Where do purlins sit?

[stoneme]

I’m putting up a small summerhouse, cavity walled natural stone outside with thermalite inside. Wooden framed frontage. Thought I might sit the small pitched slate roof (3m x 3m) over particle board over a couple of purlins, rather than use rafters; ie the side walls bear the load, rather than the front which is wooden and glazed.

I’d rather not have the purlin ends exposed to the elements (sometimes you see them sticking out). Is it ok to set the purlins flush into the thermalite blocks? They are ‘standard’ size/shape.

Any helpful advice appreciated as I’ve never built a roof before!

 

[Notch7]

Wont you need rafters anyway as the tile battens will need something to fix to?

If you want a vaulted ceiling and the load to be taken on the gables ends, I wouldve thought a structural ridge would be the solution

 

[stoneme]

Thanks yes, that’s sort of the idea, but I wondered about just pinning battens to the particleboard - they did this next door to us on a single storey pitched roof about 6m long, with membrane on top of the board under the battens.

Hoping for the load to rest on gable ends, ie on purlins, but can you help a complete novice - what’s a structural ridge?

 

[Notch7]

Thanks yes, that’s sort of the idea, but I wondered about just pinning battens to the particleboard - they did this next door to us on a single storey pitched roof about 6m long, with membrane on top of the board under the battens.

Hoping for the load to rest on gable ends, ie on purlins, but can you help a complete novice - what’s a structural ridge?

A structural ridge is simply ridge that can support the weight of the roof.

Conventionally a thinnish ridge is fitted, the load on the roof is transferred from the rafters to the wall plate -which only works where there are ceiling joists acting as collars to prevent the walls from spreading.

If you have a stiff ridge, it cant sag so the rafters cant make the walls bow out -where there arent any collars of ceiling joists, in the case of a vaulted ceiling.

You could use purlins on your small summer house roof -but you will need somewhere to fix you tile battens.

 

[^woody^]

but I wondered about just pinning battens to the particleboard

Ignoring the lack of penetration of the fixings, how will the water that will penetrate the tiles drain, and how will this water be prevented from getting through the fixing holes?

 

[stoneme]

Ignoring the lack of penetration of the fixings, how will the water that will penetrate the tiles drain, and how will this water be prevented from getting through the fixing holes?

Good point, and persuades me to change back to plan A. I’m wondering though what the pro builders did next door with the particle board base, covered with membrane then battens, then tiles... I don’t think it’s leaked yet but I can see why it might.

 

[Notch7]

Good point, and persuades me to change back to plan A. I’m wondering though what the pro builders did next door with the particle board base, covered with membrane then battens, then tiles... I don’t think it’s leaked yet but I can see why it might.

Sounds like a sarking board, but didnt that get fitted over rafters?

 

[stoneme]

Yes, sarking board is what it was, I was thinking I could do that but over a couple of purlins on stone gable ends instead of trying to support rafters on wooden frontage. But probably won’t.

 

[^woody^]

If you want to tile over boards then you need to do what is known as "counter-battening".

This requires fixing a batten running up the roof (parallel to and fixed directly above the rafters) then laying the membrane as normal and then fixing the tile battens as normal.

 

[John D v2.0]

I don't follow the whole thread, edit: now I do, so my post is not useful