Hi david,

Could you post a picture of what you describe. It seems a little like a scheme I've been mulling over for the eaves and verges of a bungalow.

1. Tyvek membrane, held by counter battens (2)
2. counter battens possibly 2 x 2
3. Outer leaf "wall plate"
4. Inner leaf wall plate

The rafter (some 350mm deep Finnjoists or similar) rests on the inner leaf wall plate (4). The overhang is formed by the counter battens (2) which also hold down the Tyvek (1) and provide the vent/drain space under the tie battens. These counter battens pass over the cavity, rest on an angled timber "wall plate" on the outer leaf before cantilevering out past the outer leaf.

The tyvek passes over the top of the cavity and is fixed in place with the counter battens being nailed into the outer "wall plate" and by the plastic "comb" (to stop insects etc getting under the tiles) between the counter battens.

When I suggested it to the architect he was not keen, citing the lack of strength in the counter battens. Fair enough if they are 1" deep and 2" wide but 2"x2" should be stiff enough for 200-300mm overhang.

As I said, this is only an idea at present.

Apologies for the semi-hijack if this is not similar to the construction you've outlined.


B

Hi David,

Are you Constructing the roof on site? I'm trying to find a good supplier of Masonite type trussed rafters to reduce the labour component. Are you full filling with rockwool?

Cheers,

Ben.

How about cedar for the fascias rather than ply (which tends to delaminate)

You could also consider fitting dummy rafter ends on the outside to make it look more traditional?

Ben

Below is an outline of the A frames formed of 350mm Masonite rafters & solid timber tie beams. Note that the 2nd floor deck & joists provide the main tie beam. The 2nd floor is assembled first to allow the airtight membrane to be wrapped over the joist ends. The rafters ends are cut to the vertical to maintain the full insulation depth to the junction with the cavity wall insulation & to allow a windtight perimeter board to be fitted.

David

David, your eves detail looks flimsy! also the underside of the bottom of the bottom tile will be too low causing kicking of the ones above.

Tony & Floyd

I agree it looks a bit flimsy. However, the plywood is continuous from the sarking board stop batten over the outer leaf wall plate. It is screwed to the sarking board stop batten, the 47x47mm mini-rafters & the outer leaf wall plate, & 50x38mm counter battens are screwed through it to the mini-rafters. So I think it will be able to take a high load before starting to buckle. The only load it needs to take is the bottom row of tiles & the arris rail to which the gutters are fixed.

I'm trying to avoid putting any structure in the cavity, while having windtight barriers at the inner & outer leaves. I could beef-up the mini-rafters and run them over the outer leaf, but then I'd need noggins between the mini-rafters. This leads to a lot of junctions all of which would need making windtight, whereas leaving the plywood to oversail the outer leaf wall plate gives one linear junction to seal.

Perhaps I should screw fix false rafter feet to the underside of the plywood, fitting tight to the outer leaf wall plate? These could be 100x47mm or more.

Tony

It may not look like it from my drawing, but the arris rail & batten cavity vent strip are sized to close the batten cavity, while keeping the bottom row of tiles in the plane of the roof.

David

David,

I've had the "it looks flimsy" thrown at me too!

Can I ask the purpose of the woodfibre sarking? Is it because you don't feel the tyvek is windproof enough? I had assumed that the tyvek would be enough to prevent thermal bypass of the wool. If not I'll have to have a rethink!

I've attached a PDF of my roof buildup and the construction method. Hopefuly this makes it a bit clearer.

One of the funnies about my roof is that there are no ceiling joists to take the side loads from the rafters bearing on the wall plate.

I don't want to hijack the thread, so I could tart another thread but it seems that my construction is very similar to davids so it may be constructive to share thinking!

(it's a pdf, but with a .rar, so download then replace the .rar with .pdf, then open in a pdf viewer)

B

David,

That is an excellent question!

The answer is that we have an unusual cross section. The ground floor rooms have a half flat bit at normal height before going up to the underside of the roof mid way. The section should clear things up.

The ground floor rooms (in this case the east and west bedrooms) are in blue

The upstairs 'office' space in a fetching shade of brown

The two box purlins referred to in the pdf (pink) form part of the loft walls.

They are supported by the end walls of the building (plus an intermediate wall).

This arrangement give me:
a) a greater feeling of height in the rooms
b) a bit of "wow" factor (bit subjective this one)
c) a slightly easier time sealing the usually tricky rafter/joist/wall area

Unfortunately it does mean that there are no joists to take any out thrust loads directly.

My plan involves a bit of stiffening of the wall plates to transfer the side loads to points that can take the out thrust loads (the aforementioned intermediate walls) and a roof structure that doesn't create out thrust. That's the last bit of the pdf, trying to get this across to builders/architects etc is rather difficult.



Now i'm not sure about my chain of reasoning in the structure, so if anyone can spot a flaw in my reasoning shout!

B

Posted By: beelbeebub My plan involves a bit of stiffening of the wall plates to transfer the side loads to points that can take the out thrust loads

Shouldn't need to do that if you have a structural ridge beam supported on gable or internal walls.

Sounds like it should be ok.

Got plans for a skylight? When we lived in Belgium we found a house which had a roof with strange shape skylight. The roof was a standard pitched roof but there was a skylight on both pitches that met at the ridge. I imagine it was a great room up there - sun all day long. Looked something like this. No ridge board or beam visible although there was a regular glazing bar along the top (eg it wasn't "frameless")...

@Cwatters;
Wow, that Belgian roof sounds pretty spectacular! I've got 4 roof lights (Velux or similar) but nothing so dramatic. I think the local planners (and conservation officers) would do their nut! I'm in two overlapping conservation areas and an area of outstanding natural beauty so I'm really keeping my fingers crossed that I don't have to go with "consrvation" roof lights ( aka. black Velux's with a bar in the middle and a big price tag)

@davidfreeborough:
you raise some good points about sarking. I've not considered it simply because it's so uncommon around these parts. I believe in Scotland it's routine.

If I may i have some observations/questions on the points you raised. I'm not criticising the sarking only trying to get a better grip on if it is something I should try.

1) I'm thinking that my tyvek (aka sarking membrane) will be run from eave to eave up over the ridge (i.e. parallel to the rafters) rather than the more common horizontally (i.e. perpendicular to the rafters). Each lap between the runs of tyvek will be sealed by the counter batten that nails to the top of the rafter, forming a sandwich of batten, left tyvek, right tyvek, rafter. I had thought that this would keep the joints air and water tight, what do you think? I could tape the joints then nail them?

2)It would act as a bridge free layer. In my design the rafters form thermal bridges through the wool. However the U value of 350mm of plywood is around 0.35 and the bridges are 10mm out of every 600mm. The U value of 350mm of wool is around 0.10 (using k=0.032 wool), so taking into account the bridges of the rafters I end up around the 0.14ish mark. Maybe I need to run the numbers for a thinner layer of wool, with a layer of wood fibre (k=0.04?) but with the slightly reduced thermal bridging.

3)Would the wood fibre keep the upper surface of the rafters dry? I can understand that condensation may form in the outer (upper) parts of the wool. If it does and it is next to a rafter top the rafter top will get wet. Mind you the rafter top will be warmer than the wool as it is a better conductor. Wouldn't the wood fibre get wet? Is it better at absorbing the water and holding it until it can be evaporated than the rock wool? This is one aspect of wood fibre (and breathable/hydro construction) that I struggle with a little. My understanding is that the wool won't absorb water into the fibres, it will just hold it in droplets between the fibres. The water is then evaporated when the temperature rises and drifts out through the membrane. Obviously any water next to the rafter tops will wet and "suck in" to the wood. If there is a layer of wood fibre, does the water stay as liquid between the fibres? Does it get absorbed into the fibres? Does the liquid water get spread around the wood fibre? I'm not sure what's going on to be honest, only that various people tell me that it's either a good or a bad thing (the same thing!)

4) If the tyvek works do I need a secondary wind barrier? To be honest I would feel a little better if there was a slight bit of low velocity air movement in the upper layers of the wool just to help move any water out.

5) Can the wood fibre act as a rain barrier i.e. is it imperious to liquid water and happy to spend a lot of time damp? My understanding was that the primary barrier (tiles) was to keep 99% of the rain out. The only water that gets through is little splashes and dribbles of wind driven rain. The secondary barrier then allows this 1% to drain away. If my primary layer fails so that more than 1% gets through (say a cracked tile) then the secondary layer will keep me water tight but only until I repair the tile, it's not meant as a permanent solution. Ditto if my secondary layer fails, I need to get up there and repair it. If there is a semi water resistant tertiary layer is there not a risk that damage to the tiles or tyvek goes un-noticed for long enough for serious structural damage to occur? (As an aside, i rented a house where the loft water tank overflow emerged from the ceiling directly over the bath - the explanation given for this curious arrangement was a succession of previous tenants had failed to notice or report an overflow that ran down the outside wall until it had gone on long enough for serious damage to occur. During the refit the landlord decided to re-route the overflow to a place that couldn't be ignored!)

6) Are you referring to the noggins needed between the rafters to prevent the twisting over? I've been pondering this exact problem. Are you saying that by having a ply layer under the rafters and a wood fibre layer linking the top of the rafters the need for shear noggins is removed?

B

"(it's a pdf, but with a .rar, so download then replace the .rar with .pdf, then open in a pdf viewer)"

The third page crashes acroread

Posted By: beelbeebub
3)I'm still trying to get to grips with the water movement in buildings. I understand the need for allowing water to leave a structure. It seems to me (at the moment) that a lot of worry is directed at the movement of water through a structure (people saying that if this doesn't happen the house will become really unhealthy) when the bulk of water/VOC etc transfer happens via air movement in and out of the building.

Personally I feel ventilation is essential but that the breathability of the structure itself is much less important. I've nothing particular against breathable design but it has to be done right. My concern is that it has the potential to become "fashionable" and for people to insist their builder omits vapour barriers on structures that are not designed to be breathable causing a risk of interstitial condensation.

Posted By: beelbeebub1) One question though, does the presence of a number of joints not increase the air penetration? I notice that they recommend dry jointing above 18deg pitch, are the joints really tight or would it be better to add a smear of glue anyway?
3)I understand the need for allowing water to leave a structure. It seems to me (at the moment) that a lot of worry is directed at the movement of water through a structure (people saying that if this doesn't happen the house will become really unhealthy) when the bulk of water/VOC etc transfer happens via air movement in and out of the building.
4) Do you think the wind is getting through the Tyvek it self or through the gaps.

1) The joints are quite tight. However, as you might have guessed by now, I'll be going for the belt & braces approach of woodfibre sarking board & Tyvek (or similar). The Tyvek will be taped to give the windtightness. The woodfibre should stop it flapping around or making farting noises.
3) For me breathability is just about making a robust design which will stand-up to the unexpected. You still need to make it airtight & ventilate it. The cheapest air barrier is 0.2-0.3mm polyethylene and this also acts as a pretty good vapour control layer.
4) I think the wind is getting through the laps & around the eaves & gable perimeters.

David