Warm Flat Roof - Airtightness and Drainage Channel/Costs

[AliG]

Hi,

 

We have a warm flat roof design.

 

This is causing two issues - Cost and airtightness

 

1. Cost for the Bauder Thermoplan covering is coming in at just shy of £150/sq metre fitted, based on 145sq metres of roof. Considerably more than I expected.

 

The breakdown is 

Flat areas £10,000

Perimeters and roof light upstands £6000

Outlets £1000

Drainage channels £4000

 

A couple of questions spring to mind. Are the numbers quoted online for flat roofs, for totally flat roofs and is it normal to charge for the upstand separately? In fairness the upstand is 3-400mm tall.

 

Are drainage channels actually necessary on top of a fall in the roof? These have been specified by the architect, but I cannot find them in standard details?

 

We have a considerable amount of this type of detail, where the gutter area is priced at over £100 per linear metre by the looks of things.

 

 

 

 

2. Airtightness.

 

The builder has questioned how we achieve airtightness under the roof as there will be a number of penetrations through the ceiling. Spotlights, cabling and MVHR.

 

He also asked how do we tape the airtightness membrane  with nothing above it. 

 

My thinking is that either we put airtightness membrane across the joists and then create a service area below so the only penetrations are the MVHR which goes between the joists and could be taped up.

 

The alternative is to use two layers of plasterboard with a service void as it would be easier to seal this for the MVHR penetrations and we would tape the joints and tape the wall membrane to it in the service void.

 

Thoughts?

 

Looks like the airtightness question came up earlier in the year but there were no responses.

 

 

 

[Dave Jones]

have a look at kingspan thermataper, this allows you to have a flat buildup from the joists. 

 

Your build up needs to be 

 

Joist

OSB decking

Alutrix VCL brought up sides of walls. You are watertight at this point.

Insulation

Final weather cover EDPM etc

 

This buildup gives you zero penetrations into the buildup as its outside the build envelope.

 

 

 

[AliG]

I wondered about that.

 

So do you just tape the wall membrane to the OSB then and not have to do anything else at the roof?

 

 

[Dave Jones]

essentially yes. The Alutrix needs to cover the whole roof and lap 150mm onto the walls. Insulation then goes on top. You want zero holes in the alutrix.

[AliG]

I spoke to MBC and they said that their standard detail is to attach airtightness membrane to the underside of the roof joists using battens to create a service zone. This they find better as you do not have to deal with sealing the end of the joists.

 

They said that you can put it on top of the joists also in a warm roof construction, but the wall membrane only goes up to the underside of the joists so accounting for the area in-between is difficult.