Designing dormer in loft conversion

[Diles]

Hi, I am currently embarking on a loft conversion for which i have done most the design and drawing work myself but have had a colleague at work provide the structural calculations needed.

The conversion will be in a detached 1920's house with a hipped roof. The intention is to position the new staircase above the existing, which will mean that we need a dormer for the headroom as it rises against one of the external walls. I would really like to extend the dormer the whole way along that side of the house now to give a bigger space inside the room not just for the staircase, however, my structural engineer says that because we have gone with timber flitch beams (a mild steel flat between 2 timbers) running either side of the stair opening, which would also be be below the dormer cheeks we wouldn't be able to make the dormer any bigger without having to change to steeel sections instead of the flitch beam. This is something I ideally want to avoid at all costs. Would anyone be able to advise me if by extending the dormer it would increase the load going through the flitch beams? The way i had envisaged it was the dormer ceiling joists would be bolted to the existing rafters, then at the other end they would be fixed to the front face vertical timbers that sit directly over the existing wall plate upon the external load bearing wall. Where the dormer abuts the pitched roof the structure would connect to doubled up rafters at each end which would again be ssupported by ridge beam at one end and wall plate at the other. At which point does the load transfer down through the dormer cheeks onto the flitch beams? The only load i understand that would be transmitted onto the flitch beams is the weight of the dormer cheeks walls themselves, which wouldn't change by extending the width of the dormer. If anyone can help i would be extremely grateful. The only thing that it might be the issue is we need to replace the 45deg. purlin posts with verticals, which will form dwarf walls. These posts have been designed to transfer the load from the existing roof & presumably half the load from the dormer ceiling through the new floor joists which could be where the load is coming from but is extending the dormer an extra couple of meters going to mean that the flitch beams get substantially more loading?

Thanks in advance if anyone can help me out.

Dan

 

[^woody^]

You'll have to upload a sketch or photo to illustrate.

Is all about loadings and if the loadings can be transferred. But I can't picture how a beam at the side of stairs can affect a dormer cheek which is fitted to rafters

 

[Diles]

Dormer typical construction arrangement

• Diles
• 5 Jul 2012
• 1

Plan on new joists & beams in attic room

• Diles
• 5 Jul 2012
• 1

Hi thanks for posting, I've attached a typical arrangement of the dormer construction and a plan showing the joists/beams in the attic room, the dormer cheeks will be over the line of FB2 & FB3 flitch beams. Hope this gives you an idea. Many thanks

 

[^woody^]

The drawings are a bit faint and its still not clear to me.

It looks like some sort of dormer with sloping roof sides, but I still can't work out the cheeks, or how they are loaded to the beams and not to rafters as normal

Have you got an external elevation showing the dormer on the roof?

 

[Diles]

sorry about that, the website only accepts image files which blow out as soon as you zoom in. The image you see of the dormer was only drawn so i could get an idea of materials, and will not be giving it to the carpenter to build. I'll attach a couple of extracts from a simplified drawing which the builder will have, that you can look at. I'm not intending to specify, in detail, the dormer construction, i would hope the carpenter knows what he's doing in that respect. So maybe a better question to ask is how would a dormer normally get constructed? There's no particular special requirements for this one, it's just a standard dormer circa. 3.5m wide and rises to a level just below the ridge of the roof, a window on the left hand side as you look face on, the roof has sloped ends (as shown) to soften the look but will have a flat roof between, finish to be tiles to match existing. Its really a blank sheet as far as the structure is concerned, as long as it ends up looking the way we want it to, it can be built however. If there's a way of doing this to avoid putting additional loads down to the beams then this would be the way to go. Thanks again

Side elevation

• Diles
• 5 Jul 2012
• 1

Front elevation

• Diles
• 5 Jul 2012
• 1

 

[tony1851]

Can't you build the gable up completely (in brick?) making it a true hip-to-gable onversion, and extend the existing ridge out? Might be a bit simpler structurally.

 

[^woody^]

That's better thanks

By the looks of things, the beams are only there to take the floor load around the staircase, and are nothing to do with the dormer or roof

The dormer cheeks would be loaded onto a double rafter, whether they are where they are now or are moved over to make the dormer wider

I can't see what your engineer was thinking of regarding the need to upgrade the beam to widen the dormer. Unless I'm missing something?

BTW, have you sorted out the detail where the pitched roof meets the flat roof on the dormer?. Can it be constructed like that, at those levels, and sealed at the flat/pitch junction?

 

[Diles]

Well the joists have been designed to take a load from the new vertical purlin props but that's only on the opposite side of the roof. On the side of the dormer there will be no purlin to worry about, between the dormer cheeks that is. The rest of that side, ouside the extents of the dormer, will have a similar detail but all the joists run parallel to the dormer cheeks so the joists taking purlin loads are not connected in any way to the dormer area. If we extend the dormer this purlin should go completely. I have a feeling my engineer may think the dormer ceiling joists run perpendicular to the cheeks so all the load of the roof goes down through the cheeks, all the way to flitch beam. Even if the ceiling joists did run this way they would be supported by doubled up rafters not the flitch beam, like you say. I spoke to him today and he's going to draw a sketch to demonstrate how the load transfers down to them, which should help to understand what he means. If he has already applied a load from the dormer to these flitch beams then there's a good chance they've been over-sized.

I can upload the calcs for the loading break down and also one of the flitch beams, maybe you can make some sense of it.

I have been thinking about the pitch/flat roof junction and i'm not entirely sure to be honest but i assumed it could be probably be done in lead or an over hang of the flat roof over the first line of tiles? Its something i need to discuss with the builder. Do you have any ideas?

Loading calcs

• Diles
• 5 Jul 2012
• 1

flith beam pg1

• Diles
• 5 Jul 2012
• 1

fb pg2

• Diles
• 5 Jul 2012
• 1

fb pg3

• Diles
• 5 Jul 2012
• 1

fb pg4

• Diles
• 5 Jul 2012
• 1

 

[^woody^]

The dormer load would go to the rafter and to the ridge and the external wall. None of it would go to that floor beam

You need to speak with him to clarify his thinking

As for the flat/pitch roof junction, you need to be sure exactly how you are going to seal it. You don't want the carpenter to build it as drawn and then find that the joint ends up higher than the main ridge, or that the flat roof should have been lower

I would not seal flatroof felt to tiles as there is the risk of movement and spliting. With lead, you need to deal with potential wind up-lift, and if using ridges, its the height issue

Unless you have a carpenter who knows what to do, then I would not leave it to him to guess

 

[Diles]

Thanks for all your help I will try and clarify things with him tomorrow but this has certainly helped.

I will give the dormer roof some more thought, there may be scope to change the whole roof to tiles and have the flat section sloping away from the ridge beam and then I could use ridge tiles along the joints instead.

Thanks, really appreciate your help

 

[Diles]

my engineer is saying its the doubled up rafters that can't cope with the increased loadings and if the dormer gets extended, doubled or tripled rafters would fail, theres nothing timber wise that would cope he says.

 

[Diles]

I think what he's getting at is to stop the doubled up rafters from failing they need to be propped by the flitch beam below. He did specify min. 10mm thk ply for the dormer cheeks, this i think would act as diaphrams.

 

[^woody^]

If that's the case then you could increase that rafter depth, increase the timber grade or just make another flitched beam for the rafter

It also seems that you could insert another flitched beam in the floor further along. Although this would presumably mean that a post or structural stud would need to be below the cheek, but you could possibly design this into some sort of eaves storage

 

[Diles]

In this case i dont think upgrading the timber or doubling up would work, i want the dormer to be abt 6.5m in length and a lot of the weight from that will come down them end rafters.

As a rule if you take one rafter out you should double up and we're looking to remove 15 or 16!

I think we've sorted it, i suggested we run another flitch beam at each end of the dormer but make it 25mm shorter so it doesn't take any floor load, it would take just a single point load from a column propping up the ridge beam of the new dormer at each ends, with possibly a third if needed, as we can tuck it behind the staircase enclosure. Glad i perservered with it now, hes going to do the calcs this weekend for me and let me know but hes got a good feeling it will.

Are you a structural engineer then Woody?

 

[Diles]

Can't you build the gable up completely (in brick?) making it a true hip-to-gable onversion, and extend the existing ridge out? Might be a bit simpler structurally.

Hi Tony - anything with bricks normally gets expensive and we're on a tight budget however its the best looking way to do and if we had a few more pennies it would be the way to go.