Bamboozled by loft conversion!

Hi I'm looking for some advice as how to tackle a loft conversion project please. My original intention was to install a staircase above the existing one & complete an already boarded out loft.
On inspection of the loft space I've found that I have a king post with no struts & the rafters sit on a wall plate on top of the external walls (mid terraced house). The joists run perpendicular to the rafters so I'm assuming the purlins stop the rafters from spreading?
I've found that the ceiling joists meet on the tie post under the king post. I cant tell if they're notched or not.

Large purlins appear to sit on the party wall & the roof ridge is in good condition. I'm wondering what is actually holding up the ceiling joists? They are staggered on the tie post & nailed to the wall on a very thin piece of wood. Im amazed the ceiling hasn't dropped when I was up there !
Ceiling joists are notched onto a 10mm thick piece of timber. The joists are an odd thickness (50mm x 65mm)
& this is the wall I'd like to remove from above the stairs;

I knew it was just void space there.
Here are some rough plans for the house as well;I was hoping to cut strengthen joists and cut out space for stairs, strengthen joists where required & the onto party wall. Sister ceiling joists & re board floor. Do electrics etc & then get roof felted in 2020. I seriously doubt that the ceiling joists will take anything other than light storage. Unless I'm missing something there must be a hidden structural stud wall under the tie post which means the wall I want to remove might be a semi structural stud wall? It's an old house but i am completely baffled by what approach to take. I'll be getting a structural engineer in to confirm things before i start any work but for now any suggestions would be appreciated.
Thanks,
J

Joromy said:

The joists run perpendicular to the rafters so I'm assuming the purlins stop the rafters from spreading?

Click to expand...

maybe the ridge and purlins are taking the roof load.

in which case that king post is supporting the ridge at mid span

nb -Im only guessing Im no roofer or SE!

My usual type of comments, but get an architect or architectural technician to survey the property and produce existing and proposed plans and submit a Full Plans Submission for Building Regulations approval. If you are employing a structural engineer they should liaise with the architectural designer on the bldg. regs application and advise on structural alterations and provide structural calculations as necessary. That is the correct way to do it and better than trying to make it up as you go along All the building regs related matters should then be sorted before you start work on site (there is more to a loft conversion than just structural considerations of course )

Hi Leofric, I'm not sure where I said I wasn't going to get a structural engineer in before I start any work. I cant see the problem with trying to understand the existing structure of my house & considering my options. I'd prefer if we stayed on topic and discussed the existing structure instead of veering off into architects, structural engineer & building surveyor territory please. I wouldn't insult your intelligence so please don't make assumptions about what I might or might not do.
Im wondering if anybody has seen this type of roof or loft construction before? The property was built from the 1916 - 1920 period according to the council but I'd imagine towards the 1919 - 1920 period.

Joromy said:

Hi Leofric, I'm not sure where I said I wasn't going to get a structural engineer in before I start any work. I cant see the problem with trying to understand the existing structure of my house & considering my options. I'd prefer if we stayed on topic and discussed the existing structure instead of veering off into architects, structural engineer & building surveyor territory please. I wouldn't insult your intelligence so please don't make assumptions about what I might or might not do.
Im wondering if anybody has seen this type of roof or loft construction before? The property was built from the 1916 - 1920 period according to the council but I'd imagine towards the 1919 - 1920 period.

Click to expand...

Leofrics comments appear to be appropriate, your first line in the first posts is asking for suggestion which is what he gave .

I'm surprised that space above the stairs (which appears was boxed in), wasn't added as a cupboard to the small bedroom in the original build rather than waste the space.

I've thought about that in our house (not a terrace but the hallway is the same width). Trouble with it is any cupboard put in there would have to start halfway up the wall in the small bedroom. From looking at that floorplan too, they would be right behind an opening door. In ours' the area is left open and not boxed in, which made painting the ceiling/walls difficult for somebody with vertigo.

foxhole said:

Leofrics comments appear to be appropriate, your first line in the first posts is asking for suggestion which is what he gave .

Click to expand...

Thank you foxhole.

Joromy said:

I'm not sure where I said I wasn't going to get a structural engineer in before I start any work.

Click to expand...

Where did I say you weren't getting a structural engineer in .

Joromy said:

I'd prefer if we stayed on topic and discussed the existing structure instead of veering off into architects, structural engineer & building surveyor territory please. I wouldn't insult your intelligence so please don't make assumptions about what I might or might not do.

Click to expand...

I was giving recommendations on the best way of dealing with it , I never mentioned a building surveyor, but do what you like . Your structural engineer will advise on the structural aspects in any case. What's all this about insulting intelligence and making assumptions, I don't need to come on here to be insulted
Anyway ,if that is your attitude sort it out yourself

There may be a ridge beam - as opposed to a ridge board - which is a structural component that can support the rafters and therefore prevent the walls spreading. That might explain the apparent absence of an intermediate support wall that is structural, and why the ceiling joists are not (apparently) tied to the rafter feet - which is a common way of preventing spread.

If you pay an SE (usually £200 or so) he/she will visit and carry out an initial assessment and that should provide the explanations you seek on the structural issues (and identify any fundamental problems with your plans).

I'm a diyer, so just some pointers.

Blup

blup said:

There may be a ridge beam - as opposed to a ridge board - which is a structural component that can support the rafters and therefore prevent the walls spreading

Click to expand...

Thats what I thought although it wasnt commented on so perhaps the OP haas discounted that option.

I think he should get an architect or arcitectural technician in to draw it up for him....I dont know why he hasnt thought about it already

This construction is pretty much identical to mine, though I have a brick wall supporting the King Truss in the centre.

Without the spine wall, the forces in the two triangles of the King Truss will resolve differently, but essentially the top elements of the King Truss are resisting the sag of the bottom chord, which is going to be Victorian over-engineering anyway. In theory your ridge board is supported onto the King Truss, but in reality it's supported by the pairs of rafters either side, which are being restrained by the lats above and are nailed to the purlins with massive brads. With the spine wall, the King Truss is stopping the Purlins sagging over time, but nothing else.

Your ceiling joists are a lot stronger than they look, particularly as they're tied together by the laths, they'll take a lot more than you think.

Obviously you need a Structural Engineer to confirm what you need to do, but also how you do it so that everything remains appropriately supported as you do the work, so the above is posted in that context.

We've just completed the first elements of our loft conversion, we've put in a new floor, opened up for new stairs and removed the King Truss. Process was broadly:

1. Install new floor, bolted to gable walls, steel supported on the spine wall, crucially we retained the bottom chord of the King Truss as it's holding the ceilings up like yours. We used 160 x 10mm steel plate either side to convert the chord into a flitch beam and used the top as the reference for our finished floor height.
2. Structural stud walls on both gables taking load to the new floor via the wall plates
3. 280 x 90 Glulam ridge beam lifted as high as possible without cutting the King Truss
4. Acrow props to top Purlins, with scaffold linking the props. Jack the purlins to leave a 2mm gap to King Truss - definitely no load on it now.
5. Cut out first diagonal section and top of vertical post.
6. Lift new ridge beam into place on structural stud walls.
7. Remove the rest of the King Truss (We've retained part of it for now to support the rear roof with an acrow as the whole roof is coming off at the back for Dormers).
8. Reinforce the existing rafters with deeper timbers, layout depending on velux windows etc. I did this using an acrow to support the top purlin and then cut it away working left to right one rafters at a time so I was only fighting one nail at a time. New timber on, then move onto the next rafter.
9. Add a post from the lower purlin to the top of the bottom chord/flitch arrangement. I used a piece of the King Truss for this as it would sit nicely between the steel plates, then added some C24 either side.

Where we cut the existing ceiling I added vertical blocks to the new floor joist to hold the cut ends, then supported the new floor downwards where it doesn't run to the gable wall. I massively over-engineered the cantilevered section, you could stand on it unsupported, though probably not jump up and down, so now it's sat on a beam below it's rock solid.