A Question of Strength from the Professionals Please

[freddiemercurystwin]

I'm putting together a quote for drawing up the Planning and Build regs submissions for a job. Its to add a storey and pitched roof onto a single storey pitched roof double garage adjoined to a 2 storey detached house. I have some of the drawings from when the house was built in '97. The information I have has enabled the sketch of the existing wall to be produced. Let us assume for now that it was built as per the drawings.

The existing garage walls are single skin 3.5N dense blockwork and 7N below the floor as per the drawing. Ideally I would want to add an inner skin and a cavity internally as per the proposed drawing or alternatively do it in timber frame although I don't think that will make a difference to the answer to my question. The observant ones will notice that the proposed inner skin is bearing down on the B/B floor which has nothing immediately below it. I suspect that the B/B floor will not tolerate a loading of the new floor/walls/roof from above at least not on paper. I spoke briefly to an engineer today who confirmed my suspicions that it is unlikely to work on paper which BC will obviously expect to see evidence of. What is the opinion of the regulars please? I don't need a definitive answer just an opinion that can be comunicated to my client. Whether we think it'll work or if he's gonna have to knock his garage down and start again which may kill the job of course!

Existing

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• freddiemercurystwin
• 20 Nov 2009
• 1

Proposed

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• freddiemercurystwin
• 20 Nov 2009
• 1

 

[noseall]

Why not build as 225mm thick wall above, i.e. add another skin with no cavity and insulate internally - thus building directly above the foundation masonry below?

Surely this will alleviate downward shear prob's?

 

[Shytalkz]

If this is just single storey non load-bearing infill stud built off the pot and beam will be fine.

 

[stuart45]

He is talking about adding another storey and a pitched though Shytalkz.

 

[freddiemercurystwin]

Thats right, it'll be two storey, ground + first, with a tiled pitched roof above.

 

[Shytalkz]

It probably would still be ok if all in timber, but would require reverting to the manufacturers for some calculations.

Nose's suggestion is the best one viz another block leaf to make it 9", then insulate internally.

Of course, you'd then have to think what you were going to do at first floor level, or ensure that the floor joists are designed to take a point load from the external wall and roof - but this shouldn't be a problem if the inner leaf is timber stud.

 

[freddiemercurystwin]

It probably would still be ok if all in timber, but would require reverting to the manufacturers for some calculations

Hmm, trouble is I have no idea of the manufacturer, drawings state a proprietry system so could be anyone.

What about this then with a bit of corbelling?

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• freddiemercurystwin
• 21 Nov 2009
• 1

 

[noseall]

We were able to gain 60mm or so by using a 6mm thick galvanized steel plate to bear the brick and block-work, when building a first floor extension over a 215mm ground floor building.

The steel plate measured 260mm or so and sat atop the 215mm wall then our masonry above occupied the entire width of steel.

 

[lostinfens]

Any chance of bearing the new internal skin on steel beams? Even if strengthening the walls at the load points is required, it would seem a cost effective & relatively faff-free solution. To be honest, I have some reservations about the corbelling method.

 

[freddiemercurystwin]

We were able to gain 60mm or so by using a 6mm thick galvanized steel plate to bear the brick and block-work, when building a first floor extension over a 215mm ground floor building.

The steel plate measured 260mm or so and sat atop the 215mm wall then our masonry above occupied the entire width of steel.

Is this what you mean nose?

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• freddiemercurystwin
• 21 Nov 2009
• 1

Not sure why its different to corbelling?

@ lostinfens, there are some piers dotted around the perimeter which could prob take a steel beam bearing although thats a faff in its self. why do you doubt the corbelling method?

@ everyone: I know I'll likely need an engineer on this whatever method ends up being used when its underway, I'm just trying to make sure the jobs a goer at this stage. Engineers generally aren't too keen on getting their calculator out unless they've been appointed!

The client is actually a garage conversion 'expert' and would like to be able cost it in both timber frame and masonary for the home owner which is why I'd like to find a method for a masonary solution. I say expert, he's only done internal garages up to now rather than adding another storey to a singel storey garage.

 

[noseall]

Precisely.

Except the blockwork overhung slightly less.

 

[lostinfens]

As I understand it Freddy, you mean to corbel from a b&b floor and build 2 storeys of blockwork including bearing an upper floor on it, is this correct?
I just have reservations about that much on that little. I have built many unusual corbellings in the past, but none I think that underpin the main structure in this way. I also feel that a b&b floor is never the best thing to build on except where there was a clear intention to do so initially. I've found in some cases, inner skins built on b&b floors can transmit considerable vibration- not a good thing for corbels. It depends on the original construction, and you would have to rummage about to check it out before doing anything above. Using beams would obviate the need for any sub floor work and save on ground floor blockwork, which would help meet any cost of steel etc.

 

[jeds]

Can you see the B&B floor - i.e. do you know its true dimensions? Usually they are higher spec than standard floors. If it is an upgraded floor I would have thought it will do the job. Still needs confirming with the original floor manufacturer though.

 

[freddiemercurystwin]

Can you see the B&B floor - i.e. do you know its true dimensions? Usually they are higher spec than standard floors. If it is an upgraded floor I would have thought it will do the job. Still needs confirming with the original floor manufacturer though.

No there is a screed and I have no idea who the manufacturer was.

 

[jeds]

I think I'd ask a manufacturer to do a design and spec based on your proposal and see what they come up with. At some point though you will need to find a way to compare that with the existing. Maybe remove an air vent, or cut out a brick.