Prefab roof vs cut roof

Paudi04

Anybody got any input to this. It would be a room in attic prefab truss. Prefab is faster although more expensive but cut roof on site has more labour involved so my view is that factoring in the labour, the cost would nearly equal??

Any structural difference?

Thanks.

topcatcbr

Paudi04 wrote: »

Anybody got any input to this. It would be a room in attic prefab truss. Prefab is faster although more expensive but cut roof on site has more labour involved so my view is that factoring in the labour, the cost would nearly equal??

Any structural difference?

Thanks.

A cut roof would be more forgiving and I would prefer it for this. Also with labour as plentyful and cheep I would like to think I could help my local economy a little by providing a bit more employment and keeping money local.

Paudi04

topcatcbr wrote: »

A cut roof would be more forgiving and I would prefer it for this. Also with labour as plentyful and cheep I would like to think I could help my local economy a little by providing a bit more employment and keeping money local.

Sorry when you say forgiving what do you mean? Is it that you have more control over the design etc?

topcatcbr

Paudi04 wrote: »

Sorry when you say forgiving what do you mean? Is it that you have more control over the design etc?

I mean its more tolerant of small errors in construction or maybe a small error in measuring. With truss roof you have to get every aspect right first time. This is not a bad thing and works well when you have many homes of the same design but with a single build I'd prefer to cut a roof. It's not a big issue either way tbh.

bennyineire

Cut roof can be open to more design flaws and construction flaws, if you go with a cut roof you had better have a fine carpenter that specializes in roofs, you will have a lot more room in the attic with a cut roof if that's what your looking for

topcatcbr

bennyineire wrote: »

Cut roof can be open to more design flaws and construction flaws, if you go with a cut roof you had better have a fine carpenter that specializes in roofs, you will have a lot more room in the attic with a cut roof if that's what your looking for

I agree a cut roof by its nature is more open to being done wrong. It's important to get a "qualified" carpenter and not a laberour who has worked on roofs.

This is true for all work. Now days there is no shortage of qualified tradesmen. For too long unqualified workers are made responsible for work they are not educated in. If your dedicated in your work why would you not get qualified in it. It's about time we started to do things right.

OBrother

Is a carpenter "qualified" to design a cut roof. if there is a possibility of something been done wrong, consider the effect on you mortgage draw down and resale.

topcatcbr

OBrother wrote: »

Is a carpenter "qualified" to design a cut roof. if there is a possibility of something been done wrong, consider the effect on you mortgage draw down and resale.

Yes. And have been doing so for thousands of years. How long have engineers been designing truss roofs.

I don't understand the problem. Cut roofs rarely give problems. The most problems I have seen have been from truss roofs being modified to provide attic conversions.

bennyineire

Modern roofs have far greater spans than traditional roofs, there was a reason why Irish cottages were built long and narrow(not hard to design), todays roofs have far greater loads and can easily cause horizontal cracking just below the wall plate and this can be very expensive to fix, if I was getting a cut roof of any substantial size I would get a truss engineer or structural engineer to design it and get a qualified carpenter to construct it, in the current climate it would cost that much to get it properly designed.

newbie2013

bennyineire wrote: »

Modern roofs have far greater spans than traditional roofs, there was a reason why Irish cottages were built long and narrow(not hard to design), todays roofs have far greater loads and can easily cause horizontal cracking just below the wall plate and this can be very expensive to fix, if I was getting a cut roof of any substantial size I would get a truss engineer or structural engineer to design it and get a qualified carpenter to construct it, in the current climate it would cost that much to get it properly designed.

What do you mean when you say about horizontal cracks being expensive to fix. My roof was built using attic trusses and ive a few cracks under the wall plate. I have been told that these are just settlememt cracks and nothing to worry about because it usually takes a house 10-20 years to settle.

OBrother

See attached, as an example.

Paudi04

So basically you would want to have the measurements down to a tee if going for pre fab roof. The builder approached me and asked would I have anything against it and I said as long as I am not Losing any of my attic space. I will be looking to convert at a later date. He assured me that I would have the exact same space. He is working on a contract basis so he said he would rather this to shorten time on site.

Thanks for all replies!!

newbie2013

OBrother wrote: »

See attached, as an example.

O yes i see. I was somehow thinking it was vertical cracks you were talking about . How are these cracks caused, im thinking who ever done this didnt tie the wallplate to the block with steel straps. The cracks in my own house are nothing like these in the pic, mines are just a few hair line cracks. What has me worried about them thou is that they keep apearing after i paint over them. I would have thought that the paint would cover them

OBrother

I don't want to comment on, or specify structural components, rather to made the point that whatever the roof type it needs to be properly designed.
Where different materials meet, because of expansion or shrinkage / moisture movement cracks can occur, and cause hairline cracks.

BarneyMc

Got prices back for a truss V cut roof for a 2 storey house and they're coming it bang on the same. It seems most people in this case would go with cut?

fclauson

We went truss - once item which was mentioned after is that it uses a lot less timber (if you want to improve you green credentials)

sydthebeat

BarneyMc wrote: »

Got prices back for a truss V cut roof for a 2 storey house and they're coming it bang on the same. It seems most people in this case would go with cut?

No. Most people would go truss of prices are equal.

BarneyMc

sydthebeat wrote: »

No. Most people would go truss of prices are equal.

Why is this - speed of construction?

There seems to be an opinion that a cut roof is better in that it is tailored to fit exactly to the position of the walls. Surely a good truss company will measure correctly before manufacturing the trusses?

Mahogany Gaspipe

BarneyMc wrote: »

Why is this - speed of construction?

There seems to be an opinion that a cut roof is better in that it is tailored to fit exactly to the position of the walls. Surely a good truss company will measure correctly before manufacturing the trusses?

Aside:

Any geometrically correct roof, cut or truss, should never be tailored to suit block work that suffers poor workmanship/setting out.

When constructing a either type of roof setting the wallplates at the same level, parallel and square to one another is where all adjustments should be made to correct for discrepancies in the preceding construction phases.

sydthebeat

Reasons a truss roof is better.

1. Less tolerances leads to more homogeneous roof plane.
2. superior speed of construction
3. Engineered and guaranteed constructIon.

Mahogany Gaspipe

sydthebeat wrote: »

Reasons a truss roof is better.

1. Less tolerances leads to more homogeneous roof plane.
2. superior speed of construction
3. Engineered and guaranteed constructIon.

Not denying sydthebeats points just countering them as roofing structures is something I know a bit about.

1. Less tolerances leads to more homogeneous roof plane.
2. superior speed of construction
3. Engineered and guaranteed constructIon


It may be easier to achieve an overall better standard of roof construction using trusses versus cut roof if the carpenter contracted is of mediocre ability.

However IMHO a well specified and faithful constructed cut roof is hands down the winner in regards of cost, life span, finished quality and future proofing for alterations.

Carpenter

Mahogany Gaspipe wrote: »

Not denying sydthebeats points just countering them as roofing structures is something I know a bit about.

1. Less tolerances leads to more homogeneous roof plane.
2. superior speed of construction
3. Engineered and guaranteed constructIon


It may be easier to achieve an overall better standard of roof construction using trusses versus cut roof if the carpenter contracted is of mediocre ability.

However IMHO a well specified and faithful constructed cut roof is hands down the winner in regards of cost, life span, finished quality and future proofing for alterations.

+1

sydthebeat

Mahogany Gaspipe wrote: »

However IMHO a well specified and faithful constructed cut roof is hands down the winner in regards of cost, life span, finished quality and future proofing for alterations.

Taking an all things being equal viewpoint, which i have for my reply, the only one of cost, life span, finished quality and future proofing that a cut roof could conceivably have over a truss roof is future proofing. Cost obviously is equal as per the op. Life span is actually guaranteed in a truss roof whereas in must cases it's not for a cut roof. Finished quality, well that's in the eye of the holder. Truss roofs are homogeneous in nature, cut roofs aren't so i suppose it's whether you prefer a turner or a dali.

Mahogany Gaspipe

sydthebeat wrote: »

Taking an all things being equal viewpoint, which i have for my reply, the only one of cost, life span, finished quality and future proofing that a cut roof could conceivably have over a truss roof is future proofing. Cost obviously is equal as per the op. Life span is actually guaranteed in a truss roof whereas in must cases it's not for a cut roof. Finished quality, well that's in the eye of the holder. Truss roofs are homogeneous in nature, cut roofs aren't so i suppose it's whether you prefer a turner or a dali.

I missed the cost as per the op, so for the project we can rule that out.

Regarding the other points:
Cutting and assembling the members of a cut roof shouldn't be any different than those of a truss roof. Its wrong to say truss roof are homogeneous in nature and cut roofs are not.
Both types of roof are open to poor workmanship resulting in finished roofing planes out of kilter. You would be mistaken in assuming a set trusses must be homogeneous, or at least more so, than a cut roof because trusses are manufactured under factory conditions. As I said in my earlier post the quality of the finished roof depends on the competency of the tradesman, regardless of construction type. Many times I seen rouge trusses in a set of supposed homogeneous trusses.

Regarding lifespan.
Modern roof truss design takes advantage of engineering principles that allow minimum structural requirements to be satisfied using the most economic use of materials; that is through using minimum sized truss components that are structural acceptable.
Cut roofs are frequently over engineered. Where roof member can not be as strategically position to resist forces and loads they are over sized thereby exceeding minimum structural requirements.
I'm not say that truss roofs don't do a better job of doing more with less timber; but a properly specified and constructed cut roof has the edge on longevity.

Now having said all that, truss roofs frequently appear, and indeed are, truer than a cut roof and I suppose what I'm trying to argue is that this is due to the ability, skill and consideration of the tradesman.
I suppose you could say that the mean value of truss roof is about a 7; and that a cut roof may be anywhere between a 3 to a 10, if that makes any sense!

fclauson

Mahogany Gaspipe wrote: »

Modern roof truss design takes advantage of engineering principles that allow minimum structural requirements to be satisfied using the most economic use of materials;

not to go too far off track - (and I like watching carpenters work) but from a sustainability view point trusses must win - just think if you took all the cut roofs and calculated how many trusses you could have built out of the same amount of timber

BarneyMc

One thing with the cut roof (in my case anyway) is the internal walls have to be brought up into the roof space. I be asking my engineer if the lite blocks can be used to reduce thermal bridging.

fclauson

BarneyMc wrote: »

One thing with the cut roof (in my case anyway) is the internal walls have to be brought up into the roof space. I be asking my engineer if the lite blocks can be used to reduce thermal bridging.

Not so we only partially rested on the internal walls in just a few places - the truss company will have the s/w to model this

BarneyMc

BarneyMc wrote: »

One thing with the cut roof (in my case anyway) is the internal walls have to be brought up into the roof space. I be asking my engineer if the lite blocks can be used to reduce thermal bridging.

My energy consultant looked at this and said the heat loss, if using lite blocks would be minor, although better to go with trusses for this reason.

Coin toss I think :rolleyes:

cork2

As a carpenter by choice a cut roof 90% of the time. Trusses are fast to put up and they will span further, but they're a danger in a house in my honest opinion. I went to look at a job last Friday where a 3rd year apprentice was doing a foxer and cut out the braces and props in all the trusses in an attempt to convert an attic. You could do the same in a cut roof but at least the rafter would be a six or seven by two as apposed to a four and a half by two! Also the amount of houses I've worked in where a truss has been cut to fit an attic ladder, it's ridiculous. trimming and bolting and propping or not you never ever ever cut a truss and people don't seem to get it. In any of these instances it's so much easier to fix a cut roof. Also unless there's beams to be sized a carpenter can design the roof and if he can't tell him to go away home.

BarneyMc

Does this mean it's harder to repair a truss roof in case of damage?

deviking

NEVER EVER CUT A ROOF TRUSS...Have a look at this video on you-tube
http://youtu.be/B7KfGW_2UgU

BarneyMc

What's the normal spacing of roof trusses? The company I'm looking at space theirs at 400mm (16 inch) centres and claim that most standard ones are spaced at 600mm (2 feet) centres.

fclauson

depends - mine are 2ft - but that's an engineering decision

newbie2013

BarneyMc wrote: »

What's the normal spacing of roof trusses? The company I'm looking at space theirs at 400mm (16 inch) centres and claim that most standard ones are spaced at 600mm (2 feet) centres.

Any ive worked on are 600mm apart. Some can even be spaced 1800mm apart

newbie2013

deviking wrote: »

NEVER EVER CUT A ROOF TRUSS...Have a look at this video on you-tube
http://youtu.be/B7KfGW_2UgU

You can cut them for an opening for a dormer window.

cork2

newbie2013 wrote: »

You can cut them for an opening for a dormer window.

You're not supposed to by right. You Should drop a double truss either side of your dormer and infill the middle yourself. If you cut it for any reason goodbye guarantee on trusses. You get a truss schedule with the trusses and I'd be amazed if it showed a cut truss for dormers.

newbie2013

cork2 wrote: »

You're not supposed to by right. You Should drop a double truss either side of your dormer and infill the middle yourself. If you cut it for any reason goodbye guarantee on trusses. You get a truss schedule with the trusses and I'd be amazed if it showed a cut truss for dormers.

Sorry, i forgot to mention the double truss at each side. You can have an opening as wide as 2.4 m if you put 3 trusses at each side then cut the trusses that are in the 2.4m space.

archtech

newbie2013 wrote: »

Sorry, i forgot to mention the double truss at each side. You can have an opening as wide as 2.4 m if you put 3 trusses at each side then cut the trusses that are in the 2.4m space.

You can have any arrangement ( by way of spacing, dormer windows etc) you want once its been factored into initial truss design by the truss designer.

newbie2013

archtech wrote: »

You can have any arrangement ( by way of spacing, dormer windows etc) you want once its been factored into initial truss design by the truss designer.

So tel me this, when someone doubles up or even has 3 trusses beside each other, would these be taking the main load seeing as the trusses inside these can be cut.