Board on Board on Board

An Eccentric Anomaly: Ed Davies's Blog

As previously noted, I'd got most of the easy bits of the east-gable cladding done by the end of August but it still took me most of September to finish off the lower part.

Having learned a few lessons from the west gable I'd did some things a bit differently. Firstly, I gave the boards two coats of paint. One covered well enough in most places but, particularly where sawing was smooth, it was a bit thin so I felt happier with two.

Secondly, to reduce the risk of the boards splitting I avoided nailing them too close to the ends:

There's a 90mm overhang at the bottom and the batten under the edge at the top isn't actually nailed, it'll just support the end of these back boards and the ones above, the nails are in the batten just below.

The east gable is different from the west one in that it doesn't have the porch/greenhouse attached so the boards need to go to the bottom of the wall rather than the level of the “finish” floor in the p/g. As with the east gable I used a board “spreadered” into position at the bottom to support and help align the boards. In this case, I nailed 90mm of packing to the top of this to keep it in the right place.

Also unlike the west gable, the boarding here will be unbroken from the bottom to the top which is longer than the available boards (4200 mm) so a join is required. I've cut the front boards down by 100 mm to create a bit of an overlap in the hope that this will make the joint a little less vulnerable.

I tried to arrange that the cut ends, which will not be so well treated, are at the bottom where they'll be less vulnerable to water pooling but I'm not sure I've got that right in every case.

The interesting bit for the east gable, though, was how to match up the board-on-board cladding with the Velux window. That took quite a bit of puzzling but I'm mostly fairly happy with the result.

The point of the board-on-board is that the wood is well ventilated: only small parts of the wood are in direct contact with other surfaces (the neighbouring boards and the battens) and those are close to parts which are open to the air so the whole lot should dry out pretty quickly when it gets wet. Around the Velux, though, that had to be compromised so this wood is likely not to last as long. At least with board-on-board if a couple of boards need replacing it should be possible to do without having to remove more than one extra front board, unlike with some sort of tongue and groove where things are likely to go further.

First, full boarding below the window…

then a chamfered batten above that with a board nailed on…

covered with vapour-open membrane:

Then fit the Velux “skirt” part of the flashing:

Then wake up in a cold sweat the next morning realising you've completely forgotten about the need for a firestop round the window. Try fitting it below the battens (under the boards at the bottom) but find that's not going to work so remove the skirt again and tuck it in directly under the window.

and put the skirt back using a few clamps to squish the firestop.

One to hold it roughly in place while placing three more to actually squish the firestop then a fifth as a “mobile” clamp while feeding in the bar which is used to screw the skirt in place.

Fit the side gutter parts of the flashing, trim the batten above to both make room for the top part of the flashing and support it… and fit the flashings which was one of those jobs which is easy once you work out how to do it but a bit of a puzzle matching up the bits in your hands with the diagrams in the instructions.

Then firestop round the window (and board the north bit, to the right of the window), making rather a mockery of the Velux drain piece above the window but it'll still help a bit, I suppose.

So then the question of how to make the boards fit around the window being reasonably well supported despite the gap between the ends of the battens and the sides of the windows (i.e., the width of the firestop and the gutters). Velux's instructions say to allow 30 to 60 mm between the edge of the window and the cladding.

I spent quite a lot of time standing in front of this window waving my hands and a tape measure around trying to work out a plausible scheme. A bear with a slightly bigger brain would have pitched the boards to make them symmetrical around the window in the first place but I got lucky and they weren't that far off being so anyway.

First a narrow back board to fill the gap immediately to the left (south) of the window and another back board on the pitch where the next back board would go lined up with the leftmost of the boards below the window but cut short so it stops overlapping the Velux skirt below the window.

These were nailed to the battens “properly”. Next a full-height front board with a small cutout to go round the end of the skirt and another front board which was a bit more cantilevered out, again stopping on the skirt but because it's further forward it can go a bit further down on the skirt.

Then add a small piece of board between these two front boards cut off at 60° left to right and also at 22.5° front to back for at least some hope of drainage. Similarly another front board also cut off at an angle (the one behind the right-hand clamp (spreader) here).

Then a “front-front” board (i.e., board on board on board) cut at 60° and chamfered at 22.5° to cover much of the ventilation/drainage gap created

together with a narrow one to its right to cover the rest of the gap. Here with the left (south) side done and starting the odd-shaped boards on the right.

The version on the right is a little better as the end of the Velux skirt is curved up to help prevent any water that gets in from splashing off to the right.

It would have been better to angle the bottom cut below that curved-up piece of the skirt to stop water pooling but I only thought of that just after it was all nailed in place.

The right-hand side was pretty similar though the alignment was about 15 mm different so I had to make a small cut out in the front board down the side of the window.

Done, as closely as anybody's likely to look at it:

All I need to do now is stuff some scrunched-up fibreglass mesh in the gaps so to reduce splashing inwards and to discourage insects.

As with the west gable, the top bit needs doing of course. The roof will take priority, though.