so from each end.
Structurally, the full depth doesn't have to apply over the full length - ignore your architect and speak with the SE.

 

[^woody^]

How will you get the new rafters over the wall plate at the bottom end?

They only need the seat cut to sit on the plate.

The only problem then, is if the new rafters are so deep so that effectively only the top third is loading the plate and may be liable to splitting.

 

[Tony Bolger]

200mm from one end would make life much easier!
Thanks for that, will definitely speak to SE about it.

 

[Tony Bolger]

Got a point there, and it’s quite a shallow pitch so possibly less than a third will be actually sat on the plate.

But if they can be stopped short of the plate anyway ....

 

[bobsuerita]

ignore the architect for the moment and wait an see what the s.e comes up with if your having steels for the new floor and ashlar wall it will all come down to the span of rafter from ashlar to ridge