Bungalow extension roof design

[garyo]

Hi Chaps,

I'm in the middle of designing a side extension to our dormer bungalow. I'm currently at the planning permission stage. To keep the (diy) build simple I want to keep the floor levels and roof planes in line with the existing buildings cross section (shown below) As you can see - the rafters aren't actually directly tied in to floor joists. Instead they're relying on the 'sheer strength' of around five courses of bricks, plus the dark brown stud work preventing any downward force pushing outwards. Although the bungalow has a gable wall at each end, there isn't a structural ridge beam - just a thin board.

Presumably the stud work that I've coloured in a darker brown is doing a lot of the work in preventing roof spread (and a contribution from the collar tie)

In the extension, I want to remove these darker brown studs.

I'll get my SE to produce calcs when I make a full plans submission, but at this stage I'm trying to establish that my design is feasible. The plan would be:

* Replicate the structure below in terms of ridge height and floor level
* Move the brown uprights to where I've shown the green upright.
* Use something beefier such as 50mm x 200mm C24, to cope with the longer rafter span
* Bolt in the smaller uprights to the joists & rafter with dog washers.

Is my SE going to laugh at me?

 

[Ian H]

The area behind the brown uprights wouldn't add much useable space to the room. You could leave them in, board them and use the area between the brown and green as storage.

The room would still be a good size.

 

[garyo]

The area behind the brown uprights wouldn't add much useable space to the room. You could leave them in, board them and use the area between the brown and green as storage.

The room would still be a good size.

Having looked around a few friends/families town houses that have similar top floor constructions we've decided that we dont want to keep them, or that we'd move them to the position of the green example shown.

I agree the space isn't particularly usable but it gives a feel of visual floorspace.

 

[^woody^]

My first thoughts are that you have funny legs

My second thoughts are that there is no bracing and your "uprights" wont be doing any bracing or any supporting in that location. And you cant have structural stud walls just built off joists.

 

[Ian H]

Ha ha!! Looks like a possible deep vein thrombosis in the left leg.

 

[^woody^]

Ha ha!! Looks like a possible deep vein thrombosis in the left leg.

Gary needs to use an SE who has a sideline in hip replacements

 

[garyo]

So it's just the few courses of brick providing restraint against spread then?

The joists that the studs are built off are 170mm deep over a 4 metre span. Maybe that's just how they did it back in the 60's with all the free love between builders and BCOs?

 

[^woody^]

So it's just the few courses of brick providing restraint against spread then?

What are those brown uprights doing?

 

[garyo]

What are those brown uprights doing?

My assumption is they're providing support to the rafters prevent spread, and also allowing the original builder to get away with only using 100mm rafters.

Asking the question another way - can the following be built without resorting to a structural ridge beam: An attic truss but without the rafter feet being tied to the floor joist ends, but implemented as cut roof:

 

[^woody^]

If it can be proved by calculation that the eaves are restained and th eridge wont drop, then it can be built. But those little stud walls right down the bottom are not doing a thing.

 

[garyo]

I think I'll start saving up for crane hire....