Timber frame extension - Question

[smurph306]

We are planning a timber framed 4m x 6m single storey extension. Having not done one before we have a few questions. Our architect is away for a while and we cannot contact him. Im not sure whether he has much experience with timber frame anyway.

 

He provided drawings for the remodel of the main house which were fine and then he did construction drawings for building control relating to the extension. They had a poured foundation and then 150mm cavity. So weve done all that but now when im looking online I am seeing videoes of timber frame construction without a cavity foundation. Just a solid strip of blocks and then a sole plate anchored into that and away they go.

 

We plan to have a timber frame with either a cement type board with render or horizontal planks of some kind as an exterior finish once an architect has drawn that up.

 

I have just had a quick look on youtube etc and i see in this vid these guys just flapped the walls up in no time and bolt throught the DPC into a two rows of solid concrete block and the inspector liked that. Is it as simple as that? Should I fill in the cavity and follow this type of build?

 

 

 

[MG-Wales]

The cavity at foundation level is usually required if you are building a timber frame with masonry outer leaf or at least a brick plinth/dwarf wall on the outside of the timber frame, below the cladding.

 

If you are saying you plan to use render board or other cladding directly to the timber frame, and there is no brick plinth/dwarf wall at the base, then I would say that the cavity construction below the DPC is not required.

 

The other suspicious thing is that you say that the cavity is 150mm which is more typical of masonry construction (because of insulation applied on the outside of the inner masonry) than timber frame where the cavity is typically 60mm only (50mm clear cavity plus 10mm allowed for the sheathing).

 

What size of blocks has the architect specified on the inside of the cavity? If it is 100mm then it sounds like it has been detailed for masonry, rather than timber frame. For timber frame he would have specified a 140mm wide block.

 

In any case, I suggest you speak with your local building inspector about it. As a timber frame designer, I see many times buildings detailed for masons at Building Regs stage that suddenly turn into timber frame construction. Both systems are very well known, and for a small extension I am sure Building Control will be sympathetic with whichever system you want to use and able to advise on the specific requirements.

[Big G]

Hi, i'm glad i found this post as i'm in exactly the same position, My drawings have been submitted to the planning department but i'm looking at these drawings and thinking why? why a cavity only to fill it up with cement at finished ground level. My proposed extension will be made up with Timber with cement board external and finished with plaster or some kind. The strip under build to DPC is 100mm block outer / 60mm cavity and 140mm inner block.

I questioned my architect on the matter and he came back with a reason "to give strength".....really? he's also drawn in some kind of ugly weather drip edge to cover the block outer leaf of the under build.

Can anyone advise me if these drawings are indeed correct, but probably more important can anybody advise if after the plans are passed can i change the strip under build by simply talking to the building control officer.

 

[Russell griffiths]

@Big G get that changed that outer detail is horrible. 
@smurph306, yes it’s up, yes they made a YouTube video of it, but is it good construction with good methods used. 
then I would say no. If you want to chuck up a quick extension then do what they did, if you want it better, with less  problems then ask some more questions on here. 

there’s a lot of talk on here recently of small timberframe extensions and BC being happy with them, I’m wondering how long it’s going to be before they start rotting out at the sole plate, due to poor details and poor maintenance.