American Deck House.

[David Allen MISVA]

I am inspecting an American timber frame kit house by Acorn Deck House (formerly Deck House Inc) built in 2003/6 with a glulam frame and timber frame walls.  They are 100mm timber with insulation, a vapour barrier and plasterboard on the inner face and what  the drawings describe as wall panel (ply or particle board?), and infiltration barrier (no idea what it is), and a smooth lime render externally.  There are lots of cracks in the lime render, particularly on board joints.  There is no obvious dampness or condensation internally, nor any anomalies when viewing with an infrared camera that might suggest cold damp areas.  I am thinking that if the infiltration barrier is intact (whatever it is) then there is only a small risk of water entry through the cracks, but it may be difficult to repair if the render needs to be removed and replaced.  Also the very large roof is covered with bitumen felt ’tiles’ about the size of a plain tile, and they are not, to my knowledge, available over here – it all came from America.  Anyone have any experience of these (I think there are only 6 in the country) or any thoughts on the above?

[Stephen Quirk MISVA]

A deck house seems to be a transatlantic term for our term “Timber framed buildings”. Normal/ UK construction would require the timber frame panels to have an internal surface of plasterboard followed by a pvc vapour barrier (or integral with the plasterboard) the insulation, racking board (to prevent twisting of the frame), a “breather membrane”, a cavity and then the cladding. I suspect that the “Infiltration barrier” is the breather membrane as it also serves to keep moisture out of the frame but allows moisture to escape. If the render has been applied to the surface of the panel on top of the breather membrane then it will also need to have expanded metal or similar to allow the render to adhere to the panel. If the render has cracked at panel joints it is possible that the expanded metal has not been continued across the joints and is thereby causing a weak point. A better detail would have been to have a space between the breather membrane by fixing a vertical batten (protected with a pvc dpc) and a layer of expanded metal to which the render could be applied. Suspect some “additional investigation” may be asked for?

Just for the record my first job out of college was for a company know as Timber Structures (Oxford) Ltd who used the “Minox” system of timber framing.

Hope this helps.

[Alan R Young MISVA]

As highlighted by Stephen Quirk this is an inferior form of construction as the cladding is not separated from the structure by a drained cavity. This being the case water exclusion is reliant firstly on the render and more importantly on the breather membrane (Infiltration barrier), especially where cracks have developed as these will tend to draw in water by capillary cation.

If the render is of breatherable lime, this should allow moisture to evaporate unless it has been sealed with a plastic paint?

It therefore appears to be the case that the serviceable life of the cladding is determined by the infiltration layer, and unless the material is known the remaining serviceable life cannot be predicted.

Interestingly modern comparable materials such as Tyvek Structure Guard are said to have “a lifetime equal to the building element in which it is installed” However 20 years ago things may have been different in the USA.

The fact that the render has cracked in multiple places suggests that thus is overly rigid and/or un-reinforced (metal or plastic) as is now considered to be good practice when render is applied to a flexible background (ie the sheathing layer of a timber-framing building).

Given the above I would suggest reporting that the remaining serviceable life of the cladding is limited and that periodic maintenance will be needed until it is eventually replaced. In the meantime, the existing cracks should be repaired using a pliable material to match the original materials, which may include plastic mesh to prevent re-occurrence.

Personally, I believe the best form of cladding for timber-framed buildings is timber (eg weatherboard) but thats another issue, but could be considered if and when the walls are eventually re-clad. This is the usual cladding for timber-framed  “Colt” houses in the SE.

The roof tiles definitely have a limited remaining serviceable as bitumen generally is not a long-lasting material.  Even high-performance bitumen lasts no longer than about 40 years. I would check these close hand to assess the condition and whether or not they are becoming brittle.

Hope this is of  interest

[David Allen MISVA]

Many thanks for these constructive comments, which broadly follow my own thoughts.  The client decided that there were too many potential downsides, as well as the fact there may be a limited market when selling due to the unusual nature of construction, and decided not to proceed.

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