Roofs

guideanna · 07-01-2010 12:27pm #1

Can anyone please help explain the difference to me between a "thrust" roof that comes specially made from a workshop, to a normal room that the carpenters make onsite?
Is there any pros r con's for the different types, and does the price vary much?

It's for a 1.5 dormer.

Thanks

gman2k · 07-01-2010 12:41pm

guideanna wrote: »

Can anyone please help explain the difference to me between a "thrust" roof that comes specially made from a workshop, to a normal room that the carpenters make onsite?
Is there any pros r con's for the different types, and does the price vary much?

It's for a 1.5 dormer.

Thanks

A Truss roof is a prefabricated product which should benefit from factory tolerances and computer aided design. The truss will be designed for your particular layout, and should be structurally sound. There is more strength in a truss roof, and rafter spacings can be greater than the onsite - cut roof.
Advantages for a builder is speed of erection, but a crane or lifting device is needed to lift the trusses into position.
A roof can easily be lifted into position in half a day.
Cannot comment on prices, but would reckon you would pay a premium over getting a carpenter to make you a cut roof.
If you are getting a carpenter to make your roof, it's essential to employ the services of an engineer to ensure a design is worked to on site.
The truss roof company will have their own engineer input at design stage.

sydthebeat · 07-01-2010 12:48pm

guideanna wrote: »

Can anyone please help explain the difference to me between a "thrust" roof that comes specially made from a workshop, to a normal room that the carpenters make onsite?
Is there any pros r con's for the different types, and does the price vary much?

It's for a 1.5 dormer.

Thanks

1.5 dormer???

do you mean 1 + 1/2 storey, or a dormer..... theres a difference. If its 1+1/2 storey you may require steelwork for a cut roof construction. Ask your engineer / architect. Dont even think about starting the construction without consultation with a professional.

"cut" roofs are made on site by carpenters and include individual elements such as joists, rafters, purlins, struts, runners, collar ties, etc to create a structural latice.

a "truss" roof is made off site, generally in a factory, and is specifically enginnered to the project. Individual "trusses" are made which are structurally designed to support the roof covering. These are erected on site and tied together by braces.

Cut roofs generally suit awkward shaped roofs with different ridge and eaves heights which requires a lot of measuring and cutting on site. Build time on site is long because of this.

Truss roofs generally suit simplified regular rectangular shaped roofs with as little on site cutting as possible. Build time on site is short because of this.

Poor Uncle Tom · 07-01-2010 12:50pm

Also, just a little point, a trussed roof can not be easily modified at a later date for conversions, extension, exposures, etc.

guideanna · 07-01-2010 12:53pm

it's a 2 story house but it's dormer as in it has the slanted walls uptairs. I thought this was called a 1.5 dormer?
Thanks for the quick replies, i'd be interested to hear if anyone compared prices for the two types.
Sounds like we could save ourselves time doing the engineered way alright!

sydthebeat · 07-01-2010 12:56pm

guideanna wrote: »

it's a 2 story house but it's dormer as in it has the slanted walls uptairs. I thought this was called a 1.5 dormer?
Thanks for the quick replies, i'd be interested to hear if anyone compared prices for the two types.
Sounds like we could save ourselves time doing the engineered way alright!

is the first floor foot print the same as the ground floor??

sounds like you have whats called "one and a half storey"... (not 2 storey or dormer, these are different).

These roof tend to have a structural force which pushes out the wallplates due to the fact that the triangulating ceiling joists are so much higher than the wall. Generally steel is required to stop this spread. If pricing a cut roof included for this.

A truss roof for this situation seems practical.

guideanna · 07-01-2010 1:05pm

Yea thats what our architect is saying i just wasn't sure what the difference was or why we should go for an engineered one.
The floor plan is different upstairs than downstairs so not sure what difference that makes.
Architect did mention if we go for the engineered roof we wouldn't need as much steel support so like what your saying i guess. Hopefully this will save us a bit on materials even if it does work out more expensive to have made and then fitted.

slowharry · 07-01-2010 1:47pm

From my experience a truss roof is only of benefit in a traditional A frame roof.
It can work for a 1.5 storey roof depending on dimensions ( cant remember off hand but its some thing like if the distance on the slope from wall plate to ceiling joists is less than 1/4 the distance from wall plate to ridge your ok). You may still require roof steels and/or a concrete band/tie beam to prevent spread at wall plates, this goes for both cut and truss options
Trusses to be efficient generally use the space that you want to use in the dormer roof situation.
Put a bit of a sketch together hope it explains something

Splinter Cell · 07-01-2010 11:38pm

sydthebeat wrote: »

These roof tend to have a structural force which pushes out the wallplates due to the fact that the triangulating ceiling joists are so much higher than the wall. Generally steel is required to stop this spread.

Hi Syd,
Can you clarify when you say the force pushes out the wallplates please? Because the weight of the roof is not supported by the the joists sitting directly on top of the wallplates, I would have thought that the slope of the rafters sitting on the wallplates would push them inwards (or towards each other) instead? To stop this, do you mean steel in some sort of ring beam?

I have seen some guidelines for 2 story showing that the traditional block on flat is no longer used as a cavity closer at the wallplate, with the roof insulation meeting the cavity insulation. On a 1.5 story, this seems like a lot of force for the top block on edge of the inner leaf to bear - are there any guidelines for this?

Thanks, SC.

sydthebeat · 08-01-2010 12:27pm

Splinter Cell wrote: »

Hi Syd,
Can you clarify when you say the force pushes out the wallplates please? Because the weight of the roof is not supported by the the joists sitting directly on top of the wallplates, I would have thought that the slope of the rafters sitting on the wallplates would push them inwards (or towards each other) instead? To stop this, do you mean steel in some sort of ring beam?

I have seen some guidelines for 2 story showing that the traditional block on flat is no longer used as a cavity closer at the wallplate, with the roof insulation meeting the cavity insulation. On a 1.5 story, this seems like a lot of force for the top block on edge of the inner leaf to bear - are there any guidelines for this?

Thanks, SC.

to clarify, im talking about a storey and a half dwelling. the middle sketch that slowharry has attached.

in both the 2 torey and dormer case where the joist is fixed to the rafter at the eaves, a triangulation occurs where the outward force caused by the bending motion of the rafter is negated by the tensile strength of the joists ie the rafter wants to push out but the joists pulls it back.

this bending motion is caused by the weight of the roof covering effecting downward forces onto the rafter. this bending motion is strongest in the rafter at the point furtherest from a support or tie, therefore it creates and outward force at the base of the rafter where its fixed to the wall plate. Its a common structural fault to see a crack in 1+1/2 storey dwellings about 400mm down from the junction of the wall and roof. The cavity closer and first block course are being pushed outwards and basically tilting out.

Steel beam supports at junction of ceiling joists and rafters transfers the majority of force down to the earth without having to affect the wall plate. i always recommend this for 1+1/2 storey. A ring beam is designed to stop the outward spread of teh wallplate, but has add on issues such as how its insulated etc. Always choose steel first.

the detail you have seen where there is no cavity closer comes from the "acceptable details" issued by the DOE. These are still controversial details because there appears to have been very little structural input into them. the details are almost carbon copy of the UKs accrediated details which, interestingly, state at this particular details that it is designed to deal with thermal performance and air tightness only. Most notably its not designed to deal with the structural aspect.

i would still be using a cavity closer in this instance.

Splinter Cell · 08-01-2010 10:48pm

Hmmm, that makes sense ... you learn something new every day. Thanks Syd!

dahamster · 18-01-2010 10:55pm

sydthebeat wrote: »

Its a common structural fault to see a crack in 1+1/2 storey dwellings about 400mm down from the junction of the wall and roof. The cavity closer and first block course are being pushed outwards and basically tilting out.

Syd this describes a crack that has been bugging me for a number of years in my house. Is my house in danger of falling down? Is there a retrofit fix for this?

englert123 · 18-12-2014 9:34am

Truss roof are more beneficial then the roof made made indoor as they build in factory under the guidance of experts and have specific design according to the scale. The metal roofing structure design specification was up to standard so the reliability of roof is high.

hexosan · 18-12-2014 12:40pm

englert123 wrote: »

Truss roof are more beneficial then the roof made made indoor as they build in factory under the guidance of experts and have specific design according to the scale. The metal roofing structure design specification was up to standard so the reliability of roof is high.

I'd say after 4yrs since the original post he's figured it out by now.

Roofs

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