Shed Roof Problem

by John Varcoe
(Cornwall, England )

Boards used for shed roof

Boards used for shed roof

I have recently discovered rot in the roof of the log cabin in our garden which is used as an office. I think the pitched roof is constructed with sterling board - well it is a large-chipped board. The boards are covered with a thin black membrane (which I was told was waterproof) and strips of self-sticking felt roofing shingles. The builder botched his work with the shingles and did not fix two overlapped layers on the bottom rows. He fixed only one layer and I have now found that the sterling board is rotting underneath the gaps in the shingle strips along the full length of both edges. Wood lice are under the shingles which are eating through the black membrane.

The pitch of the roof is 24 degrees. I will be doing the repair work myself and need to keep costs down. Depending on your advice, I will either replace the rotten areas with sterling board or replace all of the ten sterling boards (size 8' x 4' x 3/4'') on the pitched roof.

Could you please advise on the best materials for this job. Should I use better quality 8 x 4 boards such as WPB external plywood or marine ply?

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Mar 03, 2010
Shed Roof - Drip molding
by: Randy Wright

I never build a roof without using galvanized or aluminum drip edge molding. It has two edges to direct water drips. I agree with my brother's comment that capillary action will kill your decking without it. Well worth the investment.

I don't know what style you have in the UK, but I'm sure it's similar to the Yank stuff.

Oct 29, 2009
Re. My Shed Roof Problem
by: John Varcoe

Thank you John and Greenboy for your very useful comments. You both recomment WBP plywood - the spacing of the beams is 2 ft so should I use 12mm or 18mm boards?

I note that you have not commented on the builders use of the breathable membrane under the bitumen shingles (that has been damaged by wood lice) and Greenboy has recommended bitumen roofing felt under the shingles. Does this mean that the membrane is wrong for this sort of roofing job?

The shingles that are currently on the roof are self adhesive. Are these OK in the kind of weather that Cornwall has during the winters, lots of rain and occasional strong winds up to 60 mph.

Lastly, I am considering cutting off the rotten lower edges of the boards - maybe by about 2 feet and replacing with new boards. Would this be a satisfactory repair or is it just not on?

Oct 29, 2009
Get the right grade of OSB
by: John - Admin

Hi John,
For construction a roof I would tend to use WBP ply. If you do decide on cost grounds to use OSB be aware that it comes in three grades with different degrees of resistance to moisture the one to use wher it is for a critical element such as a roof is OSB3. You may need to order this specially or go to a specialist timber supplier, general builders merchants (eg Travis Perkins) may give you a blank look when you ask what grade it is they stock.

With regard to the thickness it depends on the span between supports. If they have used 18mm on the existing shed I would go along with that.

The other tip is to use a drip edge so that water cannot find it's way back into the board through capillary action.

I hope this helps.


Oct 29, 2009
by: GreenBoy

As you are trying to keep the cost down i would advise against marine ply - this is very expensive. And not totally necessary.

Prsonaly i would go for standard WBP plywood in either 12mm or 18mm (dependant on the strength of the supporting structure)

Ensure you but joint the boards tightly and nail securely to the supports to avoid movement. dont skimp on this - yes it will make it difficult in remove in the future but the intent is not to have to do that isnt it?

Now this is optional - lay one layer or roofing felt with overlaps as epr manufacturers instruction. Use a cold cure bitumen to stick the edges down and dab a little over all nails used to fix the felt - ensure you use the correct type of nail - they are designed to not rust and have a large head to avoid tearing through the felt.

Then lay bitumen shingles as per the instruction as above use a cold cure bitumen to seal the tiles on the edges to stop the wind getting under the shingle and ligting it. It also stops wind blown water ingress. Use boards to protect the shingles as you lay them to avoid tears. Mend any tears immediately as its easy to forgett them later.

Grab a beer, relay and enjoy your lovely cabin - and it looks a beauty.... lucky sod! :-)

Oct 29, 2009
nice cabin
by: Anonymous

love the real stuff!

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