Timber Framers Guild Forums Tools Timberframe Design Conservatory Roof Design Help

Good Day,

I am completely new to timber frame, but not carpentry. I am designing a self build timber frame conservatory on the South coast of the UK, and I was hoping for a bit of guidance in the roof design, if that's okay? I have sourced a number of books by the likes of Steve Chappell but these deal with bent frames.

I am hoping to build a 3 meter x 3 meter oak framed 'lean to' type conservatory into an 'L' shaped rear/side elevation to my house. The space is such that one side will be a full 3 meter wall but the other side will extend 1 meter beyond the 2 meter side elevation of the house. The roof will be a mono pitch roof with a 15 degree slope, 25% clay tiles and 75% glazed. I am planning on using 6" x 6" oak timbers for the frame, and then 5 air dried oak rafters supporting 4 glazed units continuing out from the tiles.

For the deadload of the roof I have calculated 200Kg for the tiles and glazing. Live load for snow, using an Sk value of 0.37 of about 270Kg.

I have 2 areas of uncertainty; the dimensions of the rafters, and the load on the ridge beam supporting the rafters adjacent to the wall of the house.

I plan on using a 6"x6" timber for the ridge beam. The problem is that the door to my house is right up against the corner of the rear/side elevation to the property and the rear elevation had a large window in it. This precludes a vertical timber being used under one side of the beam although the end leading off from the outer wall can be supported by the timber frame. I am thinking of letting the other end into the brickwork of the side elevation, but this would be above the door lintel, whilst the use of anchor bolts along the span would transfer load above the window lintel. I'm a little bit uncomfortable with this, as by time I add the weight of the rafters in, I suppose I will have a potential uniform load approaching 800Kg on the ridge beam.

What would you say the dimensions of the rafters would need to be to support the load of the roof over a 3 meter span with no intermediate support?

Because of the window, I am also precluded from utilising a truss type design for the rafters. I am thinking of extending the Sill plate that leads off from the side elevation right up to the door. Whilst it will run abut the wall by 1 meter, I will be able to install a vertical timber adjacent to the door jamb, and then run a 3 meter purlin under the rafters from side to side. This then results in a 2 meter span from the forward wall plate to the purlin, and a 1 meter span from the purlin to the ridge beam along the rear wall to the house. Presumably I can then decrease the scantlings of the rafters to save weight.

So, would this arrangement be structurally sound, safe for the ridge beam and window/door lintels?

And what should the rafters be, 4" x 4"? 4" x 6"?

Thanks for any help and advice.

Richard