How to build walls with a pitch

[trevorbayliss]

I am going to do a mono pitch extension. The back of the extension which connects with the house will be about 300mm higher than the front of the extension. The distance from front to back is about 7.1m. So the roof is going to be a flat style.

I think you will normally run your timber joists from the back to the front of the extension, but i am wanting to run mine the opposite way. The reason being i will have 4 walls from which to hang my joists and so the load will be spread over these walls and not on the back of my house and the front of the extension.

So when i build my 4 walls (see plan) how do i go about getting the correct pitch?

Do i build up all the walls level up to a certain point and then complete the front of the extension where all the windows will be. I then have my finish level and so now need to go from this point back towards the house and finish about 300mm higher up. I will be setting onto the top of the walls a wood sole plate (oak probably because it will be seen in the main room on one wall).

So do i use a string line to go from this point to a point on the back of the house and infill with brick or block ?

 

[oldbutnotdead]

Pretty much (same way I did my garage). Got to say, that shallow pitch on such a narrow space might look a bit rubbish, depends how the rest of the structure works. What's the plan for roof covering- EPDM or similar? (Don't think you'll get any tile or slate that'll work at that fall)

 

[^woody^]

think you will normally run your timber joists from the back to the front of the extension, but i am wanting to run mine the opposite way. The reason being i will have 4 walls from which to hang my joists and so the load will be spread over these walls and not on the back of my house and the front of the extension.

Non of that makes any sense at all

 

[trevorbayliss]

if you look at my diagram you will see i am going to rest my joists ACROSS the extension rather than front to back.

But i need to make my walls have a small fall. How do you do this accurately ?
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Yes i will be using fibreglass for the roof finish.

 

[oldbutnotdead]

if you look at my diagram you will see i am going to rest my joists ACROSS the extension rather than front to back.

But i need to make my walls have a small fall. How do you do this accurately ?
\
Yes i will be using fibreglass for the roof finish.

As your plan- wall top to whatever finished height at the rear edge is going to be. String from that rear wall top to the front wall, put a nail in the wall 300mm above your rear wall height. Pull string tight.
For that length you'll want to prop the string up at least at midpoint (150mm) and probably at a couple of points otherwise you'll end up with a delicate curve.....use trig to determine string height at the intermediate points.
It'll be quicker to use blocks, you'll need a stihl saw or a 9" grinder with a diamond disc to cut the blocks to the line you want. If you build the wall a little short (5-10mm) it'll give you room for a mortar bed under your wall plate which means you don't have to be spot-on with your block cuts.
Probably best to do this wall first then set the line for the other wall plate from it. I'm still not sure what that fall will look like (from the outside) running that way but I can see why you'd want to do it that way (avoids guttering over the boundary)
How are you going to close off the rafter ends- bargeboard? If so then you'll have to be careful about rafter length (don't want to overhang the wall) and if access from outside the boundary is restricted/not available you'll have to do that job from above

 

[trevorbayliss]

Thanks for the info OBND.

Access is ok and yes going to use boards, maybe in oak, as i am having some oak framing around the french doors onto the patio.

Hoping to build it a little short and then use the mortar bed to iron out the differences.

So old string line it is. Will need to read up about the trigonometry.

 

[^woody^]

Your talking about pitched roof, mono pitched roof, but refer to joists and fibre glass covering.

So what type of roof is it flat or pitched?

 

[trevorbayliss]

It is a small pitch, and i am going to be using 4 oak purlins to give me some oak feel in the main room created. On top of these purlins i will sit my joists. Between the 125mm joist will be the kingspan. On top of this will be OSB board. Then the fibre glass system.

The oak purlins will run from the garage / sitting room wall over to my new wall next to my neighbours wall. So the ceiling joists will actually be running from front to back over these oak purlins, but the garage will have normal ceiling joists running from side to side.

Here is an extra drawing.

 

[^woody^]

It is a misconceived idea, but that's not the question.

You would set up the rafters, set a line off these on the face of the block wall and then external wall, and then build to those lines.

 

[trevorbayliss]

Ok, so my idea is not that good. What would be a different way of doing this ? Using the oak purlins means i can fit in 2 large roof lights (1.5 x 1m non opening). Plus i get that nice oak feel above the head.

 

[tomfe]

Wouldn't it be easier to build it with joists 90 degrees to the house and then add fake oak beams in?

 

[trevorbayliss]

Probably, but the load would be on the rear wall of the house and i have a big opening with a steel (i imagine) in the kitchen area. This was done years ago before us. I would prefer to put loads on the new extension.

 

[^woody^]

You are not building a pitched roof. 1:20 is less than 3° so its a flat roof. A flat roof does not have raking brickwork - in fact it can't have because of the way it must be designed and built.

Have you had your joist and oak beam idea checked by an engineer? I can't see how it can work as the timber sections will mean you will need to raise the roof else be bumping your head on those oak beams.

Also, your concept of not loading those walls appears to be flawed, as the proposed purlins will be loading the internal walls, so if you change that around, you will end up loading the walls you say you don't want to load

 

[trevorbayliss]

I WANT to load the new walls i am building. What i don't want to load on is the rear wall of my house because it has a big chunk missing in the kitchen area (steel in there somewhere i suppose).

The oak joist depth/width is going to be about 9" x 6" i think from memory.

Two purlins will be spanning an open area of 5.6m and the other two will span 4m open, then rest on the wall separating the utility and lounge area. If needed i may have to make the 2 longer beams into flitch beams. So two pieces of oak with steel piece in the middle.

Yes i need to get an engineer to look at this and get calcs. Just got these plans and the neighbourhood consultation has been submitted. So got to wait 6 weeks now for the green light.

Yes it is a flat roof really. The height difference from front to back is about 35cm and the distance travelled is 7.1m.

How come i can't have slopped supporting walls ? I spoke to my mate last night and he is a builder and it is possible to do this.

I think i am getting there with this extension. Lots of planning still. Good to hear all your input as it helps the clarity.

 

[trevorbayliss]

What about making a flat roof woody as you mention. So the walls are level and the beams sit on the walls level. Then on top of the beams i sit my 125mm rafters plus the furrings on top of these. A 7.1m distance needs a minimum fall of about 18cm (40:1).

A problem i have is on the back wall of my house where i have 2 big old sliding sash windows. From internal floor finished height these reach to 2.7m. So above this i need to accommodate 225mm oak beam, 125mm rafter, 18mm osb sheet, fibreglass kit. So need a height above of 40cm approx, plus i need to add a fall. Hence i have a roof height at the back of the house of 3.35m. Hence why the sloping wall idea developed.