Cross Gable Roof Extension Help! 5

[sithlord382]

Hi I have been looking at a house (circa 1960s) to purchase that has an approved planning application attached to it, for a side extension that will result in a cross gable roof structure. I went to view the property today and went into loft to find some sort of truss system instead of the open space rafter configuration which I am normally used to seeing.
I am interested in purchasing the property, carrying out the proposed works and re-selling but I need to roughly figure out how much the proposed works would cost before I go ahead, to essentially see if it is worth it.
My question is how would the new gable roof (which is perpendicular to the existing gable roof) be constructed? The existing roof has a W shaped truss system and surely you can't just snip away at these trusses to make way for the new roof structure as it will effect the structural integrity. Would the new roof construction need to be a truss system (therefore do i need to get a truss specialist involved in determining a quote) or can it be open plan with the standard ridge beam, roof rafters etc.

Thanks in advance

 

[jayrod12]

If the existing roof and ceiling is intended to stay as is, they would just overbuild the new roof over the existing. Most truss designers don't specifically account for additional dead load from this overbuild.

If you want to cut open the ceiling in the existing property to make it vaulted or something like that, you would have some significant rework to do.

Either way, hiring a structural engineer in your area will be necessary.

 

[phamENG]

For estimation purposes, I'd count on replacing the trusses where the new addition will join. As jayrod mentioned, they weren't designed for the overbuilt condition and reinforcing them will be challenging at best. A good designer may be able to come up with a creative solution, but at this stage while trying to find your potential ROI, probably better to be conservative and consider replacement cost.

 

[KootK]

Whether done as trusses or stick built framing, it's likely to look something like the setups below.

The overbuilt framing itself is nearly weightless. What's impactful is the roofing and, to a lesser extent the sheathing. it is quite common for truss designers to account for a future re-roofing in initial truss design so that helps some. What also helps is that the valleys are usually only meaningfully wide down near the ends of the existing trusses where that load will have a minimal impact on overall bending within the truss.

This is a very common type of renovation and I've never heard of a residential one involving the removal of the trusses beneath the buildup unless the goal is open attic space or something under there which is rare. For what it's worth, I was a designer of these kinds of trusses before and during college.

 

[sithlord382]

Thanks for all the responses and apologies for the delayed response.
If everything was to go to plan and I did take on the property to develop I think I would aim to have minimal disruption to the existing house and its roof structure.
I spoke with a roofer who completed the roof on my actual house extension a few months ago. He has said that I have 2 options to construct this:

1. I do what you guys have said and go for an over build. (Thanks KootK for the image above). Because the side extension (cross gable will be at least 4.4m from the existing roof ridge to the existing side wall) he has advised I get a truss specialist to come to site and measure the existing trusses to see if they can take the load of a new roof. As you guys have said the weight of the valley set truss frame will be minimal and most of the weight will come from the roof tiles etc?
2. I do an over build out of the generic ridge beam and rafters (is this a stick built frame). He sent me over a crude mark up which I have attached below. He is saying an SE will need to design possibly a flitch beam that will sit on 1 post on the existing left hand side wall (shown as a dashed line) and that will run all the way to the existing ridge and the SE will need to design another post for this beam to sit on, which in turn will have to some how be supported at loft floor with another beam. Essentially make the ridge beam independent to the existing truss system as I wouldnt be able to hang it off the existing if that makes sense?

Ideally i would like to do option 2 if possible but I cant seem to picture how to support the new ridge beam and construct the cut valley onto existing roof

 

[KootK]

Drifted snow in the new valleys may also add load. I like valley trusses more than ridge and rafter as the valleys do a better job of spreading out the new load. That said, you could also put posts intermittently under your rafters to achieve a similar result. Post and beam will help a good deal although it's up to you if that's too invasive.

 

[jayrod12]

Regarding additional load due to roofing, I'd be looking at removing the existing roofing where you are installing this new framing, therefore you should be pretty close to the original loading again on the existing trusses all said and done.

I'd be doing valley set trusses and not really thinking twice about it.

 

[dhengr]

Sithlord382:
Watch out for the (half valley?) slope, drainage and flashing at the high exterior wall and the existing lower roof, which is the continuation of your dashed line to the north on the drwg. There is a lot of roof area draining into that new high wall and existing roof slope area/detail.

You might be able to put a post under that E-W high ridge beam, right on the dashed line existing wall below. Then that E-W high ridge beam would be a two span cont. ridge beam over that post, which would significantly reduce the beam reaction at the existing ridge to the east. Then the new high roof to the east of the post might just be overbuilt.

 

[JStructsteel]

Could you use a ridge beam, and find a location where you can put a post down to the ground? That would take a bit of the load of the overframe. Then use rafters as needed.

I would find it very hard to analyses the trusses. Perhaps boxing them in with plywood to give them more strength?

 

[sithlord382]

Dhengr - Thats what my roofer was saying as well, definitely have a post on the dashed line and to the left of that post have a 'stick built frame'. My only question is even if the new roof over existing roof (right of the post on dashed line) is overbuilt, would you not need another post where the 2 ridges meet?

I have just calculated a very very rough calc for the ridge beam spanning from post on dashed line to existing roof ridge junction:
4.4m span of ridge beam, Perpendicular length to ridge beam = 4.5m
Uniform Load= DEAD - (4.5m x 1.2kN/m^2)LIVE - (4.5m x 0.6kN/m^2) therefore END REACTION = approx. 18kN (by using wl/2)

18kN as an end reaction of the ridge beam, overbuilt, onto an existing truss frame without incorporating a post just doesn't sit well with me

What are your thoughts?
I think KootK and jayrod12 have proposed a solution where I might have to pay a couple of thousand £ extra in terms of materials for the roof structure ie. Having a new truss frame as opposed to a ridge beam but there will be less hassle

 

[sithlord382]

J StructSteel - Although that would easily solve my issues, I want to try and avoid disrupting the existing house as much as possible. It is in great condition and honestly needs no internal work to it. Bringing a new post down to a pad footing within the existing footprint is going to exceed my budget overall i think. I have considered maybe taking that post onto a beam that is spanning either front to back of left to right within the loft space, but then there is a lot of manoeuvring around the existing trusses which I know the steel installers will be very miffed about

 

[KootK]

I think KootK and jayrod12 have proposed a solution where I might have to pay a couple of thousand £ extra in terms of materials for the roof structure ie.

Between us, jayrod12 and I have more light frame construction experience than Jesus. Pre-eng trusses haven't almost killed off hand framing entirely because the trusses are more expensive. If these two things are true, I can pretty much guarantee that valley trusses will save you $$$:

1) The existing roof can take the load and;

2) You'll have pre-eng trusses on the project at other locations.

 

[dhengr]

Sithlord382:
But, you shouldn’t be using (wl/2) for the existing ridge reaction, it is more like (3wl/8) due to the new ridge beam continuity. And, even a little less given the longer, more heavily loaded west span of the new ridge beam. The center post will see something more like (5wl/8) from each side span. Take a look at the formulas for a two span cont. beam, and the methods for analyzing a two span cont. beam with different span lengths and loads. Also, the load on the new cont. ridge beam diminishes as you move up the valley, so that reduces the reaction too.

Your carpenter offers that solution because it means more of his typical framing work for his time cards. But if you have other roof trusses and maybe a crane on the job already, trusses will probably be less costly, in total time and materials, than stick framing that portion of the roof.

 

[sithlord382]

Honestly guys, this forum has helped me squash all those worrying questions that pop up in my head randomly during the middle of the night! So i can't thank you guys enough!

KootK I spoke with a truss specialist this morning and he said he would typically see valley set truss at junctions if the entire roof was new. He would expect an overbuilt solution for this example.

dhengr - I also spoke with my SE this morning and he was amused to find I was asking him questions about continuous beams and triangular loads etc. and jokingly questioned if I had been speaking to another SE behind his back. Like what you have said, he said I can either go with continuous ridge beam spanning from west to the existing ridge junction, or split it into 2 separate beams. However the load of the ridge beam will be relatively small at the new ridge to old ridge junction as it is a triangular load.

I know i am being pedantic considering I haven't even purchased the property but I am a worrier and want to produce an accurate construction quote as possible. Plus I am really taking an interest into learning new engineering/construction methods (with the help of google and you guys!)

Now just waiting on my roofer to get back to me with a quote but I think the options are between the following:

1. The new extension is a truss frame (need to check with my missus, as she has all of a sudden become an interior designer, if she thinks loft space is a must)
2. The new extension is a stick built frame system built on site with the generic rafter/ridge frame.

Both ways the cut valley where the existing meets the new will be done with a ridge board, lay boards and jack rafters. As jayrod12 said, the existing roof tiles etc. will be stripped and i could possibly re-use them to try and save money but we will deal with that if it gets to that.

Thanks for all your help!

 

[jayrod12]

I have my doubts that putting a post from a ridge beam down on a single existing truss will be easier in the long run than valley framing. Cheaper materials, sure, but you're going to have to reinforce that truss, and that is time consuming and highly labour intensive. And then you still need to stick build the roof after the reinforcing which is significantly more labour intensive than putting trusses down. Especially if you're putting new trusses for the rest of the new build. Just seems like you're overcomplicating the construction to save a few bucks on materials.

 

[sithlord382]

Jayrod12 - My SE has calculated an end reaction at the junction where the old and new ridges meet of 6kN. He has advised that no post is needed there and that will be a detail the roofer will have to come up with where the ridge beam meets the existing truss and new layboard within the cut valley. As it depends on where the ridge beam lines up with regards to the existing trusses. If it sits on an existing truss then will have to hang the proposed ridge beam off the truss. if it falls inbetweeen 2 trusses then some sort of bridge will have to be constructed for the ridge beam to sit on.

Surely 6kN is minimal?

 

[jayrod12]

Yes, and no. All depends on the scale.

I don't have dimensions and loads to know for sure.

That could be a 2% increase on the truss, in which case I would agree with your engineer, it could be 20% increase in which case I wouldn't.

Where I practice, a 9m long truss spaces at 600 o/c supports a total load of 15 kN. So that's a almost 50% increase in load on that one truss. So I would disagree with your engineer.

 

[KootK]

Surely 6kN is minimal?

No, not for this kind of construction. 6 kN = 1350 lbf = 125% of a typical end reaction on a 24' truss. And I can guarantee you that load is going to land:

1) Between trusses and;

2) Between truss panel points.

In my mind, the ridge beam solution is dubious if the load is still to be resisted by the existing roof rather than structure down to below. That just concentrates the new load in a few spots on the roof rather than spreading it out. And you don't need to be Fazlur Khan to know which of those is better, particularly given that the truss solution is probably cheaper. For the same kinetic energy, would you rather be hit with a bullet or a sack of granola?

 

[dik]

...and remove the existing roofing underneath... it likely weights half of the addition. Also with home construction, many jurisdictions do not require increase in snow load due to valley conditions.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Nicolas P.]

Hi, I'm currently looking to have an addition built to our current house in the same way as the OP. I've been scrolling through multiple thread, but this one caught my attention due to the "ridge beam" idea.

A ridge beam in a traditional gable roof framing allow the removal of ceiling joists or rafter ties and transfers 1/2 the weight of the roof on each end of the beam.

However, in a stick-built (no valley set trusses) overframe roof like below, I'm wondering why should the ridge be considered structural? Wouldn't the main roof continuous sheathing under the overframe where valley boards & jack rafters are nailed to act as "rafter ties", thus eliminating the need of a structural ridge, and at the same time the need for a post on each end as mentionned here previously?