Raising the roof, scissor or raised tie truss...

[bobbert]

Hi all,

So plan is to creat a new liviing space in the loft. Already have about 3.3m to the apex. We would like more head height at the front and back though (means we can have 2 extra rooms due to improved access).

So the plan:

1 Remove existing roof,

2 Raise the walls (2 options here)

    Option 1 - raise front and back and reduce the roof angle

    Option 2 - raise all walls and chimney and keep current roof angle.

3 Install three dormer windows at the front and three skylights at the back (north facing)

4 New trusses either scissors or raised tie across 5m span of the house.

 

That's it in brief - I'm looking for thoughts and experiences on trusses, type, angles, and any other considerations anyone had to make.

Just to note, everything removed will be used in a future part of this project. There is some really nice wood coming out of the roof.

[bobbert]

Another pic from the outside for some context.

[Radian]

11 hours ago, bobbert said:

Install three dormer windows at the front and three skylights at the back (north facing)

So dormers face south. You could do with working out the solar gain from those windows. Rooms in roofs suffer from overheating in the summer. One way to estimate the solar gain is to pretend the windows are solar panels and put their details into PVGIS That will give you a feel for the kind of energy input each month - bearing in mind that PV generally converts only 20% of solar radiation into electrical power.

[Dave Jones]

attic truss.

[Gus Potter]

On 19/10/2022 at 10:42, bobbert said:

So plan is to creat a new liviing space in the loft.

That's a cracking looking project you have there.

 

On 19/10/2022 at 10:42, bobbert said:

So the plan:

1 Remove existing roof,

2 Raise the walls (2 options here)

    Option 1 - raise front and back and reduce the roof angle

    Option 2 - raise all walls and chimney and keep current roof angle.

3 Install three dormer windows at the front and three skylights at the back (north facing)

4 New trusses either scissors or raised tie across 5m span of the house.

1/ Remove the roof..

Yes probably the most practical. Keep that timber!

 

2/ Option 1. - raise front and back and reduce the roof angle.

I would think carefully before you change the roof pitch drastically, you could easily change the character, apprearance.. planning issue? As a guess that looks like about 40 deg.. makes for an attractive, watertight roof.. easy on the eye (well mine anyway). Also in terms of structural design it's a nice and angle as it reduces the bending effects in the rafters as more is converted into an axial load.

 

Option 2 - raise all walls and chimney and keep current roof angle.

Now that is an option especially if you have enough matching stone to allow you to hide the fact that you have added say 300 to 450mm to the external wall height. Remember though that you just need the outside to match, you can use a cheeper material where hidden. I would try and keep the something akin to the current roof angle.

 

3 Install three dormer windows at the front and three skylights at the back (north facing)

If you raise the walls then you may not need all three dormers. Remember they are a lot of work =  cost. Can you get away with two and say put a feature window in the gable end?

 

4 New trusses either scissors or raised tie across 5m span of the house.

On paper your approach is good.. but this is an old building and they don't take kindly to the horizontal forces that raised tie and scissor trusses can generate at the wall head. Yes you can get special slip shoes to mitigate.. but... then you don't get the wall head tying effect.. see later.

 

Have you put a string line / plumb bob over the existing walls yet? You may find they have been enjoying life and are far from straight and plumb. Nothing wrong with that but it can be a challenge to marry up a modern manufactured truss with an old structure like that.

 

I would have a look at traditional cut rafter roof but with say a steel, timber flitch beam or glulam a ridge beam with simple rafters down onto the wall head. The first floor can then be independant and all that leaves you with a big space and a vaulted roof to really play about with.

 

The main thing here is that with a bit of care you can adjust the rafter lengths and so on to follow the shape of the old building to some extent. At the end of the day it depends on what floats your boat. For me.. if it was mine, (can you feel the jealousy creeping in?) I would want it not to look like I had put a modern square box roof on an old building.. I would want to keep the gutter detail looking "original" and so on... and a good joiner (chippy) can work wonders here.

 

For all:

 

I have copied a bit from and old Wolf Systems design guide which lets you see the typical types of prefabricated roof trusses.

 

 

 

The raised tie and scissor trusses can add an outwards thrust onto the old wall heads. They are old walls so SE's are cautious and that can lead to an overly conservative design... underpinning, knocking down bits you don't really need to, replacing perfectly good old oak beams and so on. These "conservative SE add ons" can by far outweigh any savings you may make using prefabricated roof trusses.

 

Over the years I have found on these conversions that the walls tend to lean out at the head. It's rare to find them leaning in. That said though, I did look at one that did but the Farmer had excavated out the inside to make a slatted slurry pit for his cattle. Somebody did buy it and turned it into a house! I suspect the pit was filled in before it went on the market.

 

Let's assume you have some walls leaning out. You are adding a first habitable floor so you put some of the floor load onto the inside of the external walls. This tends to tilt them back the right way. At the same time by using a ridgebeam you take some of the rafter roof load and shed it to the inside walls.. this is load reduction on the external walls which is helpful.

 

You also tie the cut rafter to the wall head.. so that if the wall wants to move further outwards the rafter holds it still as it's connected to the ridge beam.. the rafter acts like a tie. The load is tranferred back up into the ridge beam and taken out away from the external wall or in a place you know is good to go.

 

At the end of the day you play about with the structural design so that you try not to add any more significant weight to the old foundations and that lets you say.. hey the old founds maybe good to go without any major interferance =  cost.

 

To make all this work and make the real savings at the end of the day I would always say.. let's look at what we have, how does it behave / work and let's design around that. Every old building is different and that is part of the fun / challenge.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[joe90]

I really like the look of (decent) dormers but they are more difficult to Insulate well (thick cheeks) but maybe at your latitude you are less worried about that. Secondly a steel or glulam ridge will allow a cut roof giving a completely clear void in there.

[Gus Potter]

11 minutes ago, joe90 said:

I really like the look of (decent) dormers but they are more difficult to Insulate well (thick cheeks) but maybe at your latitude you are less worried about that. Secondly a steel or glulam ridge will allow a cut roof giving a completely clear void in there.

Yes a well designed dormer can really compliment a roof.

[bobbert]

On 20/10/2022 at 21:18, Gus Potter said:

 

Thank you for the hugely detailed response, I've been digging into the details over the weekend.

On the dormer front, I think 2 and a skylight instead of the third dormer would be just as good if it's a cost and complexity saver. The particular style of dormer we're keen on is a wall dormer. The picture of the wall dormer below is a guide to what we'd like to achieve.  

You're point of the symmetry of the house is spot on. The house itself is not square, more of a parallelogram floor plan. 

I like the suggestion of a timber flitch beam or glulam ridge beam. Feel closer to how we'd like the space to be. I need to do my research on this as it's not something I've any familiarity with.

On 19/10/2022 at 22:41, Radian said:

 You could do with working out the solar gain from those windows. Rooms in roofs suffer from overheating in the summer. One way to estimate the solar gain is to pretend the windows are solar panels and put their details into PVGIS

Were intending to get an energy simulation done that should give us a total energy performance once we've come up with a design. This will hopefully help us size windows and select where we get the biggest bang for buck on insulation, triple glazing Vs double etc.

 

 

[joe90]

4 hours ago, bobbert said:

The picture of the wall dormer below is a guide to what we'd like to achieve.  

The problem with that is you will need to raise the walls and match the stonework! (More work/expense), I would raise the dormer from the existing wall with slate hung vertical (to match the roof) below the actual window. (Can’t find a picture but have built some like this in the past). Or build the dormer back from the wall at a height to match the cill to the vertical stud wall required within the room.

Edited October 24, 2022 by joe90

[bobbert]

10 hours ago, joe90 said:

The problem with that is you will need to raise the walls and match the stonework! (More work/expense), I would raise the dormer from the existing wall with slate hung vertical (to match the roof) below the actual window. (Can’t find a picture but have built some like this in the past). Or build the dormer back from the wall at a height to match the cill to the vertical stud wall required within the room.

 

We're not necessarily going to match the stone work as we go up. We will be trying to maintain the character. Within the local area we've seen some lovely complementary styles that enhance this look rather than try to hide the change. A few ideas we're looking at are a light colour brick like old London stocks, a brick and stone blend where brick would enhance the symmetrical lines and corners and stone in between. Slate on the dormers is not something we've looked at yet but sounds like it's worth some research.

The roof finish is a whole other conversation. The tiles will be replaced but not like for like.

[joe90]

Yes, look up some pics or local houses and find something you like, that’s what really matters to start with.