How bad is a ridge board?

[Mr Blobby]

The construction detail from the architect had the insulation between the rafters connecting over the top of the ridge beam.

 

Being clueless about the potential to deviate from this construction detail, I thought it unimportant.  Oops. 

My roofers have of course installed a ridge board above the beam.  Which isn't really their fault, they were not given the ridge detail I thought unimportant.

 

The ridge board is the bit bewtween the rafters on top or the steel, like this:

 

 

... which is a bit of a shame because the ventilated roof sits on top of that ridge board. 

The loft is warm (or should be) and so the ridge beam sits inside the heated envelope.  That steel is now connected to the cold roof along its full length by timber instead of the planned insulation between the rafters.  😫

 

Short of getting a bunch of trained otters from the zoo there's not much I can do now that the roof is covered. 

So just how bad is the cold bridge created by the ridge board? Should I be naffed off to have missed this?  Please someone cheer me up.

Edited November 14, 2023 by Mr Blobby

[markc]

As long as insulation is packed tight to the ridge board it will have a similar effect as all the other rafters. Timber is a poor conductor of heat.

[SuperJohnG]

It'll be fine..don't worry about. 

[joe90]

8 hours ago, markc said:

Timber is a poor conductor of heat.

+1, just pack it with insulation 👍

[Redbeard]

Does your architect have any site supervision responsibility? If so arguably he/she could instruct the builder to remedy and build as per the dwgs. Those rafters appear to bear on a timber plate on top of the steel, and are presumably tightly butted to the ridge board, so the archo could arguably instruct the contractor to cut out between each rafter pair. They will probably meet as tightly on the bits of ridge board as they would have done on each other. Can't see how they are fixed so far anyway. Plainly the suggestions made so far would be simpler. Just depends whether OP wants it built to the plans.

[ProDave]

How are the rafters fixed to the ridge board?  I don't see any brackets.  Normally with a roof hung from a ridge beam, which is what that is with the steel beam, there is some form of substantial bracket or strap to stop the rafters sliding down away from the ridge beam.  The lack of that fixing would worry me more.

 

What do the drawings say?  and why were the builders building without the detailed drawings?

[ToughButterCup]

2 hours ago, ProDave said:

How are the rafters fixed to the ridge board?  I don't see any brackets.  ....

 

Nail gun ? Screws ? 

[Alan Ambrose]

>>> a bunch of trained otters from the zoo

 

Looking for a more skilled workforce?

 

>>> So just how bad is the cold bridge created by the ridge board?

 

You can work it out approximately, if you can be bothered (or get someone to do it for you). I would guess not very much - add maybe 3-5% to roof w/m^2?  Maybe there some internal insulation detail you can figure - the RSJ was always going to be a cold bridge anyway, so some beefed-up and specific insulation detail under that?

[Redbeard]

Ah! You added the otters and 'now that the roof is covered' after I'd read your first post, I think.

 

Obviously if the tiles/slates are on you won't be taking up my suggestion.

 

Am I right in thinking those are 150 (perhaps 200 at a stretch) rafters? What's the proposed lay-up in terms of ventilation gap on the cold side, and depth of insulation? I assume you will have insulation both between and under the rafters or (it would appear) you will have difficulty meeting the Part L U value target. (I think... probably)

4 hours ago, ProDave said:

Normally with a roof hung from a ridge beam, which is what that is with the steel beam, there is some form of substantial bracket or strap to stop the rafters sliding down away from the ridge beam.  The lack of that fixing would worry me more.

 

If the OP takes @Alan Ambrose's suggestion

 

1 hour ago, Alan Ambrose said:

Maybe there some internal insulation detail you can figure - the RSJ was always going to be a cold bridge anyway, so some beefed-up and specific insulation detail under that?

perhaps with collars and insulation above the collars, he could maybe solve both issues.

[Temp]

Did the architect specify any straps linking the rafters on one side to the other over the top of the structural ridge? Perhaps not needed as you also have structural purlins as well but perhaps check.

 

Edit: Ah I see @ProDave already raised this.

 

 

 

Edited November 15, 2023 by Temp

[SteamyTea]

Pine has a thermal conductivity of around 0.15 W/m.K

So 0.1 m will conduct 1.5 W/K (R-Value = length / conductivity).

The U-Value is 0.67 W/m².K (1/ resistivity).

 

Without knowing the exact dimensions, I shall assume that the thickness is 0.04 m.

0.67 [W/m².K] x 0.04 [m] = 0.027 W/m².K for each linear metre.

 

You say it is a warm roof, so the steel will be in the region of 20°C.  It is a very rare spell of weather where the outside air temperature is below -5°.

So.

0.027 [W/m².K] x (25) [dT°C) = 0.68 W/m

 

If I assume that the beam is 10 m long, that is 6.8W at -5°C

 

Now my kitchen has two pendant lights of 3W each and my laptop at about 10W that is 16W.

 

(It is early and I may have made a mistake).

Edited November 16, 2023 by SteamyTea

[Mr Blobby]

On 15/11/2023 at 10:09, ToughButterCup said:

 

Nail gun ? Screws ? 

 

Big nails is a good guess.  My understanding is the ridge board is not structural, just a guide to make it easier for the roofers to fit the rafters so hopefully nails is not a big deal.  There are purlins half way down supporting the rafters as well.

 

On 15/11/2023 at 13:00, Redbeard said:

Am I right in thinking those are 150 (perhaps 200 at a stretch) rafters?

 

Yes 150 rafters.

 

On 15/11/2023 at 13:00, Redbeard said:

What's the proposed lay-up in terms of ventilation gap on the cold side, and depth of insulation? I assume you will have insulation both between and under the rafters or (it would appear) you will have difficulty meeting the Part L U value target. (I think... probably)

 

50mm ventilation gap above rafters then 150mm glass wool between rafters and 130mm PIR underneath rafters.  Then VCL.

 

 

... as others have said, the heat loss throught the ridge board is not terrible as timber is a good insulator.  I'm just naffed off that I overlooked this detail. 

 

On 16/11/2023 at 07:03, SteamyTea said:

Without knowing the exact dimensions, I shall assume that the thickness is 0.04 m.

0.67 [W/m².K] x 0.04 [m] = 0.027 W/m².K for each linear metre.

 

Thank you for the calculations but my brain struggles with the size of the board and u-value.  So the board is 0.04 wide, as included in the calculation, and about 0.17 high.  Why is the 0.17 high not included in the calculation, surely the heght of the board would affect the heat loss?