Attic Conversion - Ceiling/Floor Joist min requirements

bemak

For attic conversions, where do I find the minimum requirement for the ceiling/floor joist depth? A colleague at work said that there is a minimum requirement in the Building Regs for floor joists and that I should make sure that the existing will comply with the Regs. The original house was built with attic conversion in mind so I would hope that it passes - just want to be sure. I was looking through Part A but can't seem to get a definitive answer.

Obviously the depth of the joist will depend on the span, so with that in mind, when measuring the span, is it considered from external wall to external wall or would it be from where it gets support from ground floor walls?

thanks for any advice!

Gumbo

You need an engineer to determine if the existing joists can take the new loads imposed on it from residential use. They may be tied to the roof rafters and as such you cannot just go throwing extra weight up there.

Part A of the Building Regulations will not tell you what the minimum sizes are, but it will tell you that any structural elements needs to be designed in accordance with the principles of structural engineering.

The span is bearing to bearing. The span can be broken by supporting walls from underneath but the joists will then be typically lapped (a new joist will start from the wall).

It's my opinion that you need to pay an engineers for structural inspection. It might not cost that much depending on what's required but bear in mind you also have requirements under Part B, Part K to a small extent and Part F at least.

bemak

Ya I think you're right. Best leave it to the expert to stand over it.

Thanks for your help

Calahonda52

bemak wrote: »

Ya I think you're right. Best leave it to the expert to stand over it.

Thanks for your help

OP, kc is right, as a matter of interest if the house was built with this in mind, then measure the existing joists: my guess is they are 9" by 2" or 225mm by 45mm.
The other thing to look for is the opening organised for the stairs to come up in a particular place: look for doubled up 9 by 2's

If the joists are part of a pre-made truss, as opposed to a cut roof with 9" joists, then you are into steel beams to support a new floor above the existing which eats into the head room

bemak

Calahonda52 wrote: »

OP, kc is right, as a matter of interest if the house was built with this in mind, then measure the existing joists: my guess is they are 9" by 2" or 225mm by 45mm.
The other thing to look for is the opening organised for the stairs to come up in a particular place: look for doubled up 9 by 2's

even if they are who's to say they're sufficient. I would assume they are but that's just my assumption. What I'll do is take a more detailed survey and once I have a layout in mind, the homeowner can get a report from an engineer just to cover themselves. Would only be minor but takes me off the hook for it if I suggest it's ok.

Gumbo

bemak wrote: »

even if they are who's to say they're sufficient. I would assume they are but that's just my assumption. What I'll do is take a more detailed survey and once I have a layout in mind, the homeowner can get a report from an engineer just to cover themselves. Would only be minor but takes me off the hook for it if I suggest it's ok.

What is your involvement here? I thought it was your house?

Generally with typical attic conversions, there will only be one layout option due to the location of the existing stairs. Realistically you are looking at one large room and maybe a small ensuite/closet off it. Not a lot of scope in a typical house tbh.

bemak

No I never said it was my house - it's something a friend asked me to look at. I usually deal in commercial work so this is alien enough to me.

The attic in question is a whopping 75sqm. It qualifies as being a habitable space too when calculated using Part F's guidelines. The original question was just regarding where I might information on the appropriateness of the existing ceiling joists.

Thanks

bemak

No I never said it was my house - it's something a friend asked me to look at. I usually deal in commercial work so this is alien enough to me.

The attic in question is a whopping 75sqm. It qualifies as being a habitable space too when calculated using Part F's guidelines. The original question was just regarding where I might information on the appropriateness of the existing ceiling joists.

Thanks