Joining a ridge beam

[SuperJohnG]

My friend and his dad are building a garage/new gym in the garden. So far great job and pretty meticulous detail, however I raised one area of concern last week and they were thinking the same. 

 

 

It a stick build with masonary outer leaf. They used an 8 x 2 for the ridge beam which needed to be joined as the garage is quite long. However they joined it with a nail plate and during the build putting the rafters on (they sit in line with ridge board not on it) they noticed it was sagging at that point. I had a look but I had a bit of a concern around joiningg it with a nail plate and the load applying a torsional shear around the nails. 

 

I'm a mechanical engineer and my input was to ensure the shear load was reacted by a clamping force, by adding two outer sections (cheeks) at the join of the same timber profile and bolting right through. However it is not my area of expertise. 

 

The concern being, its sagging 10-15mm already, but the roof is to be loaded out with slates, so will be heavy. There are currently no collar or rafter ties. I think they need rafter ties, to stop the walls slapying, but I wanted to ensure there was no extra deflection on the ridge beam. 

 

garage is 7m long. 4m wide.

 

any thoughts/ advice appreciated. 

Picture attached. 

 

 

[PeterW]

Definitely need some sort of tie at either ceiling or collar height. 

[tonyshouse]

Traditionally every fourth rafter, is there a ceiling tie? What supports the prop? In my view it should be a hanger not a prop , how big are the ceiling joists? 

[scottishjohn]

joining plate looks way to short anyway v and prop is only on one side not supporting both bits 

Me 

 

 I would make the rafters into roof trusses , that will  get rid of side loads on your walls from roof  and if they are close enough together there will be no need for the prop anyway 

osb tie plates at top next too the ridge  and same dimension stretcher  as rafter is made from at wall head - you then got your ceiling frame work at same time 

[markc]

I have to agree with all of the above. 8x2 wont even self support at 7m.

Unless the walls and wall plate are exceptionally strong they will bow allowing the roof to fall in.

Bottom cord / ceilng ties is the way forward.

 

[PeterW]

Unless you need the clear headroom I would put a double ceiling tie at 2.5m from either end with a king post to the ridge. That would be 8x2 doubled and bolted. Then every other rafter would be 6x2 collar truss single side and bolted at 1/3rd rafter depth. 
 

If you need headroom, then it’s going to be doubled collar ties at 1/2 depth all the way down, especially if this is a timber frame with a garage door opening at one end. 

[Temp]

Roofs should be "designed". There are two main ways to stop the roof pushing the walls out and the roof sagging:

 

1) Triangulation involving tie beams in tension joining the rafters together to form a triangle. With this approach you only need a ridge board not a beam.

 

2) Structural Ridge beam, which involves a much stronger ridge beam supported on strong gable end walls and possibly posts elsewhere. The rafters are then strapped together so that virtually the entire roof load "hangs" from the structural ridge beam.

 

There are also variations involving things like purlins that also transfer weight to gable walls.

 

Given the info in the OP I would triangulate the roof with beams bolted to the rafters at ceiling height. Every other rafter or so.

[tonyshouse]

Ridge beam to span 7m would be huge eg 450mm deep gluelam or 300mm steel depending on type of tiles and roof covering 

 

if ties you can use rods or wire ropes etc they don’t need to be wood 

[Simplysimon]

the ridge doesn't do very much, if you didn't have it and placed each rafter opposite the other, meeting at the plumb cut, would the roof sag? nope due to triangulation. the only reason the roof is sagging is the the walls are moving outwards. are you putting in joists to sheet ceiling? if so you could do a pole plate roof, best roof design you'll get.

[Temp]

5 hours ago, Simplysimon said:

the ridge doesn't do very much

 

At least not in this design . In others its a different matter. 

[scottishjohn]

8 hours ago, Simplysimon said:

nope due to triangulation

I would say due to the LACK of triangulation 

turn all the rafters into  triangulated roof trusses  and problem goes away -

look at nay old barn and if they have trusses the section of wood is much smaller than if it is rafters 

this is maybe the time you need to calculate the weight of your slates 

[Simplysimon]

5 hours ago, Temp said:

 

At least not in this design . In others its a different matter. 

fair shout, the difference between a ridge and a ridge beam