I built my two storey extension for 20% of builder's quotes

This site shows how - step by step from design to completion

Making a grp Box Gutter

I was starting to realise just how big a job my roof was. I had lifted the trusses into position much quicker than I had expected, and this changed the appearance dramatically in a short time. I think I was fooled into thinking the roof was all coming together really fast.

Unfortunately, following the trusses everything seemed to take ages. Working alone and having to go up and down scaffold slowed things down, but the fact is there just is lots to do on a roof of my design.

Next I tackled the box gutter (where the two duo-pitch roofs come together)















box_gutter_3I had designed one set of roof trusses to be cantilevered (the ones on the left) so the steel that supported all the trusses would be placed directly above the stud wall I would create below. This would mean I could easily hide the steel within the wall, and it would be invisible.

It was now time to see if all my design work came together. I had spent a long time pondering, re-designing and trying to visualise just how the two roofs, the box gutter and my little break-front in the brickwork would work, look right and feed neatly into a hopper for the guttering. It did my head in, and I was certainly not 100% confident how it would work out.

box_gutter_4I had left the bottom few battens off so that I could pull back the felt to give me access.

I would make support boards from 18mm OSB. I started by working out the width of the boards. I knew I wanted the width to be 200mm where the gutter terminated at the hopper end. Advice varies on the rate of fall for the gutter. Some suggest 1:40, some 1:60. Now I come to write this up I realise that I have actually forgotten which I ended up going for, but I am sure either would be fine!

The run from back to front is about 4M, so at 1:40 it means the far end needs to be 100mm higher than the outlet end (67mm for 1:60) I measured that distance vertically and noted where that came on the angled rafter. I then laid a string line and made marks on the rafters.


Though it does not look it from the pic because of the foreshortening, the near end is 200mm wide and the far end about 450mm. I cut boards, ensuring the joint would be over a rafter.

I fitted support struts between each set of rafters underneath to offer the board maximum support. I then added boards up the rafter line on each side to form the gutter.

I taped the joints ready for the next stage. 

There are several products on the market for lining the box gutter. Many people automatically think of lead for linings. I am sure every pro has their own preference, but such large, wide sheets of lead would not only cost a bomb, but large lead sheets can lead to problems. I checked out EPDM (a kind of rubber sheet), GRP (fibreglass) and roofkrete. I am not sure, but I think that GRP is probably the most used in this situation, so I went for that.

Some companies sell the mat and resin particularly aimed at the roofing industry, so I assume it is somehow different from that used for yachts, for example (not sure how, but I decided to get proper roof grade stuff)

I had done a little bit of fibreglassing when I was a kid - probably model aircraft and the like - but I was certainly no expert.

box_gutter_5Fortunately, I found a very very helpful guy at Matrix Roofing (I unashamedly, and without benefit, advise you contact them for advice if you need any - the guy there was very patient and offered me tons of advice on how to go about doing the job - great service!)

I have spent some time making furniture, but the advice was to forget everything about such finery. Laying the GRP is all about not messing about. The process is quite agricultural - slap it on, get it down.

I laid an extra strip over all joins and then the full 600g mat over the whole lot. I wet the boards first with resin then laid down the mat and rollered more resin through. Mix of the resin and hardener is quite critical and the ratio needs adjusting depending on weather. The helpful sheet supplied by Matrix offered all the ratios. I think I mixed at 2%

box_gutter_6Nearly done as I reached the hopper end. I was supplied with a second metal roller which I ran over to make sure all the mat was nicely soaked with resin.

Again, I know it doesn't look like it's getting narrower, but it is.

It is best to try to get the topcoat on once the resin and mat have had chance to set. If it is left, and the top coat done some days later it will be necessary to do some prep work to get a good bond.

I had a cuppa then mixed up some topcoat. The Matrix idiot sheet gives you all the times, quantitie etc.

I bought grey topcoat to look like lead. I am sure this is the most common, but if you want to go crazy I am equally sure there are a range of colours.

The topcoat is just like the resin. I mixed it with hardener and set to with a roller. I would have had a tendency to think a nice thick coat would be best - belt and braces, but apparently too thick and it can crack. The recommendation was that it was not so thick as to totally obscure the appearance of the matting beneath.

On the whole I was very pleased with the job, though I am sure a pro would be better. 

The best advice I was given was - don't mess about. Slap it down and move on. My natural inclination would have been to be more fussy, but I would surely have ended up half way down the run with a tin full of resin that was starting to go hard.

Next job - valleys - four of the buggers.