Flat roof

This page details the construction of the flat roof on the extension along the rear of the building.

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First job was to get some more scaffolding in place so I can get up to the existing pitched roof eaves and sort out where the new flat roof is going to join onto the existing roof.

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Restraint straps were added onto the wall plate and fixed in several places down the blockwork using big thick screws.

The truss clips have also been placed along the top of the wall plate in order to check the spacing and make sure I don't put a strap where a joist needs to go, these will be fixed in place just before the joists go on.

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In order to fit the flat roof onto the top of the wall the existing pitched roof will need to be trimmed back.

As I was working on the wall below this corner and needed to make sure I propped it in the right places I have removed the lower edge of the roof in preperation for getting the wall plate on this section.

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The tiles, battens, and felt were removed from the bottom section of the roof in order to access the base of the rafters.

This picture shows three rows having been removed, but a further couple of rows were also stripped off.

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Having cleared the roof overhang I removed the brickwork from between the rafters.

The bottom of the rafters were then trimmed back so they would be flush with the back face of the new wall plate.

At this point I completed the brickwork above the lintel for the new doorway in the internal wall and added a further course of bricks along the entire length of the wall to bring it up to the level where the wall plate could be installed (shown in next picture).

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The wall plate was bedded onto mortar and once that had gone off was fixed down as with the others. On top of the wall plate were added double joists of 225mm x 47mm bolted together, care being taken to get good overlaps on the joins.

Using a string line to get them level and a magnetic spirit level to get them plumb, hangers were fixed to these joists and the ends wrapped over the top and fixed down the back.

Fixing the hangers involved hammering in 632 nails (yes, I counted them). I managed to get away with only hitting my thumb twice, so I was pretty happy with that.

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Moving on a bit and the roof joists have been placed into postion. The outer ends are notched to clear the outer leaf of the wall and fixed into the truss clips, and then the noggins placed between them to keep them square.

As the span is 4.2 metres there is a set of intermediate noggins at mid span. I did start off using the metal herringbone struts instead of blocks for noggins, but because quite a few of the joists were fairly distorted (having got wet and then dried out again) the metal straps were not good at keeping them square. The ones under tension were fine, the ones under compression just bent at the fixings.

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The ends of the joists were left the length the timber arrived at until I had received all the guttering and fascia deliveries and I could work out how much of an overhang I was going to have on the outer edge of the roof.

Having decided where it was going to be, I setup a string line top and bottom. This string line was positioned so that when a block of wood was attached to the joist as a cutting guide and aligned with the string the circular saw blade would cut the joist to the correct length. It was a fair bit of messing about to set it all up, but it did make the actual cutting of the joist fairly painless.

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The original intention was to vent at the front edge of the roof but have the sides flush with the wall, however, the building inspector said he wanted some cross ventilation so that had to change.

The picture shows my slimline solution. The firring strips have been added, to provide a fall to the gutter, and counter battens have been added to the top of those to provide a cross ventilation space. This ventilation space has been carried down the sides with the blocks attached to the outer edge.

The black item at the bottom is an over fascia vent. I have re-purposed this by fitting it rotated 90 degrees from its usual position. This gives me the required ventilation with the minimum possible overhang of the roof edge. The inset picture shows how these vents are usually used.

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Some further rows of tiles were stipped off along with their battens to allow the rubber roofing membrane to be lapped up under the felt and provide a secure join between the two rooves.

The picture shows the start of the decking boards being applied. I went for square edged 18mm OSB as I could get that for a sensible price. I maybe should have gone for the smaller T&G boards as there would have been no requirement to put noggins to support all the edges and it would have been easier, but the cost and lack of availability put me off.

You can also see the roofing rubber is now on the top. This weighs 95 Kilos and was a pain to raise the final few feet onto the roof. This I did by putting it on a pallet and using a long length of 10x3 timber as a lever and bags of cement as a counter balance. Repeatedly levering the pallet up and placing further pallets and stands underneath allowed me (evenutally) to get it to roof height where I could drag it over.

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With the decking all laid it was time to sort out some more of the details.

Here's the front edge of the roof with some tile battens added to provide a base to fix the fascia and soffits. Although the thicker fascias I have don't technically need a backing, I wanted to move them forward slightly to get the soffit to fit underneath without having to cut it along its length.

The inclined batten in the middle is to take the gutter clips, again not technically needed, but belt and braces.

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And here is the same view with the fascia boards fitted and the first part of the gutter edge trim for the roofing membrane attached along the top.

Dealing with 5 metre long bits of bendy plastic single handed is a challenge, but I've long arms and some handy clamps to help so got there in the end.

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The ventilated soffit boards were located into the groove in the back of the fascis and nailed up into the batten next to the wall.

These were even more bendy than the fascias, but the biggest pain was actually getting the protective plastic off.

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With the fascia and soffit boards done the gutter clips were added and the gutter itself put into place.

There is definitely more of a slope to the gutter than is required, I probably got a bit carried away with the drop. There will be a lot of water coming off this roof though so getting it away quickly is no bad thing.

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Having added the vent to the side of the roof it has left a bit of an awkward corner.

I could have avoided this by matching the front and side fascias, but then there would be the vent below, so this was always going to be a problem whichever way I went as something would need a bit of work to make it look tidy.

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Luckily I had four corner pieces so I was able to modify them to fit into my special situation.

On the left is the standard corner piece, in the center is one modified to fit under the vent, and on the right is one modified to cover that and the actual fascia corner join.

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Here they are fitted.

Yes, this is the other corner of the roof. The lighting was better for taking a picture at this end (and it was the second one I did so a little neater).

The vertical corner cover is notched to clear the two cover strips that will go on over the rubber roof membrane when they are fitted.

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And this is the same corner from the reverse angle.

Some mastic will now be needed along the back of the soffit and to fill some of the tiny gaps around my custom corners.

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The final job for the guttering was to put the downpipe in place.

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Before gluing the roof membrane down I used some 6mm ply to create a fillet strip along the back edge. The rubber was tending to drop into a small gap between the top of the OSB decking and the face of the rafters, sagging slightly between the rafters didn't help. I was concerned this would allow water to pool under the bottom of the tiles so hence added the fillet.

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And here is the finished article. The membrane has been glued down and the finishing strips added to the edges. I've also replaced the battens and tiles along the bottom of the pitched roof.

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Some slight adjustment was required at the side as I had cut the tile battens back to the rafter and so there was nothing on which to hang the tiles right next to the wall. I stripped back to the next rafter along and added some new battens to span up to the wall, before cutting some tiles to fit and making it tidy.