Rafter span

hookrkrook

Hi all,

I had an extension built in the past 4 year which was designed and overseen by an architect. The extension is basically a 7m x 5m rectangular cavity-block structure to the rear of the existing house. The extension has a tiled hip roof, including concrete roof tiles to match existing, on 25x50 treated battens on breathable roof membrane on 225 x 50 rafters. 200mm fibreglass insulation placed between rafters with 12mm foil backed plaster boards fixed to the underside of the rafters. The roof span of the principal rafter is 5m. Some minor cracks have appeared in the plaster board (one on the line of the principal rafter and one parallel to it). Another builder recently expressed concern that the principal rafter might not be strong enough to support the load, especially over time. Should I have any concerns, and what should I do to seek reassurance?

Thanks.

muffler

I'm leaving this thread open but be very careful with any replies here folks as we don't allow any "structural component/detail" comments or the likes as set out in Section 1.5 of the forum charter nor do we allow legal advice as set out in section 6.3. Cheers

1.5 Structural items . Issues relating to the specification of structural components is prohibited. Any queries in this regard need to be dealt with by an architect/engineer privately.

6.3 Whilst the planning process, including the legalities involved may be discussed in full, please note that seeking or imparting legal advice may result in a ban.

hookrkrook

Thanks for the reply muffler. I wasn't aware of the restrictions in the charter.

muffler

hookrkrook wrote: »

Thanks for the reply muffler. I wasn't aware of the restrictions in the charter.

There's no problem with people posting up advice on what you should do by way of who you should be consulting or engaging....that type of thing

A fella called fish

You say it's a hipped roof and that cracks have appeared along the line of the rafter. I'm assuming from this that you have a vaulted ceiling? I.e, you do not have horizontal ceiling joists (ties) at wallplate level?

muffler

A fella called fish wrote: »

You say it's a hipped roof and that cracks have appeared along the line of the rafter. I'm assuming from this that you have a vaulted ceiling? I.e, you do not have horizontal ceiling joists (ties) at wallplate level?

With 225mm deep rafters I'd say that's a a fair enough assumption

hookrkrook

I do have horizontal ties. The crack has appeared along the length in the centre of the horizontal part of the ceiling, ie roughly in line with where I imagine the principal rafter is running above it. I believe the ties are 225 x 50 also.

sydthebeat

hookrkrook wrote: »

what should I do to seek reassurance?

to answer this first, i would strongly advise you engage a structural engineer to review the construction. This would mean creating an access to the attic space of this roof, if none exists. You may be able to do this through the original house.

secondly, when i hear "tiled roof" "hipped" and "vaulted ceiling" i begin to worry. While 225 x 44 rafters are decent, a lot depends on the height of the ceiling joists in relation to the eaves and ridge, ie are they 1/3 off the eaves, or halfway up? etc

also, when you say "principal rafter" do you mean there is structural trusses installed under the roof construction?

any chance of a pic?

North West

Hi Hookrkhook
Most rough structural timber can be up to a moisture content of between 17 and 20% when purchased. as your building dries out over time, the timber will loose moisture and can account for your cracks. as the timber shrinks and moves in size. Just for your own peace of mind it would be worth contacting a structural engineer to look at the job. you may have to open part of the roof to see whats happening.It could also be an effect of the fixing of plaster board ( not enough slab naiks or screws along the long edge of the slab? Also if the slabbing was not scrimmed properly before plastering?
NW

jakko86

From what i can gather from op posts the crack is along the length of the collar ties,perpendicular to the direction of the timbers. It sounds to me to be a crack along the joint in the plasterboard in the same direction as the ridgeboard. Without a pic its hard to judge what kind of a crack it is but if you have no cracks on your hip rafters i cant see there being a problem as your hips are carrying most weight and are the longest timbers in the roof and most liable to sag before anything else.Seeing as its a vaulted ceiling the collar ties are probably the shortest members of the roof and being 225 by 44 i cant see them sagging or coming under huge pressure as they are so high up as most of pressure would be at mid point of rafters rather than the top.

the keen edge

I think when the OP says principle rafter he is referring to the ridge board, or maybe the crown rafter in the hipped end? If this is the case it sounds more likely that the problem is somewhat superficial rather than structural, and is a plasterboard joint crack.

Notwithstanding this:
Regardless of rafter depth/width, this particular roof design: an open, clear, vaulted internal roof space with a hipped end must include in its design either:

• A structural ridge.
• RC ring beam at wallplate level.

If neither of these elements were incorporated into the roof during construction, you should get an engineer cast an eye, not the originally engaged engineer.

hookrkrook

Attached is a (very) rough sketch of a cross-section of the roof with red circles indicating two locations where the cracks have appeared. The cracks are hairline cracks and are very difficult to photograph as a result. The ceiling is approx 3m in length from the wallplate to the collar ties, and the ceiling below the collar ties is approx 1m wide.
@the keen edge : The design definitely doesn't include a RC ring beam at wallplate level. What do you mean by structural ridge? Looks like they might be plasterboard joint cracks.
@jakko86 : Hairline crack along hip rafters (approx 1 metre long, approx 2 metres from wall plate level)
Let me know if I can provide any further information, or whether I should be getting an engineer to look at it. There wasn't an engineer involved in the construction - just architect (engaged independently of the builder.)

landcrzr

If your sketch is proportional then the collar tie is quite high. The rafters are likely big enough to carry the concrete tiles but the position of the collar tie is not providing much restraint, it is likely the weakest link. If you are worried about it you need to get an Engineer to look at it.

jakko86

Personally I think the roof is fine with very slight hairline cracks it's probably just cracks at plasterboard joints, what you could do is get a straight edge slightly less than the length of your rafter and place it against your ceiling in a few different places to see is there any slight buckles in your rafters, you probably won't be able to do same with your hip because its plastered in a curve in the joint but if you have bellys downward in several places get onto an engineer ASAP. Have a quick look at the top of your walls and look for cracks in case of your wallplate shifting you could also look at the soffit and see that it hasnt pulled away from the fascia with pressure pushing out w/p,But I really doubt you have with the span and size of rafters it's very unlikely that they are sagging.

hookrkrook

I've just realised on reviewing the posts here that I made an error in the original post. We had originally planned a hip roof, but changed the design to a gable roof. I pasted the original spec into the original post, and am only realising my error now. The cross-section sketch is my previous post above is a cross-section through any part of the roof. The sketch is not necessarily proportional, but the collar ties are quite high, resulting in an open, clear, vaulted internal roof space with a gable end.

sydthebeat

hookrkrook wrote: »

I've just realised on reviewing the posts here that I made an error in the original post. We had originally planned a hip roof, but changed the design to a gable roof. I pasted the original spec into the original post, and am only realising my error now. The cross-section sketch is my previous post above is a cross-section through any part of the roof. The sketch is not necessarily proportional, but the collar ties are quite high, resulting in an open, clear, vaulted internal roof space with a gable end.

firstly, hair line cracks are no problem at all as long as they dont keep getting wider. General dryling out and shrinking will create these.

secondly, if you could take a measurement from:
1. internally floor to ceiling
2. internally floor to eaves
3. externally path to ridge
4. room measurements length x width

this will give us a rough idea of the shape of the construction.

hookrkrook

1. internally floor to ceiling : 3220
2. internally floor to eaves : 2170
3. externally path to ridge : 4150
4. room measurements length x width : 7000 x 5000

landcrzr

jakko86 wrote: »

Personally I think the roof is fine with very slight hairline cracks it's probably just cracks at plasterboard joints, what you could do is get a straight edge slightly less than the length of your rafter and place it against your ceiling in a few different places to see is there any slight buckles in your rafters, you probably won't be able to do same with your hip because its plastered in a curve in the joint but if you have bellys downward in several places get onto an engineer ASAP. Have a quick look at the top of your walls and look for cracks in case of your wallplate shifting you could also look at the soffit and see that it hasnt pulled away from the fascia with pressure pushing out w/p,But I really doubt you have with the span and size of rafters it's very unlikely that they are sagging.

If the collar tie is failing (and I'm not saying it is) the end of the rafter, where it connects to the wall plate could move horizontally, that is why they are called ties. It is not an issue of bending the rafter itself and from the OPs description of the size of rafter, it is unlikely to suffer from bending failure.

OP if you are still concerned, get an Engineer. No one here can say categorically what the cause of these cracks are.

sydthebeat

hookrkrook wrote: »

1. internally floor to ceiling : 3220
2. internally floor to eaves : 2170
3. externally path to ridge : 4150
4. room measurements length x width : 7000 x 5000

just did a quick sketch of this, see pdf

your collar ties are over half way up between the eaves and ridge, not a good factor!!

There could quite well be bending motions created between the colar ties and wall plate which is causing the cracking, even regardless of the 225 rafter depth.


my advice would be to monitor the cracks, if they become over 1 mm wide then apply a crack gauge and record readings over 1 year period.

it doesn't sound significant currently, but if it gets worse it may need propping and remedial works.

mickdw

Quite common to get cracks in those areas. IMO, Slab should ideally be isolated from roof structure as structure will move small amounts due to wind gusts etc causing alot of this type of cracking. Im assuming there is no sagging or other defect present

sydthebeat

mickdw wrote: »

.... IMO, Slab should ideally be isolated from roof structure as structure will move small amounts due to wind gusts etc causing alot of this type of cracking....

how would you do this mick?

rodzer

sydthebeat wrote: »

how would you do this mick?

Counterbattens.