building a lean to roof , purlins and ridge plate.

[thatbloke]

i am building a roof on my lean to. the existing roof is 10 meters by 3.15 meters and is made from corrugated cement.
the 10 meter span is interrupted internally by a clay lump wall around 3.5 meters from the end of the lean to , all the external walls are clay lump.

i have a good idea of what size of rafters to use but would i get away without using a purlin? the roof is quite a shallow pitch and i will be using ceder shingles over a felt , counter batten then batten.

also , should my ridge plate be the same size as my rafters?

thanks.

 

[Chukka63]

i have no purlins in my lean-to roof with a bigger span than you have.you can always check with the local authority.

 

[noseall]

150mm x 50mm rafters should be ok (3150mm span) but 175mm x 50's @450mm centres will eat it.

The pitching plate will need to be deep enough so that it takes up the entire depth of the rafter plumb cut. So, if you are using say 175mm deep rafters then, on a fairly shallow pitch, i would say a 200mm deep plate will be ok.

If this is habitable space you will be required to fit around 125-140mm celotex (warm deck spec) to the vaulted rafters.

What is clay lump?

 

[thatbloke]

thanks guys. clay lump is a mixture of clay , straw , small stones/gravel and sometimes lime. its a traditional building material for east anglia and some other parts of the country. the bricks are a tad larger than breeze blocks and very heavy. they were made by mixing up all the ingredients in water then putting this into a mould where it was left to dry out.

they were put together with clay slurry and sometimes lime mortar , but mostly just clay slurry.

usually they were rendered with lime plaster externally as they are quite delicate if left to the open rain etc and can easily be washed away if left exposed.

this is what they look like from inside my wood store. these are over 100 years old although the main house was built in 1520.

i am trying not to get building control involved in this as most of the time they have no real understanding of old buildings like this and because its not listed (yet) they will/might make me do unspeakable things to the house in order for it to comply with building regs , a 500 year old house just wont be up to that task.

insulating is a definite yes though , i was going to use sheeps wool or hemp to insulate but cellotex may be a better option.

 

[noseall]

Good for you, Thatbloke.

 

[thatbloke]

thankyou noseall. i am sure if i do it all by the book then if a BI does show up he/she wont have too much to grumble about.
its just the clay lump walls that they may have difficulty with ,apparently its difficult to calculate what kind of loads they can take. also my single glazing windows and uninsulated walls might get up their nose a bit as well , although i am sure they might give me a bit of lee way because of the age of the house and the way in which it has been constructed.

 

[Chukka63]

those walls are in good nick.

 

[thatbloke]

yeah they are pretty good , dome decay at the base though through damp. some eejit decided to render the outside in cement instead of lime so god knows what shape they are in underneath it.
once the roof is off i will give them the once over to make sure they are up to the job and carry out any repairs before putting the new roof on. no point in putting a new roof on dodgy walls.
i was surprised to see they are built on a brick and rubble plinth that goes down around 2 feet , i didnt think they would have put foundations in that deep.

 

[Chukka63]

i owned a bungalow built in 1929 and it was cavity construction with clay lump on the inner leaf.did some knocking down and donated the blocks i saved to the national trust.they were well chuffed.they make there own if need be.

 

[thatbloke]

i am sure they were well glad of them ,apparently second hand ones are very hard to get hold of.

this roof guys , one side butts up to a wall but the other side is just the edge of the house. i am not sure what to do with it to make it weather proof and aesthetically pleasing.
also , do i run a rafter down ontop of the wall? or should i run it down the inside of the wall then try and extend the wall upwards to the level of the battens?

 

[Chukka63]

put your rafter at least flush with the outside edge of the wall.most people would overhang the wall plate and make a "ladder" to give you more overhang over the wall for weather protection.(not looked into how i add drawings etc,but im sure someone will put a drawing up for you shortly)then face it with a barge board/soffitt

 

[thatbloke]

the thing about overhang is the roof at present does not overhang and i am assuming this is so it can sit flush with the side of the house.

here is an overhead plan of the roof's layout at present.

 

[Chukka63]

ok,put your rafter flush with the end of the wall .barge board directly onto that and then overhang your roof using "soffit" strip and then tile and batten to that,cementing the end tile in place.
if you cant go over the boundary you will need to build the extension 50mm short of the existong wall.

 

[thatbloke]

cheers fella. how would you advise to flash the other side , the roof butts up to a flat walled extension with a flat roof.
ooops , its not a brick wall, its rendered blocks.

 

[Chukka63]

lead flashing.you need to make sure the lead goes into the block and not just into the render.you will want a bell drip bead at the bottom of the render really to direct rain water away from the wall.it may be best to cut the render 150mm above the lead height to then form the bell drip bead detail.this area would then need re-rendering.

some would just chase the render/block and fit the lead and repoint and not bother with the bell drip and the need for the re-rendering.