Roof construction advice

[gms]

I am rebuilding a single story kitchen / utility rooms extension on exsiting footprint with new 1m foundations in a semi detached house built in 1900. I cannot use the party wall which has a neighbours parapet wall on topas a load bearing wall for the new roof which is replacing a flat roof.

Roof Dimensions 8.1m x 4m
Pitch of roof is 12.5 degrees
Rafters span the 8.1m

Load bearing walls:
1. Existing main house 9inch red brick cavity wall.
2. New internal load bearing 9” hollow concrete block wall on new 1m foundation @ 5.2m form the main house wall
3. New Load bearing 9” external red brick and internal celcon block 100mm cavity wall @ 8.1m.

11 rafters to be hung from main house wall on wall plate screwed to wall and joist hangers attached to a wall timber . Rafters will then run through the internal block wall to new gable wall. I will then be using counter battens for fixing tile battens.

Tiles Redland regent @ 4.6 Tones per 1000.
We are going to use 340 tiles which = 1588kg or 3500 lb

My questions are. Are the walls adequate for the roof weight?
What would be the optimum rafter dimensions at 16” centres over this span keeping in mind that it might snow again some day!
javascript:emoticon('8)')
Cool

I have considered a hipped roof but for many reasons decided against it.

Any other comments members have would always be appreciated.

Thanks

GMS

 

[noseall]

what does your architect say?

i take it 8m isn't clear span. by the way you described it, the rafters pass through a block wall, thus lessening the span?

 

[gms]

I wish I could afford architects fees ..... I know I know what your going to say: 'if it falls down I'd wish I had afforded one'('')
Sad
This rebuild is an extension I built on a shoe string about 24 years ago whaen I had less cash than I have now (although I'm still poor!) the falt roof still in situe has lasted all those yaears without any attention and is still water tight

Your right the span in total is 8m and the rafters go through a 9 " concrete block wal at 5.2m

So what do you think? I shan't sue you if it falls down!

Thanks

gms

 

[gms]

Hi Noseall

I wish I could afford architects fees ..... I know I know what your going to say: 'if it falls down I'd wish I had afforded one'javascript:emoticon('')
Sad
This rebuild is an extension I built on a shoe string about 24 years ago whaen I had less cash than I have now (although I'm still poor!) the falt roof still in situe has lasted all those yaears without any attention and is still water tight

Your right the span in total is 8m and the rafters go through a 9 " concrete block wal at 5.2m

So what do you think? I shan't sue you if it falls down!

Thanks

gms

 

[noseall]

so you have a split span of 5.2m and 2.8m? and the roof pitch is only 12.5 degrees! what are you going to use for a roof covering??

it wouldn't surprise me if you have to use some thing like 225x50 mm timbers for that span. can you not introduce another purlin at 1/3rd span?

in other words break the spans into three sections?

 

[gms]

Hi,
As the pitch of the roof runs opposite to the direction of the rafters and I cannot use the the party wall I see no way of using a purlin to break up the span.
As I said in my original post I'm using Redland regent tiles as they are the only concrete tile that will go down to 12.5 and not leak through capilary action.
I wish there was a way to show and illustration of the cad drawing of the roof as this would save a time in explaining what is clear on the drawing.

I had almost decided that 225 x 50 s were too light. Would increasing the rafter width to 150 x 75 have any beneficial effect? I say this because of the twisting effect likely when the tiles are laid in the opposite direction to the rafters. Hence the the suggested use of counter battons for the tile battons to fix to.

Thanks again for the trouble your taking.
Hey I found a way to send a drawing of the roof: look here:

http://i138.photobucket.com/albums/q266/gms_photo/roof.jpg
Regards

gms

 

[jeds]

This is effectively a flat roof for engineering purposes. Table A19 of Part A gives you 75x225 at 400cs allowing a span of 5.28m for a maximum dead load of up to 1.0kN/m² using strength class SC4. (C24 in new money)

 

[gms]

Hi Jeds,
Thanks for your comments very helpful .... had some difficulty finding the sections of the Building Regs you reffered to (Table A19 of Part A.) It always seems someone wants one to pay again for regulations that we as tax payers have already paid them to produce ''
but hey someone always come up with a way round it.''
This may be a helpful link to others looking for timber strength calculation tables for roofs and flloors: http://bauing.f2.fhtw-berlin.de/lehre/professoren/damm/conrem/Timberdesingtables.pdf

Whilst I take your point that the pitch of my roof being so shallow is almost a flat roof, in fact it is not. Consequently because I'm forced to span the rafters the opposite way to normal the Redland concrete tiles will surely have a tendency to exert a force towards the bottom of the pitch and in a twisting action too. Hence my original questions.

Any further comments would be welcome.

Thanks

gms

 

[jeds]

The loading in pitched rafters travels axially along the rafter more or less depending on the degree of the pitch. The more the pitch, the more axial transfer - so if the pitch was vertical the axial transfer would be 100%

Your rafters are not in fact pitched at all - they are side to side. So they do not benefit from any axial load transfer. They are effectively flat.

 

[markthebuilder]

so in theory if i have a larger span than say 6 x 2 rafter will strech (max in my tables ) I can use sizes taken from flat roof tables and be confident i'm on the right side.

GMS
have you considered building a load bearing wall adjacent to party wall ither timber frame styel or in block .
secondly under party wall act there isn't anything that precludes you from using the wall structurally if it is up to the job its as much your wall as your neighbours unless it is infact Their wall and it just happens to form a boundary with you

 

[gms]

Thanks jeds thats helpful. Have decided to go with 225x75 for the rafters.
Any advantage in strength and stability to be gained in cladding roof with 12mm ply?

Hi markthebuilder, yes I had considered building a skin adjacent to the party wall. The house when it was built did not have the single story kitchen built on there was merely a piched roof washroom come outhouse about 4m from the main house. The then owner added a single skin wall between the two houses that joined the house to the outhouse. This was built inside the boundary which means a brick width inside the boundary. If I add another skin I will lose more space, and space is what I'm trying to maximise hence my need to avoid using the party wall (which has a parapet above it already) as a load-bearing wall and run the rafters the opposite way round.

Thanks again or all your helpful advice

gms