Roof design

[peweuk]

I am self-building a 2 story extension and have everything designed and under control, and am contemplating cutting and building the roof on site.

Before deciding I need to do some research to see what might be involved, and wondered if anyone had any pointers to any information I can access to assist with my research before I take it further (eg decide on pre-made roof trusses or using a roof designer to design the construction).

TIA.

 

[noseall]

if you are looking for space in the loft, then forget trusses.

if you are looking for a large clear span upstairs with no steel or intermediate support then forget a traditional cut roof.

you could train a monkey to fit trusses.

you will need to sh£ll out mon£y for a cut roo£.

 

[peweuk]

I am not particularly looking to create clear space, but want to work out what would be involved with a cut roof so I can compare the costs of truss vs cut roof - hence my search for information about making a roof on site.

I have the time and facility to do a cut roof, but noted with interest your final comment - "you will need to sh£ll out mon£y for a cut roo£. " as I anticipated it may be cheaper than trusses

 

[noseall]

i would say that over a certain span a cut roof becomes untenable so a manufactured truss roof will definitely be cheaper.

the timber sizes involved in large clear spans renders a cut roof financially and structurally prohibitive. you need to involve steel and intermediate walls for a cut roof to work over large spans.

that said, some large trusses do require craning into place. also the strapping and bracing involved in trussed roof systems means a fair bit of acrobatic fixing.

i like building cut roofs. they can be erected without delay and provide a larger space, loft wise.

also, joining plasterboards on a 2" wide timber is easier than those tiny chords available with trusses. same with the roofing battens.

all in all i would say a manufactured truss roof is less expensive than a cut roof, but *i would say a cut roof is superior.

*my opinion only.

 

[noseall]

p.s. i'm not sure whether you can have an open, vaulted or joist-less roof using trusses.

if you wanted one of course!

 

[peweuk]

Thanks for your input noseall.

The roof I am working with will have a span of 6.9m across, 5.5m long.

In your experience what size timber would that require - it has been suggested that an "A" frame of 6" x 2" timber would be the starting point?

 

[noseall]

what is the roof pitch?

are you saying the roof will be orientated so that the joists will be spanning 6.9m?

with a cut roof you will need to know, joist size, rafter size, purlin size and frequency, ridge element and hip element if applicable.

if the span is 6.9m and it is a clear span, then it looks like trusses for you!

 

[peweuk]

The roof pitch has to be 46 degrees (to match the existing) and it is a clear span width (although there will be a central stud wall in the centre running the full length supported by the first floor joists), but it also has to have a raised tie as the roof plate is going to be lower than the internal ceiling height.

So I guess from what you are saying, and with the aded complications it looks like it will have to be trusses ?

 

[noseall]

clear spanning nearly 7m with standard sawn timber is a big ask.

it would require joists in the region of 13" deep.

if there had been a structural wall splitting the span then life would have been easier.

the rafter lengths will turn out around 5.3m (including soffit overhang) at 46 degrees. 4.8m timbers can be bought as standard but special machined lengths would be required for your roof.

trusses it is then!

 

[peweuk]

Thanks noseall, your input is greatly appreciated.

One consideration that had crossed my mind was to place an rsj along the centre of the span, supported on the gable wall at one end and bedded into the existing house stone wall at the other. The roof joists could then be supported on that.
Not sure if this would be a posible solution, and if so, what size the rsj would need to be - although I could probably work that out from the weight of the tiles and roof timbers.

 

[noseall]

the steel will be spanning the other way, around 5m, correct?

 

[peweuk]

Correct

 

[Deluks]

From what has been discussed so far, there are no internal walls on the ground floor?

The stud wall that will be sitting on the first floor joists, what size will these joists be? are they to also span the required 6.9m??

I feel the clear solution is being obstructed by too much information, delivered in the wrong order.

Start again, describe foundations and wall layouts, starting with the ground floor.

What are you hoping to gain from this structure, room wise? (bed/bath/gym/kitchen etc?)

 

[peweuk]

OK Deluks here we go.

There is an existing single story extension in place and I am extending it to a second floor which will be used as 2 bedrooms with en-suite.

The area is 6.9m deep by 5.4m long.
The footings are 1m deep solid fill 450mm wide.
The walls are 215mm solid aircrete block.

The ground floor is one room with a vaulted ceiling supported by a 300mm x 300mm oak beam (originally installed to support the existing single storey extension roof which currently has concrete tiles on the roof).

The joists for the first floor are intended to be 200mm x 50mm in size spanning the depth of the building (6.9m) supported in the middle by the existing oak beam.

The upstairs will be split into 2 bedrooms and there will be a stud wall running the length of the building on the joists directly above the oak beam .

Due to planning restrictions, to make the extension subordinate to the existing house the height of the roof plate will be approximately 200mm lower (depending on rafter size needed) than the upstairs ceiling at both the back and front of the extension, so the roof joists will have to be of a 'raised tie' type and the roof pitch is 46 degrees.

Is that the sort of detail you suggested?

 

[Deluks]

So the oak beam is 6.xm long, only supported at either end?

...and this will be carrying the loading of two habitable rooms?

Not entirely sure about that but anyhow the roof.

I think a steel ridge beam as you suggest, could be supported by steel rafters. A couple each side might do it. If the beams are designed and joined properly, they need not need a 'tie' although they will need secure fixing onto the walls with appropriate pads etc.
The rest of the now 'infill' rafters could be timber as normal, these may not need to be tied either

A few areas of confusion with regards to the mixing up of depth and width, and roof plate could either mean wall plate or ridge beam. Other than that, good description