# Flat roof workshop build



## joeg (27 Jan 2020)

Hi Guys,

Had a good look over the previous topics and struggling to find any information on flat roofs. 

My build is on a boundary so am limited to 2.5m hence the flat roof. 

I have a solid concrete slab that measures 5m X 8m. I have the option of making the walls from either wood or breeze block. The other option is if I use wooden joists or steel to support the flat roof. I do not want to have a supporting beam in the middle of the area so the joists would need to span the distance. 

I can find information on the wooden joists and are happy but I am struggling to size the steel option. Has anyone had any experience in this area?

Thanks in advance all.


----------



## MikeG. (27 Jan 2020)

Hi, and welcome to the forum.

My first suggestion is to allow a maximum of 4.8m for the roof timbers, including overhang, because it is tricky finding anything longer than this. If you go for a 5m span plus overhang you'll find you run into structural issues and the potential extra cost of steel. My second suggestion is to slope the roof down towards your property, and not back towards your neighbours. This will prevent any boundary issues with overhanging gutters and drains in the ground. I would definitely advocate building it in timber frame, otherwise you will need much thicker walls to fit your insulation in. Finally, the old chestnut.....put the building on a brick plinth, and not straight onto the slab. Have a look at the first link in my signature.


----------



## joeg (27 Jan 2020)

Thanks for the quick reply and for your input Mike. 

I agree, 4.8m would be much easier, it just so happens that the walls are the boundary of my property (5000 exactly). Agree on the front to back for the run off, I have sunken drainage in the slab in prep for drainage. I have looked at your plans and they are great and I think I will be going to timber walls.

The other idea I had in my head was running one steal down the middle of the long edge (8000mm) and then using smaller joists rather than using 47*225 C24 joists?


----------



## MikeG. (27 Jan 2020)

I understand the boundary thing, but what other wall constrains the dimensions to 5 metres?


----------



## joeg (27 Jan 2020)

Nothing else, just a want (rather than need) to maximise the internal space.

Due to the size, I might be constrained by using timber as the walls come to think of it? Timber is easier and cheaper but the structure is constrained to upto 30sqm isn't it?


----------



## MikeG. (27 Jan 2020)

Thirty square metres (internal floor area, remember) is about Building Regulations. If you go above that area you will need to get BR approval and have inspections during construction, which means either trench fill foundations or a structurally engineered solution such as piles or a reinforced raft. An 8m x 5m building obviously falls outside these parameters whatever it is made of. There is no difference in the qualifying area of a masonry building compared with a timber building or any other type of construction. 

Building within 1 metre of the boundary (any building above 15 sq m) means having fire resistant materials facing the boundary (some authorities interpret this as any part of the building within 1m of the boundary, so check with them first). This does not usually prevent the use of timber framing, because external fireproof cladding is commonplace. indeed, I have built a straw bale building on a boundary and been certified as in compliance.


----------



## joeg (28 Jan 2020)

Okay, I managed to do some more reading last night. 

Fortunately, my base is a reinforced raft with photos of the pour so I can evidence it. 

I have the option of using a friend to carry build the walls from block work for beers only. 

Again, thinking last night. the slab is 5000 total, minus the 100 block each side then if I put the joists on hangers I will be at the 4800 mark which is good. I had a quote from a local merchant and for 47*225 is £3.47 so much better than I thought it was going to be so I will look at using them across the span.

I have a habit of overthinking and under thinking often....


----------



## MikeG. (28 Jan 2020)

You can't put roof timbers on hangers. That would mean there is no support for the roofing material at the edges of the roof. Also, you have to decide very early whether you are doing a warm roof or a cold roof, and get your eaves detail right to suit one or the other. There are no circumstances in which roof timbers on hangers work, and even if somehow they did you would have no roof overhang, which would mean certain failure at the eaves. Have a look around at other people's builds. There must be plenty on Youtube, and I'm sure you know someone with a flat roofed garage. They're done the way they're done for good reason.

I'd be uncomfortable with a roof overhang of less than 50mm at the top, and 100mm at the bottom. So if you are using 4.8m timbers that puts the outside of your walls 4650 apart. With wall thickness of 10+95+25+30 for OSB, frame, battens, boarding gives you a wall thickness of 160, so you'll be looking at about 4330 from inside to inside of your walls. If you are trying to avoid BR approval (your slab will require structural engineers calculations otherwise, not just a photo of some A142 mesh*) then your maximum length internally is about (30 / 4.33 =) 6.9m, or 7.22m externally.

*A raft isn't a slab with reinforcing in it. It has a thickened edge, and the steel located top and bottom with a cage around the edge, at a minimum. It also has a designed specification of concrete, but most importantly of all, it has a structural engineers name on the drawing.


----------

