Extension design completed – questions about roof & trus

[kingandy2nd]

Hi all,

I’ve had the first draft of the designs back from my designer and they’re not entirely what I was expecting.

My original brief given to the designer was that I wanted to build an extension (5.5m wide, 9.5m deep on ground floor) with a pitched roof that would allow an attic room with front dormer to be built in the roof space above the extension.

There is a complication in that the only possible entrance to the new dormer room (via existing window at the top of the stairs) is approximately 3.5m from the back of the house – therefore not centered to where the highest point of the new roof would attach to the house.

Well the plans have come back now, and they’re a bit different to expected. The designer has basically drawn up all the walls to be 1.5 storeys – with a pitched roof similar to that of the existing roof (17 degrees). The new roof actually goes to the same height as the ridge of the existing property.

The design looks good (better than my original idea), but I imagine that such a design is going to cost a lot more than anticipated as not only do I have the expense of building another half storey, but I am guessing that such a design will require specialist roof trusses (scissors or double inverted – from google) in order to make the first floor space useable.

I’m going to have a chat to the designer, but would really value some independent input from you guys on here so I can form an opinion before asking for revisions to the design. The things I’m not really clear on are:

Is there any reason why the new roof cannot be at a steeper pitch than the original roof (providing it won’t be higher than the existing ridge)? – I am thinking that with a steeper pitch then roof would join the front walls at a lower height and thus reduce the need for additional brickwork on those walls.

Are specialist roof trusses considerably more expensive than standard attic room trusses? Are the any additional complications with such trusses – I read on one website that scissors trusses are more difficult to insulate sufficiently.

Could it actually be wiser to install a second staircase to access the new room attic room rather that trying to modify the design to meet up with the potential opening that’s already there?

Thanks in advance

 

[RonnyRaygun]

I'm not sure I fully understand what is being described, but can't you have a ridge beam and cut roof?

Do you have drawings you can attach?

 

[^woody^]

There is nothing to prevent a new roof with a different pitch, but the point of design is to come up with an acceptable scheme which harmonises with the existing, and not look like some random tracked on extension.

I don't understand the question about trusses. You have a truss, or if you want a room in the roof, an attic truss. There will be a cost difference.

 

[noseall]

There will be a cost difference.

And a weight/section size difference.

 

[kingandy2nd]

Hi guys,

Thanks for taking the time to reply.

I totally appreciate that part of the skill of the designer is the look/harmonisation piece which is where the designer has gone with the drawings, but it may put the build beyond my means if specialist trusses and additional wall heights are required.

Based on the drawings the walls on the first floor are approx 1.2m high and the roof starts from that point and rises up to about 3.5m at the ridge line. Therefore the bottom chord of any standard roof trusses/attic truss would be at waist height in the first floor.

Therefore via googling I've made a assumption that this design needs to incorporates trusses that don't have a bottom chord that is horizontal - otherwise I can't see how the upstairs room could be useable.

Does that make any more sense?

While I have drawings, at this stage I've only paid the design 25% up front as a deposit so I'm not sure i have any rights to publish the drawings.

To give you some idea, this is my house now and we were planning on demolishing the garage and building the extension there:

View media item 50058
And this is the kind of extension I had imagined:

View media item 62480
Cheers
Andy

 

[^woody^]

I see

Here is a quick image of a similar thing I did quite a few years ago. The two pitches were different, and the front wall was taken up as you suggest, to cill height. I went for a cut roof and there was a steel ridge beam.

Untitled

• ^woody^
• 12 Jun 2013
• 1

 

[kingandy2nd]

Hi Woody,

Yes that's the type of thing that's been designed - but with a 17 degree pitch on the roof.

Ok I see, the roof wouldn't actually be made of trusses but purlins and a ridge beam - I wasn't aware of that sort of configuration before.

I guess that this type of work is again going to increase the costs over trusses because I assume the ridge beam RSJ is going to have to extend all the way across the ridge of the extension and half the width of the original house in order to meet up with the original ridge line (around 9m) and I assume that a gable wall is going to have to be build on top of the existing outside wall of the original house in order to provide support to the rsj and roof.

This is starting to sound very expensive...

 

[^woody^]

Trusses or a cut roof will have their particular costs and pros/cons. If you want a room in the roof, then you will need to pay for it one way or the other

If you have only had some outline designs done, then you should ask your designer to look into either, and then he (or you) can work out some basic costs. You can get a truss quote with only three dimensions, and the designer can tell you what timber or steel sizes will likely be needed for a cut roof

Your designer may or may not be able to design out costs, depending on his knowledge

 

[kingandy2nd]

I knew there would be additional costs in having a room in the loft. Originally I had planned on having a double storey extension with pitched roof , but I was led to change my mind for two reasons.

Firstly, because I was told by the designer I wouldn’t be able to have a non-frosted window at the side/ rear on the first level because it overlooks my neighbour’s garden – which meant the building a double storey extension over the full floor plan of the second level that would only leave me with one 5.5m x 9m bedroom – which would have been costly compare to the value gained.

However we still wanted an extension 5.5m x 9m downstairs – for a double garage with a kitchen behind on the groundfloor – and a pitched roof .

So, secondly, as we were still wishing to have a single storey extension with a pitched roof, I was of the understanding that the costs of building a taller pitched roof to incorporate an attic room would not been significantly more expensive than building a standard pitch roof with trusses (it terms of building a waterproof shell) - and would be considerably cheaper than building the originally planned double storey extension.

Perhaps this was belief was misplaced – but I anticipated that the change from standard roof trusses to attic room trusses wouldn’t be a particularly big jump.

The design I’ve received would never achieve any such savings as it requires 1.5 stories to be built anyway, and then a bespoke roof solution too – and thus might be prohibitively expensive.

Part of me now thinks I should just ask for a re-design and just a smaller first storey room built to full height (say 6 x 5m) and just have a basic truss roof built onto that. I guess that would just need some suitable RSJs at the first floor levels to support the walls above.

It won’t look as nice, but will be a cheaper compromise to get an additional bedroom & en suite.

 

[maltaron]

You do not need a steel ridge beam on a cut roof if the base of the triangle is formed by the ceiling/floor joists. I have just done an extension and the difference in cost between trusses or cut is negligible. (there is less timber in a cut roof). The only problem for either type is that if hips or valleys are involved then you may have a problem finding a carpenter to cut it.

 

[RonnyRaygun]

You do not need a steel ridge beam on a cut roof if the base of the triangle is formed by the ceiling/floor joists. I have just done an extension and the difference in cost between trusses or cut is negligible. (there is less timber in a cut roof). The only problem for either type is that if hips or valleys are involved then you may have a problem finding a carpenter to cut it.

True, but the point is that the masonry is one and a half storeys, so the rafters would be high above the first floor joists, and there would only be nominal tying from the ceiling joists.

It should be possible to rest the ridge beam on the existing gable wall. It's possible further steel might be desirable to support dormer walls, or what would otherwise be very long joist spans.

 

[kingandy2nd]

You do not need a steel ridge beam on a cut roof if the base of the triangle is formed by the ceiling/floor joists. I have just done an extension and the difference in cost between trusses or cut is negligible. (there is less timber in a cut roof). The only problem for either type is that if hips or valleys are involved then you may have a problem finding a carpenter to cut it.

Thanks Maltaron, there are hips on the roof and as Roony mentions the floor joists are separated from the roof by about a metre of masonary.

True, but the point is that the masonry is one and a half storeys, so the rafters would be high above the first floor joists, and there would only be nominal tying from the ceiling joists.

It should be possible to rest the ridge beam on the existing gable wall. It's possible further steel might be desirable to support dormer walls, or what would otherwise be very long joist spans.

As the ridge beam of the extension would be at 90 degrees to the existing roof there isn't an existing gable wall where the extension meets the house.

I appreciate you're at a disadvantage as you can't see the drawings, but I don't think it's right for me to post them publicly without permission.

 

[RonnyRaygun]

As the ridge beam of the extension would be at 90 degrees to the existing roof there isn't an existing gable wall where the extension meets the house.

You could still support a ridge beam off a post supported on the flank wall.

The ridge beam doesn't need to go all the way to the existing roof, as the rest of the rafters could be supported on valley boards with a ridge board at the apex.