Cantilever

[eandg]

We are at the design stage and considering having our first floor cantilever out 0.5-1m beyond our ground floor (mostly due to the space/use we want but also to provide shading to downstairs living space). Is this possible with timber frame and will it be particularly expensive?

[Jeremy Harris]

Definitely possible, we were building timber framed houses 500 years ago with first floor cantilevers of this sort of size. 

 

Needs careful detailing, though, as the potential for poor thermal performance under the cantilever section will need some thought.  That section of "floor" will have a much lower potential temperature underneath it in cold weather than the ground floor, so will need both a decent level of insulation and careful design so as to reduce the impact of thermal bridging.

[Bitpipe]

Should be fine - we have a cantilever box that sticks out about 1.5m - plenty of steel holding it up inside the frame  and the frame company didn't blink when they saw it.

 

Picture here - 

 

 

 

Edited February 25, 2019 by Bitpipe

[eandg]

Thanks both - does the frame company typically take care of the structural issues (adding steel etc) or do you need to get someone else to do that? And how much additional cost might you expect (looking at a 12.5m run of 0.5/1m overhang)?

[Jeremy Harris]

It's just run-of-the-mill stuff for most frame companies, and they should just include whatever structural stuff is needed in their design, including steels, if needed.  You just need to watch the thermal detailing carefully, as it can be tricky to reduce thermal bridging where you have steels poking out from the main structure.

[eandg]

9 minutes ago, JSHarris said:

It's just run-of-the-mill stuff for most frame companies, and they should just include whatever structural stuff is needed in their design, including steels, if needed.  You just need to watch the thermal detailing carefully, as it can be tricky to reduce thermal bridging where you have steels poking out from the main structure.

 

Appreciated - will make sure we keep an eye on it!

[Mr Punter]

If they can do the cantilever in engineered timber it will make it simpler to finish and insulate, with less cold bridging. Typically the cantilever will need to project into the building at least twice the distance as it projects out and this will be concealed in the floor zone.

[jack]

We have three cantilevered sections. One is only about 600mm from memory, and isn't much of a thermal bridge because it didn't need steels.

 

The other two, however, are over a metre each, which required the use of steels. They also support a balcony, and so couldn't be within the thermal envelope. As a result, we have cold bridges at these points. Unfortunately, I didn't pick this up until it was too late. To try and mitigate this, the external steels are wrapped in aerogel blanket (supplied by the timber frame manufacturer) and I'll be further insulating with some left-over rigid insulation before enclosing these areas in the coming weeks.

 

There are ways of thermally breaking such steels if they're required. One I saw used GRP spacers positioned within the wall, between the internal and external steel work. No idea of cost though.

[newhome]

Slight off topic but I read lots of posts about steels being needed / used. I don’t think there is any steel here. Is that ok? 

 

 

[Nick1c]

There are  thermal &/or financial costs to any deviation from an ideal form factor with minimal glazing. Budget, plot, planning policy and personal taste determine where each individuals 'sweet spot' is. We are going for an over glazed, cantilevered reverse level house, we accept that it will cost more and be less efficient, but to us, in our specific case, we think it is worth it. As a result the square meterage is less than it might have been & I am doing all I can to minimise the penalties.....

Managing the thermal bridges from the steels and overheating are two of the things I am keenest to get on top of. I am hoping for clever ideas from the frame designer & architect to help with the former and relying on external shading of the south facing Windows and rooflights for the latter. 

If you are going for the cantilever for practical rather than aesthetic reasons you could consider supporting it with posts, that would be cheaper, and more efficient, but less dramatic. 

[mvincentd]

Google Farrat plate...SE should spec them anyway i’d Imagine.

[redtop]

On 25/02/2019 at 17:42, jack said:

We have three cantilevered sections. One is only about 600mm from memory, and isn't much of a thermal bridge because it didn't need steels.

 

The other two, however, are over a metre each, which required the use of steels. They also support a balcony, and so couldn't be within the thermal envelope. As a result, we have cold bridges at these points. Unfortunately, I didn't pick this up until it was too late. To try and mitigate this, the external steels are wrapped in aerogel blanket (supplied by the timber frame manufacturer) and I'll be further insulating with some left-over rigid insulation before enclosing these areas in the coming weeks.

 

There are ways of thermally breaking such steels if they're required. One I saw used GRP spacers positioned within the wall, between the internal and external steel work. No idea of cost though.

and only now realised we have exactly the same thing with balcony, flat roof overhangs, steel posts from ground up through floor and onto support upstairs.  And not a thermal break in site. great. I should be able to insulate steels inside and outside house, do I go with eps, spray foam, need a vcl? arghhh

[Ferdinand]

On 25/02/2019 at 15:57, eandg said:

We are at the design stage and considering having our first floor cantilever out 0.5-1m beyond our ground floor (mostly due to the space/use we want but also to provide shading to downstairs living space). Is this possible with timber frame and will it be particularly expensive?

 

You need to check how far it needs to project wrt the angle of the sun in your location at the time of year of interest.

 

I have one with I think an approx 1m projection at a height of maybe 2.6-2.7m, and it is not quite enough for all occasions, but it was done in 1970 so I forgive them not doing it online in the sun simulator. This photo shows the sun angle at one point. The overhang faces due south. Check what you need, or at least what you will get for x mm cantilever at y height.

 

0.5m is unlikely to be enough, and I would be guestimating 1-1.5m may be necessary.

 

 

Edited February 25, 2020 by Ferdinand

[JOE187]

My cantilevered balcony  here and all calculations done by floor i  joist engineer. Lots of details in ijoist specifications.