roof renovation - outbuilding, not square

[Mr Chibs]

Wondering if I can get some advice from you knowledgeable lot.

I would like to renovate an old outbuilding into a workshop, it currently has an old roof that's been bodged (not by me) over the years.
I would like to add a velux window, possibly two and replace some of the defect bits of timber,
I intend to take the tiles off replace where needed, replace battens and felt to breathable membrane.

On a quick measuring I found the that the building tapers slightly and is 300mm wider at the gable end than the other.
How do you go about allowing for this, as I presume it's too much to loose in the tiling?

 

[catlad]

How is it done at the moment? what sort of tiles do you have now or do you mean slates!

 

[Mr Chibs]

It's currently 75mm square rafters, sat on a 100mm square purlin, with some raised collars (300mm deep) every 4th rafter.

I will take a measurement of the span of the wall plate from each end, currently I measured the walls, but I can't tell how the roof deals with the 300mm difference, in my very basic understanding, say I wanted to replace the roof and it's currently built out of square, won't it kick up the tiles when I put them back on?

Where/how do you make up for the 'out of square' bit.
Building footprint is 4.5m x 5.5m roughly a 35 degree roof, with concrete double pan tiles.

 

[datarebal]

do a sketch and some photos

 

[catlad]

This workshop is it a lean to roof?

 

[Mr Chibs]

Sorry guys, will try to take some pics, it's an old outbuilding with a pitched roof, at the right gable end it's a 4.5m span, and the left gable end is 4.8m.
it's rafters x 14 with every fourth one with a raised collar. There are no ceiling joists.

 

[catlad]

Ok well you will need to take a datum point from the ridge if you can't mimic what is already in place.

 

[Mr Chibs]

Not able to take pics today, excuse these, but they are not the best, the felt hanging off is the front of the building and is the wall the left of the other image (gable wall) if you call it that

You can just make out a wall plate that was sunk into the wall water damage LHS (where it's black).
The crappy mezzanine will be coming down.

This building is around 5m/5.5m tall.
span is 4.5m
width of building 5.5m
Construction is double skin brick, apart from topmost gable end, single skin (could beef up)

I'd like a vaulted type roof, but due to the height, don't mind add a few more raised collars for structure, I just won't board to make a ceiling.

Sorry again for the poor images, will post better once I've had chance.

Thanks

 

[Mr Chibs]

Spotted this recently, and it's the type of thing I'd like to try and have, but with 2 or 3 velux.

https://www.aldworthroofing.co.uk/2016/02/cut-pitch-roof-timbers/

 

[datarebal]

got any bats? ecological survey?

 

[Mr Chibs]

No bats, only a few butterflies

 

[catlad]

Well you will notice that roof under construction has a flitch beam.

 

[Leofric]

If one end of the building is wider than the other and the ridge is level you will have sloping eaves. Not sure what you are asking on the other points - do you want to know how to do the tiling to allow for this out of square on plan situation ? If you want a roof similar to the photos you can have a steel or timber ridge beam of appropriate size, but I would add timbers below the ridge beam tying the tops of the rafters together. Somebody posted some photos of ridge beam roofs on here recently but I can't remember where.

 

[Mr Chibs]

Thanks again all.

I will research sloping eaves, not something I've heard of before.

Part of what I was not sure about is, what is the proper way to work with an out of square building if I was going to replace part/all of it.
Do I slope the ridge to accommodate or should this be level and I sort it with the rafters (is this possible?)
I will take a photo of the roof and post, perhaps you pro's will be able to see where/how the 300mm has been dealt with.

 

[catlad]

Definitely a level ridge or you will have problems when your tiles get to the ridge.