Loft Conversion on Cob Walls

[greyhound]

Hi,

I'm a new member and I'd be grateful for any discussion about the problems (and hopefully solutions) involved in creating a loft room in a terraced house with (chalk-based) cob walls. (Unknown date.)

We seem to have building control who are happy (in theory) for us to do this, but the plans discussed so far involve a lot of weight being put onto the walls in the form of steel beams or heavy extra timber, solid purlin walls, etc.

We're looking into lighter alternatives to replace the beams that held the roof up (downward V's and cross pieces). Hoping for a solution that doesn't need a purlin wall on both sides to support the roof but has other, beam-based solutions, which would also allow more space in the room. We're looking into using beams for that.

Perhaps allowances are sometimes made by building control to use smaller floor joists to reduce weight. Is anyone ever permitted to lay a floor on the ceiling joists in order to reduce the loading on cob walls? (Perhaps we can limit the number of people who are allowed to use the room. It will be a single bedroom.)

If anyone has any ideas, or experience of this, I would be pleased to hear from you.

(Also - how exactly is the strength and condition of cob walls determined, without damaging them?)

 

[MatthewC]

Welcome Greyhound. You seem to have posted a difficult if not impossible question for the forum, as normally you'd have had 30 replies including a lot of off-topic chat and a request for photos by now. I can't help with your question as I have no experience of cob, but you might need to engage a specialist structural engineer, I think. However, this will bump it back to the top so let's see if anyone has any ideas!

Matthew

 

[MdB]

Welcome.

As MatthewC says, you need a structural engineer to come up with something sensible. BC won't accept anything else if isn't something standard that they know will be ok. They also won't accept anything along the lines of reduced strength because you will limit the number of people. It will be strictly, a room of this size needs to be able to support this amount of weight.

 

[AMc]

Vote 3 for a structural engineer. Check they have knowledge of cob before you engage them. I don't see why cob shouldn't be strong enough to support a relatively small increase in load of a room vs. an unoccupied loft but you really will need those calculations and the insurance backed word of an expert, esp. if your property is mortgaged.

 

[Penners]

Greyhound - I don't know whether anyone else has mentioned this ( :wink: ), but I think you need the services of a structural engineer.

Building control are going to need full loading calculations in order to give you formal consent, so although the expense of consulting an engineer is probably unwelcome, I don't think you're going to be able to avoid it. As MdB mentions, I'm afraid it's not sufficient to promise to limit the people in the room - you may be able to enforce that promise yourself, but others are likely to live in the house after you. So the loft room is going to have to comply with standards.

 

[Pford75]

Another vote for a structural engineer...but I will venture some of my own 'unqualified' thoughts as well.

I assume that the rationale behind you wanting to reduce the weight of the floor joists is to in turn reduce the weight onto the walls?

I recognise that this is slightly different since you are talking about a loft conversion (therefore I assume that there are two floors below the floor joists?), but in our previous property we had to insert horizontal steels to support a chimney stack which was removed on the ground floor only. Since the walls were clay lump, our structural engineer advised that these horizontal steels (in the ceiling of the ground floor) be supported by two vertical steels running down from each end. These vertical steels were then braced with another horizontal steel 3 ft into the floor which was then set in reinforced concrete. Basically, this created a steel square and prevented the load from spreading the walls. Might seem like overkill, but this was what they advised.

Only downside is that in your case, I'm assuming the steels would have to be huge if they were to reach the ground floor?