Advice required: Increasing existing window opening to take bifolds

[B52s]

Guys,

 

I could do with a bit of advice as I am doubting the structural details provided by my appointed Architectural & Structural Engineering Practice. My only point of contact is the architect who seems reasonable to deal with.

 

The first issue I had was on Detail D1 (below) that shows a Naylot lintel across the top of the non-loadbearing block outer skin (timber framed house). The architect agreed with me that this was not required and said that this was merely a standard detail that has been added to the drawing by the SE and is not applicable in this situation. I can sort of accept that as a reasonable answer.

 

The second issue was the location of flitch beam F1 positioned directly below the existing horizontal roof support (as shown in Detail D1). I pointed out to the architect that the flitch beam can’t be located below the existing horizontal roof support as this would be in conflict with the bifold door opening height. He said he would speak to the SE to see what could be done; I’m still waiting to hear back from him. He went on to say that the builder would sort this out on site, which puzzled me a little.

 

However, in reflection, I’m now seriously questioning the reason for adding the flitch beam in the first place. The current 1800 window sits below the underside of the existing horizontal roof (truss) support as shown on the photo (below) of the existing arrangement. This opening is to be widened to 2850 to accommodate the proposed bifold doors. I can understand that 2 or 3 of the existing 95x45 vertical studs will have to be removed and therefore an alternative vertical support arrangement will be required, but surely this should not require the addition of a flitch beam, or am I being a proper “doubting Thomas” and should just accept that the SE knows best?, after all, structural calcs have been done which apparently support the need for a flitch beam with goal post supports.

 

Any advice/thoughts would be very much appreciated.

 

 

 

[Mr Punter]

The flitch beam is to support the roof over the wider opening.  You could locate it further up.  I assume the Naylor lintel is to finish to the top of the doors and will be rendered to match the rest.

[B52s]

Thanks Mr Punter, yeah, if the wider opening requires a flitch beam, then I can accept that. It appears to be over-engineered (in my opinion) but that seems to be the way of things in terms of today's structural design requirements. With regard to the Naylor lintel, I would have thought that the top of the doors might be better being been finished in timber cladding to match the existing window openings. See below photo showing the existing 1800 window which is to be replaced by the bifolds.

[Mr Punter]

If there is currently no lintel across the masonry leaf I can't see why you would need one with the wider opening.  You may need to mess around with the downpipe as well.  And relocate those air bricks.

[Gus Potter]

9 hours ago, B52s said:

I could do with a bit of advice as I am doubting the structural details provided by

Looks like you may have a bit of a "disconnect" between what you actually have and the drawings so can see why you are raising some queries.

 

Starting with the roof. Looks like you have prefabricated fink trusses which are resting on the timber frame with a stud thickness of 89 -95mm. I'm guessing at the span of the trusses from the photo but over the existing window you'll have maybe have a timber lintel made up of 2 no 145 or 170 deep timbers or a Kerto beam (some composite) of a similar size. The soffit is level all the way round so that tallies with a standard kit. The timber frame is the skeleton of the structure and holds the masonry in place although they both interact to resist wind loading. With that type of roof detail I would not expect to see a lintel on the outside masonry leaf.

 

Now when you widen the opening you get much more deflection, bifolds are very sensitive to this hence the flitch beam. But the flitch beam looks too deep! How do you fit the doors and what is under the concrete lintel shown?

 

I would ask someone to go in the attic and check to see how the eaves are formed and then make sure what is drawn can be built and hold the roof safely wthout excessive deflection. The steel grade of the plate suggests that this may be an old generic detail that is recycled and not maybe applicable to what you have as does the lack of a bottom truss chord in detail D1. That said I can't see all the drawings hence it's just suggestion.

 

 

 

 

 

 

 

 

[ETC]

The engineer’s drawing is wrong. You don’t need the outer leaf lintel (there’s nothing above the window on the outer leaf) and the beam is shown too low in the inner leaf - the bottom of the beam should be the top of the bi-folds. The architectural detail looks like it has everything in the right place. For bi-folds and the stability of the wall panel I would expect some sort of goal-post arrangement - a couple of posts at each side and a steel beam as a lintel to make sure the bi-folds don’t buckle.

 

I wouldn’t go off at the deep end with your engineer or take too much advice from a public forum but I do think you need to tell him that you need site specific - not generic details for the build.

 

Good luck.

[B52s]

Thanks Gus Potter and ETC for your helpful input.

 

I have received an updated structural drawing from the architect this morning (with revised Detail D1 and an elevation detail showing the flitch beam and goal posts). It would appear that things are now falling into place, if you can forgive the unintended pun.